Concept #1: Calculating Net Work

Practice: A 3-kg box is on a flat surface. The box-floor coefficient of friction is 0.6. When you pull horizontally on it for 10 m, it moves with 2 m/s^{2} . Find the net work on the box. (Start by finding the magnitude of all forces acting on the box)

Practice: A 2-kg box is on a rough horizontal surface. When you pull horizontally on it, it moves with 3 m/s^{2} . The magnitude of your force and the box-floor coefficient of friction are unknown. What is the net work on the box across 5 m?

Concept #2: The Work-Energy Theorem

Practice: A 4-kg object has speed 6 m/s at point A, and speed 10 m/s at point B.

(a) How much work was done to it between A and B?

(b) If –32 J of total energy is done to the object between B and C, what speed does it have at C?

Concept #3: The Work-Energy Theorem,ually

Practice: You lift a 3 kg object from the floor to a height of 2 m. Find the:

(a) work done by you;

(b) work done by gravity;

(c) net work done on the object. You then walk horizontally with the object for 10 m.

(d) How much work do you do?

A mass of 1kg begins at a position (–3 m, –4 m) at a speed of 4.6 m/s. If it moves in a straight line and ends up at a position (2.5 m, 3 m) at a speed of 3.2 m/s, how much work was done on the mass? What is the net force acting on the mass?

The horizontal force FP = 150 N force pushes the block of
5.1 m
up along the 32
incline? A coefficient of friction
exttip{mu _{
m k}}{mu_k} = 0.12. [Hint: Work-energy involves net
work done.]How much work is done by the horizontal force FP?How much work is done by the gravitational
force on the block during this displacement?How
much work is done by the normal force?What
is the speed of the block (assume that it is zero initially)
after this displacement?How
much work is done by the friction force?

A gardener pushes a 16 kg
lawnmower whose handle is tilted up 37 above horizontal. The lawnmowers coefficient of rolling friction is
0.21. How much power does the gardener have to supply to push the lawnmower at a constant speed of 1.3 {
m m/s} ? Assume his push is parallel to the handle.

An 8.1 kg crate is pulled 5.1 m up a 30 incline by a rope angled 18 N above the incline. The tension in the rope is 130 (Pages 222 - 223) and the crates coefficient of kinetic friction on the incline is
0.25.You may want to review (Pages 222 - 223) .
For help with math skills, you may want to review:
The Vector Dot Product
How much work is done by tension, by gravity, and by the normal force?What is the increase in thermal energy of the crate and incline?

A 16-kg sled starts up a 28 incline with a speed of 2.3 m/s . The coefficient of kinetic friction is exttip{mu_{
m k}}{mu_k} = 0.26.How far up the incline does the sled travel?What condition must you put on the coefficient of static friction if the sled is not to get stuck at the point determined in part (a)?If the sled slides back down, what is its speed when it returns to its starting point?

Proper design of automobile braking systems must account for heat buildup under heavy braking.Calculate the thermal energy dissipated from brakes in a 1500 kg car that descends a 18 hill. The car begins braking when its speed is 95 km/h and slows to a speed of 40 km/h in a distance of 0.31 km measured along the road.

You are a member of an Alpine Rescue Team and must project a box of supplies up an incline of constant slope angle exttip{alpha }{alpha} so that it reaches a stranded skier who is a vertical distance exttip{h}{h} above the bottom of the incline. The incline is slippery, but there is some friction present, with kinetic friction coefficient exttip{mu _{
m k}}{mu_k}.You may want to review (Page).
For related problem-solving tips and strategies, you may want to view a Video Tutor Solution of Using work and energy to calculate speed.Use the work-energy theorem to calculate the minimum speed you must give the box at the bottom of the incline so that it will reach the skier. Express your answer in terms of exttip{g}{g}, exttip{h}{h}, exttip{mu _{
m k}}{mu_k}, and exttip{alpha }{alpha}.

In the following figure assume that there is no friction force on the 20.0-N block that sits on the tabletop. The pulley is light and frictionless.Calculate the tension
T
in the light string that connects the blocks.For a displacement in which the 12.0-N block descends 1.20 m, calculate the total work done on the 20.0-N block.For the displacement in part B, calculate the total work done on the system of the two blocks. How does your answer compare to the work done on the 12.0-N block by gravity?If the system is released from rest, what is the speed of the 12.0-N block when it has descended 1.20 m?For a displacement in which the 12.0-N block descends 1.20 m, calculate the total work done on the 12.0-N block.

An old oaken bucket of mass 6.75 kg hangs in a well at the end of a rope. The rope passes over a frictionless pulley at the top of the well, and you pull horizontally on the end of the rope to raise the bucket slowly a distance of 4.00 m.How much work do you do on the bucket in pulling it up?How much work is done by the gravitational force acting on the bucket?What is the total work done on the bucket?

A 46 g particle is moving to the left at 21 m/s . How much net work must be done on the particle to cause it to move to the right at 42 m/s ?

A factory worker pushes a crate of mass 30.1 kg a distance of 4.55 m along a level floor at constant velocity by pushing horizontally on it. The coefficient of kinetic friction between the crate and floor is 0.26.What magnitude of force must the worker apply?How much work is done on the crate by this force?How much work is done on the crate by friction?How much work is done by the normal force?How much work is done by gravity?What is the total work done on the crate?

A factory worker pushes a 30.0 kg crate a distance of 3.3 m along a level floor at constant velocity by pushing downward at an angle of 30 below the horizontal. The coefficient of kinetic friction between the crate and floor is
0.25.What magnitude of force must the worker apply to move the crate at constant velocity?How much work is done on the crate by this force when the crate is pushed a distance of 3.3 m ?How much work is done on the crate by friction during this displacement?How much work is done by the normal force?How much work is done by gravity?What is the total work done on the crate?

Two blocks are connected by a very light string passing over a massless and frictionless pulley (the figure ). Traveling at constant speed, the 20.0-N block moves 76.0 cm to the right and the 12.0-N block moves 76.0 cm downward.During this process, how much work is done on the 12.0-N block by the tension in the string?During this process, how much work is done on the 20.0-N block by the normal force?Find the total work done on 12.0-N block.During this process, how much work is done on the 12.0-N block by gravity?During this process, how much work is done on the 20.0-N block by gravity?During this process, how much work is done on the 20.0-N block by the tension in the string?During this process, how much work is done on the 20.0-N block by friction?Find the total work done on 20.0-N block.

A 0.805-kg ball is tied to the end of a string of length 1.61 m and swung in a vertical circle.During one complete circle, starting anywhere, calculate the total work done on the ball by the tension in the string.During one complete circle, starting anywhere, calculate the total work done on the ball by gravity.Repeat part (a) for motion along the semicircle from the lowest to the highest point on the path.Repeat part (b) for motion along the semicircle from the lowest to the highest point on the path.

You push your physics book 1.80 m along a horizontal tabletop with a horizontal push of 2.60 N while the opposing force of friction is 0.650 N . You may want to review (Pages 173 - 177).
For related problemsolving tips and strategies, you may want to view a Video Tutor Solution of Work done by a constant force.How much work does your 2.60 N push do on the book?How much work does the friction force do on the book?How much work does the normal force from the table do on the book?How much work does gravity do on the book?What is the net work done on the book?

A batter hits a baseball with mass 0.150 kg straight upward with an initial speed of 28.0 m/sHow much work has gravity done on the baseball when it reaches a height of 26.0 m above the bat?Use the work-energy theorem to calculate the speed of the baseball at a height of 26.0 m above the bat. You can ignore air resistance.Does the answer to part B depend on whether the baseball is moving upward or downward at a height of 26.0 m ?Explain.

An old oaken bucket of mass 6.75 kg
hangs in a well at the end of a rope. The rope passes over a frictionless pulley at the top of the well, and you pull horizontally on the end of the rope to raise the bucket slowly a distance of 4.00 m.How much work do you do on the bucket in pulling it up?How much work is done by the gravitational force acting on the bucket?What is the total work done on the bucket?

A woman stands in an elevator that has a constant upward acceleration while the elevator travels upward a distance of 18.0 m. During the 18.0-m displacement, the normal force exerted by the elevator floor does 8.25 kJ of work on her and gravity does
-7.35 kJ of work on her.What is the mass of the woman?What is the normal force that the elevator floor exerts on her?What is the acceleration of the elevator?

A 255 kg load is lifted 22.0 m vertically with an acceleration = 0.155g by a single cable.Determine the tension in the cable.Determine the net work done on the load.Determine the work done by the cable on the load.Determine the work done by gravity on the load.Determine the final speed of the load assuming it started from rest.

Use the work-energy theorem to solve each of these problems. You can use Newtons laws to check your answers. Neglect air resistance in all cases.A branch falls from the top of a
95.7-m-tall redwood tree, starting from rest. How fast is it moving when it reaches the ground?A volcano ejects a boulder directly upward
526 m
into the air. How fast was the boulder moving just as it left the volcano?

You throw a 3.00-N
rock vertically into the air from ground level. You observe that when it is
15.1 m
above the ground, it is traveling at
25.1 m/s
upward.Use the work-energy theorem to find the rocks speed just as it left the ground.Use the work-energy theorem to find its maximum height.

In the Bohr model of the atom, the ground-state electron in hydrogen has an orbital speed of 2190 km/s.The mass of an electron is 9.1110-31. What is its kinetic energy?If you drop a 1.0-kg weight (about 2 lb) from a height of 1.0 m,
how many joules of kinetic energy will it have when it reaches the
ground?Is it reasonable that a 30-kg child could run fast
enough to have 100 J of kinetic energy?

A cable with 21.0 N of tension pulls straight up on a 1.50 kg
block that is initially at rest.
You may want to review (Pages 251 - 252).What is the blocks speed after being lifted 2.00 m? Solve this problem using work and energy.

A pile driver lifts a
350 kg
weight and then lets it fall onto the end of a steel pipe that needs to be driven into the ground. A fall of 1.5 m
before striking the pipe drives the pipe in
45
cm.What is the average force exerted on the pipe?

The gravitational attraction between two objects with masses m1 and
m2, separated by distance
x, is
F=Gm1m2/x2, where G is the
gravitational constant.How much work is done by gravity when the separation changes from x1 to
x2? Assume
x2&lt;x1.If one mass is much greater than the other, the larger mass
stays essentially at rest while the smaller mass moves toward
it. Suppose a 1.5kg1013 kg
comet is passing the orbit of Mars,
heading straight for the 1.99kg1030 kg sun at a speed of
3.7
m/s104 G. What will its speed be when it crosses the orbit of Mercury?
The orbit of Mars is 2.28m/s108
m km, the orbit of Mercury is 5.79km107
m km, and
G = 6.67km10-11
m Ncdot m^2/kg^2.

Youre fishing from a tall pier and have just caught a
1.3 kg
fish. As it breaks the surface, essentially at rest, the tension in the vertical fishing line is 16 N.Use work and energy to find the fishs upward speed after youve lifted it
2.0
m?

A ball shot straight up with kinetic energy K0 reaches height
h.What height will it reach if the initial kinetic energy is doubled?

You throw a 5.5 g
coin straight down at 4.0 m/s
from a
55-m-high bridge.How much work does gravity do as the coin falls to the water below?What is the speed of the coin just as it hits the water?

The figure shows how the force exerted by the string of a compound bow on an arrow varies as a function of how far back the arrow is pulled (the draw length). Assume that the same force is exerted on the arrow as it moves forward after being released. Full draw for this bow is at a draw length of 75.0 cm.If the bow shoots a 0.0250-kg arrow from full draw, what is the speed of the arrow as it leaves the bow?

How many joules of kinetic energy does a 750-kg automobile traveling at a typical highway speed of 65 mi/h have?By what factor would its kinetic energy decrease if the car traveled half as fast?How fast (in mi/h) would the car have to travel to have half as much kinetic energy as in part A?

The mass of a proton is 1836 times the mass of an electron.A proton is traveling at speed
V
. At what speed (in terms of
V
) would an electron have the same kinetic energy as the proton?An electron has kinetic energy
K
. If a proton has the same speed as the electron, what is its kinetic energy (in terms of
K
)?

On April 13, 2029 (Friday the 13th!), the asteroid 99942 Apophis will pass within 18600 mi of the earth-about 1/13 the distance to the moon! It has a density of 2600 kg/m3, can be modeled as a sphere 320 m in diameter, and will be traveling at 12.6 km/s.If, due to a small disturbance in its orbit, the asteroid were to hit the earth, how much kinetic energy would it deliver?
Take the speed of impact with the earth to be 12.6 km/s. Neglect the work that would be done by gravity.The largest nuclear bomb ever tested by the United States was the "Castle/Bravo" bomb, having a yield of 15 megatons of TNT. (A megaton of TNT releases 4.184 1015 J of energy.) How many Castle/Bravo bombs would be equivalent to the energy of Apophis?

The space shuttle Endeavour, with mass 86400 kg, is in a circular orbit of radius 6.66106 m around the earth. It takes 90.1 min for the shuttle to complete each orbit. On a repair mission, the shuttle is cautiously moving 5.50 m closer to a disabled satellite every 16.5 s .Calculate the shuttles kinetic energy relative to the earth.Calculate the shuttles kinetic energy relative to the satellite.

Compute the kinetic energy, in joules, of an automobile of mass exttip{m}{m} traveling at a speed of exttip{v}{v}.By what factor does the kinetic energy change if the speed is doubled?

T. Rex. The dinosaur Tyrannosaurus rex is thought to have had a mass of about 7000 kg.Treat the dinosaur as a particle and estimate its kinetic energy at a walking speed of 4 km/h.With what speed would a 70.0-kg person have to move to have the same kinetic energy as a walking T. rex?

At room temperature, an oxygen molecule, with mass of 5.31 10 - 26 kg, typically has a kinetic energy of about 6.21 10 - 21 J.How fast is it moving?

Which has the larger kinetic energy, a 10 g bullet fired at 500 m/s or a 75 kg bowling ball rolled at 5.5 m/s ?

If the kinetic energy of a particle is tripled, by what factor has its speed increased?If the speed of a particle is halved, by what factor does its kinetic energy change?

At what speed does a 1600 kg compact car have the same kinetic energy as a 1.80×104 kg truck going 24.0 km/hr ?

A mother has four times the mass of her young son. Both are running with the same kinetic energy.What is the ratio vs/vm other of their speeds?

One car has twice the mass of a second car, but only half as much kinetic energy. When both cars increase their speed by 7.5 m/s , they then have the same kinetic energy.What were the original speeds of the two cars?

A 3.0 g locust reaches a speed of 3.0 m/s during its jump.What is its kinetic energy at this speed?If the locust transforms energy with 40 % efficiency, how much energy is required for the jump?

A 33.0 kg crate is initially moving with a velocity that has magnitude 4.01 m/s in a direction 37.0 west of north.How much work must be done on the crate to change its velocity to 6.02 m/s in a direction 63.0 south of east?

If a particles speed increases by a factor of 5, by what factor does its kinetic energy change?

One mole (6.022310J atoms) of helium atoms in the gas phase has 3700 -27 of microscopic kinetic energy at room temperature. Assume that all atoms move with the same speed. The mass of a helium atom is 6.682310^{-27}
m kg.What is the speed of a helium atom?

Adult cheetahs, the fastest of the great cats, have a mass of about 70.0 kg and have been clocked at up to 72.0 mph (32.2 m/s ).You may want to review (Pages 177 - 182).
For related problemsolving
tips and strategies, you may
want to view a Video Tutor Solution of Using work and energy to calculate speed.How many joules of kinetic energy does such a swift cheetah have?By what factor would its kinetic energy change if its speed were doubled?

About 50000 years ago, a meteor crashed into the earth near present-day Flagstaff, Arizona. Recent (2005) measurements estimate that this meteor had a mass of about 1.4 108 kg (around 150000 tons) and hit the ground at 12 km/s.You may want to review (Pages 177 - 182).
For related problem-solving tips and strategies, you may want to view a Video Tutor Solution of Using work and energy to calculate speed.How much kinetic energy did this meteor deliver to the ground?How does this energy compare to the energy released by a 1.0-megaton nuclear bomb? (A megaton bomb releases the same energy as a million tons of TNT, and 1.0 ton of TNT releases 4.184 109 J of energy.)

A moving electron has kinetic energy K1 . After a net amount of work
W
has been done on it, the electron is moving one-quarter as fast in the opposite direction.Find
W
in terms of K1 .Does your answer depend on the final direction of the electrons motion?

A ball on a string travels once around a circle with a circumference of 2.0 m. The tension in the string is 5.0 N.How much work is done by tension?

A baseball leaves a pitchers hand at a speed of exttip{v}{v}. The mass of the baseball is exttip{m_{
m b}}{m_b}. You can ignore air resistance.How much work has the pitcher done on the ball by throwing it?

An 80 g arrow is fired from a bow whose string exerts an average force of 130 N on the arrow over a distance of 80 cm .What is the speed of the arrow as it leaves the bow?

A 53.0 kg ice skater is gliding along the ice, heading due north at 4.20 m/s . The ice has a small coefficient of static friction, to prevent the skater from slipping sideways, but
exttip{mu _{
m k}}{mu_k}
=0. Suddenly, a wind from the northeast exerts a force of 4.10 N on the skater.Use work and energy to find the skaters speed after gliding 100 m in this wind.What is the minimum value of exttip{mu _{
m s}}{mu_s} that allows her to continue moving straight north?

The barrels of the 16-in. guns (bore diameter = 16 in. = 41 cm) on the World War II battleship U.S.S. Massachusetts were each 15 m long. The shells each had a mass of 1300 kg and were fired with sufficient explosive force to provide them with a muzzle velocity of 760 m/s . Use the work-energy principle to determine the explosive force (assumed to be a constant) that was applied to the shell within the barrel of the gun. Express your answer in both (a) newtons and (b) in pounds.

When it is burned, 1 gallon of gasoline produces 1.3 108 of energy. 1600 kg car accelerates from rest to 38 m/s in 10 s. The engine of this car is only 15% efficient (which is typical), meaning that only 15% of the energy from the combustion of the gasoline is used to accelerate the car. The rest goes into things like the internal kinetic energy of the engine parts as well as heating of the exhaust air and engine.How many gallons of gasoline does this car use during the acceleration?How many such accelerations will it take to burn up 1 gallon of gas?

In the game of paintball, players use guns powered by pressurized gas to propel 33 g gel capsules filled with paint at the opposing team. Game rules dictate that a paintball cannot leave the barrel of a gun with a speed greater than 86 m/s . Model the shot by assuming the pressurized gas applies a constant force F to a 33 g capsule over the length of the 36 cm barrel.Determine F using the work-energy principle.Determine F using the kinematic equations and Newtons second law.

A sled with mass
12.10 kg
moves in a straight line on a frictionless horizontal surface. At one point in its path, its speed is
4.51 m/s ; after it has traveled a distance
2.51 m
beyond this point, its speed is
6.61 m/s .Use the work-energy theorem to find the force acting on the sled, assuming that this force is constant and that it acts in the direction of the sleds motion.

A small block with a mass of 0.0600 kg
is attached to a cord passing through a hole in a frictionless, horizontal surface
. The block is originally revolving at a distance of
0.41 m
from the hole with a speed of
0.71 m/s . The cord is then pulled from below, shortening the radius of the circle in which the block revolves to
0.15 m . At this new distance, the speed of the block is
1.94 m/s .What is the tension in the cord in the original situation when the block has speed
exttip{v_{
m 0}}{v_0} = 0.71 m/s ?What is the tension in the cord in the final situation when the block has speed
exttip{v_{
m 1}}{v_1} = 1.94 m/s ?How much work was done by the person who pulled on the cord?

A 0.2 kg plastic cart and a 20 kg lead cart both roll without friction on a horizontal surface. Equal forces are used to push both carts forward a distance of 1 m, starting from rest.After traveling 1 m, is the kinetic energy of the plastic cart greater than, less than, or equal to the kinetic energy of the lead cart?

One end of a horizontal spring with force constant
76.0 N/m
is attached to a vertical post. A
4.00-kg
can of beans is
attached to the other end. The spring is initially neither stretched
nor compressed. A constant horizontal force of
56.0 N
is then applied
to the can, in the direction away from the post.What is
the speed of the can when the spring is stretched 0.400 m?At the instant the spring is stretched 0.400 m, what is the magnitude of the acceleration of the block?At the instant the spring is stretched 0.400 m, what is the magnitude of the acceleration of the block?

A block of ice of mass 4.10 kg is placed against a horizontal spring that has force constant exttip{k}{k} = 210 N/m and is compressed a distance 2.60×10−2 m . The spring is released and accelerates the block along a horizontal surface. You can ignore friction and the mass of the spring.Calculate the work done on the block by the spring during the motion of the block from its initial position to where the spring has returned to its uncompressed length.What is the speed of the block after it leaves the spring?

A
45 g
rock is placed in a slingshot and the rubber band is stretched. The magnitude of the force of the rubber band on the rock is shown by the graph in
. The rubber band is stretched
30 cm
and then released.What is the speed of the rock?

You are asked to design spring bumpers for the walls of a parking garage. A freely rolling 1300 kg car moving at 0.66 m/s is to compress the spring no more than 7.6×10−2 m before stopping.What should be the force constant of the spring? Assume that the spring has negligible mass.

A horizontal spring with spring constant 85 N/m
extends
outward from a wall just above floor level. A
6.5 kg
box
sliding across a frictionless floor hits the end of the spring and
compresses it
6.5 cm
before the spring expands and shoots the
box back out. How fast was the box going when it hit the spring?How fast was the box going when it hit the spring?

Chris jumps off a bridge with a bungee cord (a heavy stretchable cord) tied around his ankle as shown in the figure . He falls for 15 m before the bungee cord begins to stretch. Chriss mass is 75 kg and we assume the cord obeys Hookes law, F = - k x, with exttip{k}{k} = 55 N/m .If we neglect air resistance, estimate how far below the bridge Chriss foot will be before coming to a stop. Ignore the mass of the cord (not realistic, however) and treat Chris as a particle.

An elevator cable breaks when a 950 kg elevator is 26 m above a huge spring ( exttip{k}{k} = 2.6×105 N/m ) at the bottom of the shaft.Calculate the work done by gravity on the elevator before it hits the spring.Calculate the speed of the elevator just before striking the spring.Calculate the amount the spring compresses (note that work is done by both the spring and gravity in this part).

A 6.0-kg box moving at 5.0 m/s on a horizontal, frictionless surface runs into a light spring of force constant
75
N/cm.
You may want to review (Pages 183 - 189).
For related problemsolving tips and strategies, you may want to view a Video Tutor Solution of Motion with a varying force.Use the work-energy theorem to find the maximum compression of the spring.

At a waterpark, sleds with riders are sent along a slippery, horizontal surface by the release of a large, compressed spring. The spring with a force constant exttip{k}{k} = 4100 N/m and negligible mass rests on the frictionless horizontal surface. One end is in contact with a stationary wall. A sled and rider with total mass 71.0 kg are pushed against the other end, compressing the spring 0.380 m . The sled is then released with zero initial velocity.You may want to review (Pages 183 - 189).
For related problemsolving tips and strategies, you may want to view a Video Tutor Solution of Motion with a varying force.What is the sleds speed when the spring returns to its uncompressed length?What is the sleds speed when the spring is still compressed 0.205 m ?

A car is stopped in a distance exttip{D}{D} by a constant friction force that is independent of the cars speed.What is the stopping distance (in terms of exttip{D}{D}) if the cars initial speed is tripled?What is the stopping distance (in terms of exttip{D}{D}) if the speed is the same as it originally was but the friction force is tripled? (Solve using the work-energy theorem.)

A car is stopped by a constant friction force that is independent of the cars speed. By what factor is the stopping distance changed if the cars initial speed is doubled? (Solve using work-energy methods.)

At an accident scene on a level road, investigators measure a cars skid mark to be 93 m long. It was a rainy day and the coefficient of friction was estimated to be 0.36.Use these data to determine the speed of the car when the driver slammed on (and locked) the brakes.Why does the cars mass not matter?

A train is moving along a track with constant speed v1 relative to the ground. A person on the train holds a ball of mass m and throws it toward the front of the train with a speed v2 relative to the train.Calculate the change in kinetic energy of the ball in the Earth frame of reference.Calculate the change in kinetic energy of the ball in the train frame of reference.How much work was done on the ball in the Earth frame of reference?How much work was done on the ball in the train frame of reference?

A softball having a mass of 0.30 kg is pitched horizontally at 120 km/h . By the time it reaches the plate, it may have slowed by 13 % .Neglecting gravity, estimate the average force of air resistance during a pitch, if the distance between the plate and the pitcher is about 18 m .

A 1.50 kg book is sliding along a rough horizontal surface. At point exttip{A}{A} it is moving at 3.21 m/s , and at point exttip{B}{B} it has slowed to 1.25 m/s .How much work was done on the book between exttip{A}{A} and exttip{B}{B}?If -0.750J of work is done on the book from exttip{B}{B} to exttip{C}{C}, how fast is it moving at point exttip{C}{C}?How fast would it be moving at exttip{C}{C} if 0.750J of work were done on it from exttip{B}{B} to exttip{C}{C}?

A 91 kg crate, starting from rest, is pulled across a floor with a constant horizontal force of 380 N . For the first 15 m the floor is frictionless, and for the next 15 m the coefficient of friction is 0.23.What is the final speed of the crate?

On an essentially frictionless, horizontal ice rink, a skater moving at 6.0 m/s encounters a rough patch that reduces her speed by 46 % due to a friction force that is 26 % of her weight.Use the work-energy theorem to find the length of this rough patch.

A
65 kg
softball player slides into second base, generating
750 J
of thermal energy in her legs and the ground.How fast was she running?

A 12-pack of Omni-Cola (mass 4.30 kg) is initially at rest on a horizontal floor. It is then pushed in a straight line for 1.20 m
by a trained dog who exerts a horizontal force with magnitude 36.0 N.
You may want to review (Pages 177 - 182).
For related problem-solving tips and strategies, you may want to view a Video Tutor Solution of Using work and energy to calculate speed.Use the work-energy theorem to find the final speed of the 12-pack if there is no friction between the 12-pack and the floor.Use the work-energy theorem to find the final speed of the 12-pack if the coefficient of kinetic friction between the 12-pack and the floor is 0.30.

A little red wagon with mass 7.10 kg moves in a straight line on a frictionless horizontal surface. It has an initial speed of
3.60 m/s
and then is pushed 4.1 m in the direction of the initial velocity by a force with a magnitude of 10.0 N.
You may want to review (Pages 177 - 182).
For related problemsolving tips and strategies, you may want to view a Video Tutor Solution of Using work and energy to calculate speed.Use the work-energy theorem to calculate the wagons final speed.Use this acceleration in the kinematic relationship vx2 =
v0x2 + 2ax (x - x0 ) to calculate the wagons final speed.Calculate the acceleration produced by the force.

How much work is required to stop an electron ( m = 9.11 10 - 31 kg ) which is moving with a speed of 1.60×106 m/s ?

A car passenger buckles himself in with a seat belt and holds his 21 kg toddler on his lap. Use the work-energy principle to answer the following questions.While traveling 23 m/s , the driver has to make an emergency stop over a distance of 55 m . Assuming constant deceleration, how much force will the arms of the parent need to exert on the child during this deceleration period?Now assume that the car (23 m/s ) is in an accident and is brought to stop over a distance of 14 m . Assuming constant deceleration, how much force will the parent need to exert on the child?Is this force achievable by an average parent?Is this force achievable by an average parent?

A car is traveling on a level road with speed v0 at the instant when the brakes lock, so that the tires slide rather than roll.You may want to review (Pages 177 - 182).
For related problem-solving tips and strategies, you may want to view a Video Tutor Solution of Using work and energy to calculate speed.Use the work-energy theorem to calculate the minimum stopping distance of the car in terms of v0, g, and the coefficient of kinetic friction k between the tires and the road.By what factor would the minimum stopping distance change if the coefficient of kinetic friction were doubled ?By what factor would the minimum stopping distance change if the initial speed were doubled?By what factor would the minimum stopping distance change if both the coefficient of kinetic friction and the initial speed were doubled?

Your cat "Ms." (mass 7.00 kg) is trying to make it to the top of a frictionless ramp 2.00 m long and inclined upward at 30.0 above the horizontal. Since the poor cat cant get any traction on the ramp, you push her up the entire length of the ramp by exerting a constant 100 {
m N} force parallel to the ramp.If Ms. takes a running start so that she is moving at 2.40 m/s at the bottom of the ramp, what is her speed when she reaches the top of the incline? Use the work-energy theorem.

How much work is done by the horizontal force FP = 150 N on the 18-kg block of the figure when the force pushes the block 6.0 m up along the 32 frictionless incline?How much work is done by the gravitational force on the block during this displacement?How much work is done by the normal force?What is the speed of the block (assume that it is zero initially) after this d is placement? [Hint: Work-energy involves net work done.]

Starting from rest, a crate of mass m is pushed up a frictionless slope of angle heta by a horizontal force of magnitude F. Use work and energy to find an expression for the crates speed v when it is at height h above the bottom of the slope.Doug uses a 28 N horizontal force to push a 4.1 kg crate up a 1.8 m -high, 20 frictionless slope. What is the speed of the crate at the top of the slope?

A cyclist starts from rest and coasts down a 6.0 hill. The mass of the cyclist plus bicycle is 85 kg . The cyclist has traveled 260 m .What was the net work done by gravity on the cyclist?How fast is the cyclist going? Ignore air resistance.

A block of ice with mass 2.00 kg
slides
1.61 m
down an inclined plane that slopes downward at an angle of 36.9
below the horizontal.If the block of ice starts from rest, what is its final speed? Ignore friction.

A box of mass 5.5 kg is accelerated from rest by a force across a floor at a rate of 2.6 m/s2 for 5.6 s .Find the net work done on the box.

An 200g object initially at rest slides 50 cm down a frictionless incline at a 40° angle. At the bottom of the slope, the object encounters a patch of rough surface, which causes it to come to a stop after 1 m. What is the coefficient of kinetic friction of this rough patch?

How much work must be done to stop a 1300 kg car traveling at 90 km/h?

A 145 g baseball traveling 33 m/s moves a fielders glove backward 25 cm when the ball is caught. What was the average force exerted by the ball on the glove?

A 51.0 kg crate, starting from rest, is pulled across a floor with a constant horizontal force of 240 N . For the first 11.5 m the floor is frictionless, and for the next 11.5 m the coefficient of friction is 0.19. What is the final speed of the crate after being pulled these 23.0 m?

Susan's 12.0 kg baby brother Paul sits on a mat. Susan pulls the mat across the floor using a rope that is angled 30 degrees above the floor. The tension is a constant 31.0 N {
m N}and the coefficient of friction is 0.200. Use work and energy to find Paul's speed after being pulled 3.10 m.

A 6.10 kg block is pushed 9.30 m up a smooth 38.0 inclined plane by a horizontal force of 76.0 N . If the initial speed of the block is 3.30 m/s up the plane, calculate the:(a) initial kinetic energy of the block(b) work done by the 76.0 N force(c) work done by gravity(d) work done by the normal force(e) final kinetic energy of the block

A soccer ball with mass 0.460 kg is initially moving with speed 2.20 m/s. A soccer player kicks the ball, exerting a constant force of magnitude 41.0 N in the same direction as the balls motion. Over what distance must the players foot be in contact with the ball to increase the balls speed to 6.00 m/s?

A block of mass m = 2.50 kg is pushed a distance d = 2.20 m along a frictionless, horizontal table by a constant applied force of magnitude F = 16.0 N directed at an angle θ = 25.0° below the horizontal as shown in the figure. Determine the work done on the block by(a) the applied force(b) the normal force exerted by the table(c) the gravitational force(d) the net force on the block

A 500 kg elevator accelerates upward at 1.3 m/s2 for 19 m, starting from rest.(a) How much work does gravity do on the elevator?(b) How much work does the tension in the elevator cable do on the elevator?(c) What is the elevator’s kinetic energy after traveling 19 m?

The figure shows the kinetic-energy graph for a 2.0 kg object moving along the x-axis. Determine the work done on the object during each of the four intervals AB, BC, CD, and DE.

A 400 g particle moving along the x-axis experiences the force shown in the figure. The particle's velocity is 9.0 m/s at x = 0 m. What is its velocity at x = 3 m?

A box with mass 5.00 kg is pulled up a 36.9° incline by a constant force F that has magnitude 75.0 N and that is parallel to the incline. The distance along the incline from the bottom to the top is 6.00 m. During the motion of the box, the surface of the incline exerts a constant friction force fk = 18.0 N on the box, in a direction opposite to the motion.If the box starts froom rest at the botttom of the incline, what is the kinetic energy of the box when it reaches the top of the incline?

A 2.30-kg textbook is forced against a horizontal spring of negligible mass and force constant 260 N/m , compressing the spring a distance of 0.260 m. When released, the textbook slides on a horizontal tabletop with coefficient of kinetic friction μk=0.30. Use the work-energy theorem to find how far the textbook moves from its initial position before coming to rest.

A horizontal spring with spring constant 270 N/m is compressed by 20 cm and then used to launch a 300 g box across the floor. The coefficient of kinetic friction between the box and the floor is 0.23. What is the box's launch speed?

A 10 kg object slides down an incline of 30 o. If the object starts at a height of 30 cm, and feels a coefficient of kinetic friction of 0.2 down the slope, answer the following questions:a. How much work is done on the object as it moves down the slope? b. What is the change in the object’s potential energy down the slope? c. What is the change in the object’s kinetic energy down the slope

Particle A has half the mass and eight times the kinetic energy of particle B. What is the speed ratio vA / vB?

If the human body could convert a candy bar directly into work, how high could a 76 kg man climb a ladder if he were fueled by one bar (=1100 kJ)?If the man then jumped off the ladder, what will be his speed when he reaches the bottom?

Use the work–energy theorem to solve each of these
problems.(a) A skier moving at 5.01 m/s encounters a long, rough, horizontal patch of snow having a coefficient of kinetic friction of 0.220 with her skis. How far does she travel on this patch before stopping?(b) Suppose the rough patch in part A was only 2.91 m long. How fast would the skier be moving when she reached the end of the patch?(c) At the base of a frictionless icy hill that rises at 25.0 above the horizontal, a toboggan has a speed of 12.1 {
m ; m/s} toward the hill. How high vertically above the base will it go before stopping?

A truck has four times the mass of a car and is moving with twice the speed of the car. If Kt and Kc refer to the kinetic energies of truck and car respectively, it is correct to say that
A) Kt = 16 Kc.
B) Kt = 4 Kc.
C) Kt = 2 Kc.
D) Kt = Kc.
E) Kt = 1/2 Kc.

A crate of mass 10.0 kg is pulled up a rough incline with an initial speed of 1.50 m/s. The pulling force is 100 N parallel to the incline, which makes an angle of 20.0o with the horizontal. The coefficient of kinetic friction is 0.400, and the crate is pulled 5.00 m.(a) How much work is done by the gravitational force on the crate?(b) Determine the increase in internal energy of the crate–incline system owing to friction.(c) How much work is done by the 100-N force on the crate?(d) What is the change in kinetic energy of the crate?(e) What is the speed of the crate after being pulled 5.00 m?

Jonathan is riding a bicycle and encounters a hill of height 7.30 m. At the base of the hill, he is traveling at 6.00 m/s. When he reaches the top of the hill, he is traveling at 1.00 m/s. Jonathan and his bicycle together have a mass of 85.0 kg. Ignore friction in the bicycle mechanism and between the bicycle tires and the road.(a) What is the total external work done on the system of Jonathan and the bicycle between the time he starts up the hill and the time he reaches the top?(b) What is the change in potential energy stored in Jonathan’s body during this process?(c) How much work does Jonathan do on the bicycle pedals within the Jonathan–bicycle–Earth system during this process?

Jonathan is riding a bicycle and encounters a hill of height h. At the base of the hill, he is traveling at a speed vi . When he reaches the top of the hill, he is traveling at a speed vf . Jonathan and his bicycle together have a mass m. Ignore friction in the bicycle mechanism and between the bicycle tires and the road. (a) What is the total external work done on the system of Jonathan and the bicycle between the time he starts up the hill and the time he reaches the top? (b) What is the change in potential energy stored in Jonathan’s body during this process? (c) How much work does Jonathan do on the bicycle pedals within the Jonathan– bicycle–Earth system during this process?

An 80.0-kg skydiver jumps out of a balloon at an altitude of 1 000 m and opens his parachute at an altitude of 200 m.(a) Assuming the total retarding force on the skydiver is constant at 50.0 N with the parachute closed and constant at 3 600 N with the parachute open, find the speed of the skydiver when he lands on the ground.(b) Do you think the skydiver will be injured? Explain.(c) At what height should the parachute be opened so that the final speed of the skydiver when he hits the ground is 5.00 m/s?(d) How realistic is the assumption that the total retarding force is constant? Explain.

An older-model car accelerates from 0 to speed v in a time interval of Δt. A newer, more powerful sports car accelerates from 0 to 2v in the same time period. Assuming the energy coming from the engine appears only as kinetic energy of the cars, compare the power of the two cars.

An airplane of mass 1.50 x 104 kg is in level flight, initially moving at 60.0 m/s. The resistive force exerted by air on the airplane has a magnitude of 4.0 x 104 N. By Newton’s third law, if the engines exert a force on the exhaust gases to expel them out of the back of the engine, the exhaust gases exert a force on the engines in the direction of the airplane’s travel. This force is called thrust, and the value of the thrust in this situation is 7.50 x 104 N. (a) Is the work done by the exhaust gases on the airplane during some time interval equal to the change in the airplane’s kinetic energy? Explain. (b) Find the speed of the airplane after it has traveled 5.0 x 102 m.

A 0.600-kg particle has a speed of 2.00 m/s at point Ⓐ and kinetic energy of 7.50 J at point Ⓑ. What is (a) its kinetic energy at Ⓐ, (b) its speed at Ⓑ, and (c) the net work done on the particle by external forces as it moves from Ⓐ to Ⓑ?

Review. In an electron microscope, there is an electron gun that contains two charged metallic plates 2.80 cm apart. An electric force accelerates each electron in the beam from rest to 9.60% of the speed of light over this distance. (a) Determine the kinetic energy of the electron as it leaves the electron gun. Electrons carry this energy to a phosphorescent viewing screen where the microscope’s image is formed, making it glow. For an electron passing between the plates in the electron gun, determine (b) the magnitude of the constant electric force acting on the electron, (c) the acceleration of the electron, and (d) the time interval the electron spends between the plates.

Review. You can think of the work–kinetic energy theorem as a second theory of motion, parallel to Newton’s laws in describing how outside influences affect the motion of an object. In this problem, solve parts (a), (b), and (c) separately from parts (d) and (e) so you can compare the predictions of the two theories. A 15.0-g bullet is accelerated from rest to a speed of 780 m/s in a rifle barrel of length 72.0 cm. (a) Find the kinetic energy of the bullet as it leaves the barrel. (b) Use the work–kinetic energy theorem to find the net work that is done on the bullet. (c) Use your result to part (b) to find the magnitude of the average net force that acted on the bullet while it was in the barrel. (d) Now model the bullet as a particle under constant acceleration. Find the constant acceleration of a bullet that starts from rest and gains a speed of 780 m/s over a distance of 72.0 cm. (e) Modeling the bullet as a particle under a net force, find the net force that acted on it during its acceleration. (f) What conclusion can you draw from comparing your results of parts (c) and (e)?

Review. A 5.75-kg object passes through the origin at time t = 0 such that its x component of velocity is 5.00 m/s and its y component of velocity is -3.00 m/s. (a) What is the kinetic energy of the object at this time? (b) At a later time t = 2.00 s, the particle is located at x = 8.50 m and y = 5.00 m. What constant force acted on the object during this time interval? (c) What is the speed of the particle at t = 2.00 s?

A 40.0-kg box initially at rest is pushed 5.00 m along a rough, horizontal floor with a constant applied horizontal force of 130 N. The coefficient of friction between box and floor is 0.300. Find (a) the work done by the applied force, (b) the increase in internal energy in the box–floor system as a result of friction, (c) the work done by the normal force, (d) the work done by the gravitational force, (e) the change in kinetic energy of the box, and (f) the final speed of the box.

Review. A 7.80-g bullet moving at 575 m/s strikes the hand of a superhero, causing the hand to move 5.50 cm in the direction of the bullet’s velocity before stopping. (a) Use work and energy considerations to find the average force that stops the bullet. (b) Assuming the force is constant, determine how much time elapses between the moment the bullet strikes the hand and the moment it stops moving.