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?

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 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.