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 tiny object of mass m is sliding around inside a pipe of inner radius R. When we first see the object it is momentarily at rest halfway up the side of the pipe. A moment later, it is momentarily at rest at an angle θ < 90◦ from the vertical on the other side of the pipe.
If a force of kinetic friction acted on the object as it slid, how much work did friction do in the time between the two instants of momentary rest?

An object has no forces acting upon it other than the one you exert. You apply a force over a given distance and as a result, the object winds up with speed v. Suppose you had applied only half the force over the same distance.
What would the object’s speed be?

A meteorite enters the Earth’s atmosphere with a mass of 12 million kg and a speed of 19 km/s. What is the kinetic energy of the meteorite when it enters the Earth’s atmosphere? After moving through the atmosphere, the meteorite hits the ground with a mass of 6 million kg and a speed of 12 km/s. What is the amount of work done by air resistance to slow down the meteorite?

Using the work-energy theorem, calculate the maximum height a mass would reach if thrown straight into the air at an initial speed of 7 m/s.

An object has mass m. If air resistance is taken into account, what will the kinetic energy of the object be after falling through a distance h starting from rest?a) mghb) less than mghc) greater than mghd) zero

A block with mass 10.0 kg is sliding due east on a horizontal surface. The forces on the block are gravity, the normal force exerted on the block by the surface and the kinetic force exerted by the surface. Point B is 5.0 m due east of point A. At point A the block has speed 6.0 m/s and at point B the block has speed 4.0 m/s. The worrk done on the block by the friction as the block moved from A to B wasA) 100 JB) -100 JC) 135 JD) -135 JE) 160 JF) -160 JG) none of the above answers

You do a certain amount of work on an object initially at rest, and all the work goes into increasing the object’s speed. If you do work W, suppose the object’s final speed is v. What will be the object’s final speed if you do twice as much work?
1. v /√2
2. 4 v
3. √2 v
4. 2 v
5. Still v

A block of mass 3.8 kg, sliding on a horizontal plane, is released with a velocity of 2.9 m/s. The block slides and stops at a distance of 1.5 m beyond the point where it was released. How far would the block have slid if its initial velocity were increased by a factor of 2.9?
1. 14.336
2. 15.376
3. 10.935
4. 15.979
5. 12.615
6. 7.497
7. 13.454
8. 3.328
9. 9.522
10. 12.844

The crate is pulled a distance of 9.5 m on the incline by a 150 N force. The acceleration of gravity is 9.8 m/s2. What is the change in kinetic energy of the crate?
1. 612.956
2. 576.413
3. 432.604
4. 318.752
5. 473.56
6. 702.891
7. 506.908
8. 535.894
9. 349.598
10. 478.646