1. W = F x d
If the applied force is 20N and the distance through which the force is applied is 3.5m, then:

2.
a) W = F x d = 90 N x 3 m = 270 J; the block has also acquired the same amount of potential energy, since its potential energy is its weight times its height above the ground.
b) This is an example of an inclined plane acting as a machine. At this angle, you need apply only 54 N of force, but must travel through 5 meters. Still, W = F x d = 54 N x 5 m = 270 J, as above. The mass is still only 3 m above the ground, so its PE is still only 270J, in other words, its gain in PE equals the amount of work done on it.

3.
a) KE = 1/2 mv² = 1/2 (3 kg) (4 m/s)²=
1/2 (3 kg) (16 m²/s² = 24J
b) Here, you could substitute 8 m/s for v and find that the new KE is 96 J, or you could reason as follows: I know that KE is proportional to the square of v. So, if v doubles, then v² increases by a factor of 2² or a factor of four, so the new kinetic energy must be four times greater than the original kinetic energy. Since the KE in part a) was 24J, the KE now is 4 times that or 4x24J = 96J. (see the next to last problem for a slightly different variant of this).

4.
a) Here you are told that for every 2m of rope you pull, the piano rises only 0.4 m, or in other words, you have to pull five times as much rope as you lift the piano. This means you have to exert your force through five times as much distance as you raise the piano. This means that you have to exert only 1/5 as much force as the weight of the piano.

5. Extra strectching means that there is more tension in the rubber band and therefore more b> force on the rock. Additionally, more stretching means the force is exerted through a greater distance . Remembering that Work = F x d, increasing F and d increases the work done on the rock, which increases its elastic PE, which means it will have greater KE and greater speed when the rock is released.

6. We are told that the mouse and elephant have the same KE. Remember that KE = 1/2 mv², we can then write:

7. The key to this question is the conservation of energy. The satellite will have at each point in its orbit a different speed, so at each point it will have a different kinetic energy. Also, you are told that PE increases as you get farther from the center of the Earth (as you already know). So, the total energy of the satellite at each point in its orbit is:

8. PE = mgh = 1000 N x 5 m = 5000 J (remember, Newtons measure weight, so the 1000 N equals mg).
The PE at the top equals the KE at the bottom. KE = 1/2 mv²
We can equate PE at the top to KE at the bottom to get:

9. If the hammer were taken to a height that is four times greater, it would have four times more potential energy. If it were released from that point, it would strike the ground with four times more kinetic energy. Remembering that KE is proportional to v², the speed doubles if the KE quadruples (see the third problem on this assignment).

10. The mechanical advantage is two, meaning the amount of force you need to apply is only half the weight of the object, since there are two strings support the weight in this pulley arrangement.