# Homework Help: About work done

1. Nov 5, 2007

### jack1234

Two eggs of equal mass are thrown at a blanket with equal velocity. Egg A hits the wall instead but egg B hits the blanket. Compare the work done on the eggs in stopping them:
a. More work was done on A than on B.
b. More work was done on B than on A.
c. The amount of work is the same for both.
d. It is meaningless to compare the amount of work because the forces were so different.
e. Work was done on B, but no work was done on A because the wall did not move.

I expect the answer is e, but it ends up to be c. What is the reason?

Last edited: Nov 5, 2007
2. Nov 5, 2007

### rl.bhat

Amount of work done is equal to change in kE. In this case initial KE of both the eggs is the same. And after impact both come to rest.

3. Nov 5, 2007

### jack1234

Thanks, but may I know is there anything wrong in the following statement:
"no work was done on A because the wall did not move"?

4. Nov 5, 2007

### rl.bhat

"no work was done on A because the wall did not move"?
since egg A rebounds from the wall, work is done on A by the wall. And this is due to the elasticity of the wall.

5. Nov 5, 2007

### jack1234

"since egg A rebounds from the wall"
Isn't the egg should be damages? an egg is not a ball :)

6. Nov 5, 2007

### jack1234

But frankly I still do not understand...
Work done of a force must moved through a distance, but the wall doesn't, how can it do work on the egg?

7. Nov 5, 2007

### nova-ex

Work done by a force must be moved through a distance. Yes this is true. However, you must keep in mind the fact that we are not considering work done ON the wall. We are considering work done ON the egg. The egg does moves, by a very minimal amount where the deceleration occurs.

8. Nov 5, 2007

### learningphysics

There is displacement when the egg is stopped by the wall... like nova-ex said... due to the elasticity of the wall... there is some motion within the wall... even if the center of mass of the wall doesn't move...