Physics Work Done question. Help!?

Question

A 100g ball moving at 8m/s bounces off a wall with an elastic collision and leaves at the same speed. What is the work done???

This is a multiple choice qn and the available answers are:
1. 3.2 J
2. 1.6 J
3. 0.8 J
4. 0.4 J

Help!!!

Answer 1

work done = loss in kinetic energy
= 1/2 m v^2
= 1/2 x 1/10 x 8 x 8
= 3.2 J per collision

Answer 2

Hehehe. This is a kind of trick question that involves an understanding of what is meant by 'work', 'potential energy', and 'kinetic energy'. First off, the work done in stopping the ball is, indeed, ½mv² = 3.2J. But a deeper understanding of 'work' is that work is what happens when energy is changed from one form to another. In this case, the wall exerts force against the ball to reduce it's kinetic energy to zero. In the process, the ball is deformed so that it is now 'storing' that 3.2J as potential energy. Then the ball does work against the wall to convert 3.2J of potential energy back into kinetic energy. At this point one has to ask, "How can 6.4 Joules of 'work' have been done when the system only had 3.2J of energy to begin with?" And the answer is... In a perfectly conservative system, the only 'work' that is done is either positive or negative. (You choose which sign to apply.) If the wall stopping the ball is 'positive' work, then the ball accelerating itself away from the wall is 'negative' work.

And the answer to the question is.... *No* work has been done since the final state of the system is exactly equal to its initial state.


Doug

Answer 3

the mass of the ball = 100g = 0.1kg
velocity = 8m/s
change in k.e =initial velocity - final velocity
=8-(-8) {N/B; final velocity is -ve because it is movin in the opposite direction
=16m/s
workdone =?
since work done = change in kinetic energy
= 1/2 m v^2
=1/2x0.1x16x16
=12.8 J
=I guess this should be correct

Answer 4

depends on what force is moving the ball.