# HW questions about Forces with ball examples

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1. Jul 6, 2016

### Avram Bourdeau

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Hey all, I'm don't have the firmest grasp on physics and a little help would be much appreciated!

So two questions:
1)a physics student watching a tennis match states, "while the ball is in contact with the racquet, the racquet exerts a larger force on the ball than the ball does on the racquet because the racquet has to stop the ball and then reverse its motion"

What if anything is wrong with this statement?

2) a student observes a rubber ball hitting a wall and rebounding states, "The wall exerts a larger force on the ball than the ball exerts on the wall, because the ball undergoes an acceleration but the wall doesn't move. That is, the ball goes from an initial speed to zero, and then from zero to the rebound speed but the wall does not accelerate since it is stationary the whole time".

What if anything is wrong with this statement?

2. Jul 6, 2016

### robphy

Merely transcribing the problems are insufficient.
You need to show some attempt at the problems.

3. Jul 6, 2016

### Avram Bourdeau

Not super sure bout this answer but here goes...

1) The ball and the racquet are exerting the same amount of force, but because the tennis ball has a smaller mass (and is not held) it rebounds; ball hits the racquet=racquet hits the ball.

2) Something to do with inertia? Like, they exert the same force on one another but the wall has more inertia than the ball, and thus does not move?

4. Jul 7, 2016

### Clara Chung

For (1), even the tennis ball has a larger mass, the ball and the racquet are exerting the same amount of force, this is simply by action and reaction,
m1(v1-u1)=m2(v2-u2)
. While the ball stops and reverse its motion, the speed of the motion of racquet decreases too.

For (2), ''the wall does not accelerate since it is stationary the whole time'' is wrong. The wall has been moved. By action and reaction again, m1(v1-u1)=m2(v2-u2), Let m2 be the mass of the wall, as the wall is connected to the earth, its mass (m2) is immense. Therefore(v2-u2) is very small and it seems like stationary.

5. Jul 8, 2016

### James R

What do you mean by "inertia", exactly? Where does the "inertia" come into Newton's laws of motion?

Did the ball in (2) experience any forces? Did the wall experience any forces? Is Newton's second law relevant at all?