Impact force of two balls

[rasen58]

Two balls of the same mass are thrown towards a wall and collide with it moving with a speed of 5 m/s. Ball A hits the wall and rebounds with a speed of 4 m/s. Ball B hits the wall and stops. Assume that the collisions times are the same for each ball. Compared to ball B, ball A has ____ velocity change, ____ momentum change, and ____ impact force.

 

So I figured out that A has a *greater* velocity change because it changes from +5 to -4 whereas B only changes from +5 to 0.

And I figured out that the momentum change for A is also *greater* because of the same reason above because momentum = mass x change in velocity.

And I kept getting the last blank wrong, but apparently the answer is that A has a greater impact force. I thought they'd have the same impact force since both balls have the same mass and hit the ball at the same speed. Why does A have a greater impact force?

 

Edited December 18, 2014 by rasen58

[swansont]

 

Why does A have a greater impact force?

 

How does force relate to momentum?

[rasen58]

Ft = delta p

So a greater momentum will have a greater force.

 

 

But I thought the impact force would occur before any change in momentum, so I thought that since they have the same initial momentum, the impact forces would be equal.

[studiot]

Better thought of as

 

Force = mass x acceleration

 

but acceleration = rate of change of velocity

 

so

 

Force = mass x rate of change of velocity = rate of change of {mass x velocity} (since mass does not change)

 

and mass x velocity = momentum

 

So Force = rate of change of momentum.

 

It is the changing momentum that generates the force. They are simultaneous and the force is applied for as long (or as short) as the momentum is changing.

 

Can you complete your question, now?

[swansont]

Ft = delta p

So a greater momentum will have a greater force.

 

 

But I thought the impact force would occur before any change in momentum, so I thought that since they have the same initial momentum, the impact forces would be equal.

 

The change in momentum is the same as the impact force. The impact force causes the change in momentum.

[rasen58]

I see. Thanks.