TBR physics. Chapter 4, passage 3, question 19

[yellowjellybean]

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How would the results change if the objects were made of compact material that did not deform, when equal mass balls are fired from positions A and F?

Answer: The balls would rebound directly backward with the same speed.

I don't understand how you would know the balls would rebound with the same speed. I though with an elastic collision, the balls would would swap velocities after they collided.

Thanks.

 

[Gauss44]

How would the results change if the objects were made of compact material that did not deform, when equal mass balls are fired from positions A and F?

Answer: The balls would rebound directly backward with the same speed.

I don't understand how you would know the balls would rebound with the same speed. I though with an elastic collision, the balls would would swap velocities after they collided.

Thanks.

Elastic collisions, think conservation of energy.

 

[yellowjellybean]

Elastic collisions, think conservation of energy.

I know momentum and kinetic energy are conserved in elastic collisions but it seems like the answer is assuming equal initial velocities and I don't understand why.

 

[BerkReviewTeach]

I know momentum and kinetic energy are conserved in elastic collisions but it seems like the answer is assuming equal initial velocities and I don't understand why.

It was sort of given in the passage. All launch points are 1 meter from the point where they collide, so we must assume that every launched ball travels at the same speed in order to cover the same distance (1 meter) in the same amount of time (the time from launch to collision is the same is all balls are launched at the same time).