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# A level Physics Elastic Collision Problem watch

1. The question appears to only include the vertical height and I am struggling to figure out the answers to the questions with the information given, any guidance would be much appreciated.
2. (Original post by Physicist Matt)
The question appears to only include the vertical height and I am struggling to figure out the answers to the questions with the information given, any guidance would be much appreciated.
What's wrong with doing a conservation of energy calculation on ball A equating the change in GPE to the change in KE to work out it's final velocity?
3. This is a fairly simple problem where you need the background knowledge that if two bodies of equal mass collide then the body in motion comes to rest after collision (elastic) and the body initially at rest moves with the velocity of the moving one. Here in the first part you simply have to consider that potential energy of A gets converted into k.e when it strikes B. The change in p.e of A is = mgh which only depends on the vertical height by which it falls. This is a property of conservative force like gravitational force. This mgh = 1/2 mv^2 where v is the velocity of A when it strikes B. Calculate v from here. For time taken you simply have to consider the vertical height fallen ( given 10 cm) and use h= 1/2 gt^2. Please do not forget to change the units cm to m.
4. Thankyou

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