1 momentum MC question

2008-03-23 3:34 am

Two identical balls, A and B, are moving towards each other at V1 and V2. They collide head-on. Which of the following is incorrect if the collision is elastic?
A. The balls move away from each other after collison.
B. After collision, A moves at V2 and B moves at V1.
C. Sound will be given out during collision.
D. During the impact, there are forces acting on A and B.

Ans: C

Please explain why A,B and D are correct and C is incorrect?

回答 (1)

天同

2008-03-23 6:18 am

✔ 最佳答案

Let m be the mass of the balls
By concervation of momentum
m.V1 + m.(-V2) = m(V1') + m(V2')
where V1' and V2' are the velocities of the balls after collision

Because the collision is elastic, kinetic energy is conserved
(1/2)m.(V1)^2 + (1/2)m.(V2)^2 = (1/2)m(V1')^2 + (1/2)m(V2')^2

solve for V1' and V2' gives V1' = -V2 and V2' = V1

thus ball A rebounds with speed V2 and ball B rebounds with velocity V1
i.e. statements (A) and (B) are correct

Snce the collision is elastic, kinetic energy is conserved. No energy will be released as sound Statement C is thus not correct.

Statement D is corect because there is a change of momentum for both balls,
impulse = change of momentum
an impulse exists indicates that there is a force acting.
Statement D is thus correct

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