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Question

Two blocks A and B having the same mass collide elastically with each other. the velocity of block A before the collision is 10 m/s. If block A comes to rest after collision, then what was the velocity of block B before the collision?


A
10 m/s
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B
0 m/s
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C
20 m/s
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D
30 m/s
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Solution

The correct option is B 0 m/s
Let the velocity of block B before and after collision be uB and vB respectively.
Using conservation of momentum:
m×10+m×uB=m×0+m×vB
vB=uB+10(i)
and using conservation of energy:
12×m×102+12×m×u2B=12×m×v2B
v2B=u2B+100(ii)

Using (i) in (ii):
u2B+20uB+100=u2B+100
20uB=0
uB=0

So, initially block B was at rest.

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