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A body of mass 4kg moves with a velocity of10m/sec collide elastically with a body of mass 8kg is at rest .Then the velocity ok 8kg body is

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Given 8kg body is in rest, means that body is stationary / non-moving. Consider it, you can solve as shown below,

M1=4kg
U1=10m/s
M2=8kg
U2=0m/s

V=V1+V2m/s
According to conservation of momentum,
 

M1U1+M2U2=M1V+M2V
M1U1+M2U2=V(M1+M2)
 

(4*10)+(8*0)=V(4+8)
40+0 = V(12)
40/12=V
3.33m/s=V
ie; The velocity of 8kg body is V=3.33m/s

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They are actually asking the velocity of the 8kg body after rest...It is already known that velocity was zero before collision, as body B was at rest
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