The speed of both balls after the collision is 8.79m/sec.
So, assume after the collision both have common velocity as V.
We are given that collision is elastic it means after the collision both of the balls sticks together and move with common velocity.
So, we need to conserve momentum in x-direction
We know that momentum=mass × velocity
According to law of conservation of momentum,
initial momentum=final momentum
initial momentum of 0.045kg ball=0.045×42,
initial momentum of 0.17kg ball=0,
final momentum of both of balls=(0.045+0.17)×V
=>0.045×42+0=(0.045+0.17)×V
=>1.89=0.215×V
=>V=1.89/0.215
=>V=8.79m/sec
Hence, if 0.045kg ball elastically collides with 0.17kg ball, then speed of both balls after the elastic collision is 8.79m/sec.
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