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Question

A body of mass m makes an elastic collision with another identical body at rest. Show that if the collision is not head-on, the bodies go at right angle to each other after the collision.

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Solution

According to the question, the collision of the two bodies of mass m is not head-on. Thus, the two bodies move in different directions.

Let the velocity vectors of the two bodies after collision be v1 and v2.

As the collision in the question is elastic, momentum is conserved.

On applying the law of conservation of momentum in X-direction, we get:

mu1+m×0 = mv1 cos α+mv2 cos β ...(i)

On applying the law of conservation of momentum in Y-direction, we get:

0=mv1 sin α -mv2 sin β ...(ii)mv1 sin α =mv2 sin βNow,12mu12 + 0 = 12mv12 + 12m×v22 u12 = v12+v22 ...(iii)On squaring equation (i), we get:u12 = v12cos2 α + v22 cos2β + 2v1v2 cos α cos βEquating the equations (i) and (iii), we getv12+v22 = v12 cos2 α + v22 cos2β + 2v1v2 cos α cos βv12 sin2 α + v22 sin2 β = 2v1v2 cos α cos β 2v12 sin2 α = 2×v1×v1 sin αsin β cos α cos β sin α sin β = cos α cos β cos α . cos β - sin α cos β = 0 cos (α+β) = 0=cos 90° α+β=90°

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