Explanation
When P is pushed downwards with speed v, Q moves upwards with same speed. ∴ total linear momentum = 0. After collision of P with floor, change in linear momentum of P = mv – (–mv) = 2mv. The linear momentum of Q must change by the same amount. Therefore, Q should start moving downwards with same speed v with which P moves upwards.
P.E. at X = 6m is U = 10 + (6 – 4)2 = 14 J
∴ Mechanical energy = K.E. + P.E. = 20 + 14 = 34 J.
By the conservation of momentum in the absence of external force total momentum of the system (ball + earth) remains constant.
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