First slide
The steps in collision
Question

 An object of 2.0 kg mass makes an elastic collision with another object of mass M at rest and continues to move in the original direction but with one-fourth of its original speed. What is the value of M ?

Moderate
Solution

 This is a case of head-on elastic  collision 

  v2 - v1 = u1 - u2 v2 -u4=u-0 .............1 Conserving momentum 

20×u+M×0=2×u4+M×v2 v2=3u2M ............2  From equations 1 and 2  we get   M = 1.2 kg

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