Q.

A moving block having mass m, collides with another stationary block having mass 4m. The lighter block comes to rest after collision. When the initial velocity of the lighter block is v, then the value of coefficient of restitution (e) will be                [2018]

1 0.5  
2 0.25  
3 0.8  
4 0.4  

Ans.

(2)

Let final velocity of the block of mass, 4m=v'

Initial velocity of block of mass 4m=0

Final velocity of block of mass m=0

According to law of conservation of linear momentum,

       mv+4m×0=4mv'+0v'=v4

Coefficient of restitution,

e=Relative velocity of separationRelative velocity of approach=v/4v=0.25