# A body B of mass M is at rest on frictionless straight line. A body A of mass 2kg moves at speed of 6m/s and collided with B. After collision, A moves to left at speed of v, and B moves at the same...

A body B of mass M is at rest on frictionless straight line. A body A of mass 2kg moves at speed of 6m/s and collided with B. After collision, A moves to left at speed of v, and B moves at the same speed v to the right . Let the coefficient of restitution be 1 (perfect elastic),

Find the mass M of the body B

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Before collision

`rarrV_A = 6m/s`

`rarrV_B = 0`

After collision

`rarrV_A = -v` (since A moves left)

`rarrV_B = v`

Using the conservation of momentum at `rarr ` direction;

`2xx6 = Mxxv-2xxv -----(1)`

Using the Newton's law of restitution at `rarr` direction;

`v-(-v) = 1(0+6)`

`2v = 6`

`v = 3`

From (1)

`12 = 3M-6`

`M = 2`

*So the mass of B is 2kg.*