Two blocks of masses `5 kg and 2 kg` are placed on a frictionless surface and connected by a spring. An external kick gives a velocity of `14 m//s` to the heavier block in the direction of lighter one. The magnitudes of velocities of two blocks in the centre of mass frame after the kick are respectively :

Options

A. `4 m//s` , ` - 10 m//s`
B. `4 m//s` , ` 10 m//s`
C. `- 4 m//s` , ` 10 m//s`
D. `4 m//s` , ` 10 m//s`

Answer

A

Explanation

Given, `m_(1) = 5` kg, `m_(2) = 2 kg`, `v_(1) = 14ms^(-1)`, `v_(2) = 0`, `v_(cm) = ?`
`therefore v_(CM) = (m_(1)v_(1) + m_(2)v_(2))/(m_(1) + m_(2))` = (5 xx 14 +2 xx 0)/(5 +2)`
`=70/7 = 10 ms^(-1)` in the direction of lighter one.