In the figure shown initial spring is in unstretched state and blocks are at rest. Now `100 N` force is applied on block `A and B` as shown in figure. After same time velocity of 'a' becomes `2 m//s` and that of 'B' `4 m//s` and block A displaced by amount `10 cm` and spring is spring is streched by amount `30 cm`. Then work done by spring force on `A` will be:

Options

A. `9//3 J`
B. `- 6 J`
C. `6 J`
D. None of these

Answer

B

Explanation

`W_("spring") + W_(100N) = Deltak (on A)`
`W_("spring") + (100) ((10)/(100)) = (1)/(2) (2) (2)^(2)`
`W_("spring") = 4 - 10 = -6J`