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. `-6J`
C. 6J
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`