Question

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

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

Answer 1

Answer: (B)

$W_{\text{spring }}+W_{100N}=\Delta k$ (on A)
$W_{\text{spring }}+(100)\left(\frac{10}{100}\right)=\frac{1}{2}(2)(2{)}^2$
$W_{\text{spring }}=4-10=-6J$