Question

A plank of mass 10 kg and a block of mass 2 kg are placed on a horizontal plane as shown in the figure. There is no friction between plane and plank. The coefficient of friction between block and plank is 0.5. A force of 60 N is applied on plank horizontally. In first 2 s the work done by friction on the block is

• A. - 1000 J
• B. 100 J
• C. Zero
• D. 200 J

Answer 1

The correct option is B 100 J

Maximum acceleration of 2 kg block due to friction can be $\mu g$ or $5m/s^2$.
Combined acceleration, if both move together with same acceleration would be,
$a=\frac{60}{12}=5m/s^2$.
Since both accelerations are equal, upper block will move with acceleration 5 $m/s^2$ due to friction.
In first two seconds, $s=\frac{1}{2}a{t}^2=\frac{1}{2}\times 5\times 4=10m$
and force of friction f = ma = 10 N
$\therefore W_f=fscos{0}^{\circ }=100J$.