Question

A man and a plank have mass $20$$kg$ each. Friction coefficient between man and plank, as well as plank and ground is $0.1$. If force applied by man on string is $15$$N$ horizontally, then value of net force on man is $(g=10m/s^2)$.

• A. 15 N
• B. 30 N
• C. 20 N
• D. Zero

Answer 1

Answer: (D)

Zero

$f{r}_{1max}=f{r}_{availableatmansfeet}=0.1\left(20\right)\left(10\right)=20N$
$f{r}_2=f{r}_{availableatground}=0.1\left(40\right)\left(10\right)=40N$
As force applied by man on string is $15$ $N$ which is less than the maximum friction forces for both the surfaces. Hence Box will not move.
So net friction on man is $=15-15=0N$