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 15N 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

The correct option is D Zero

Friction on man's feet:

$f_1=\mu m_{man}g$

$=0.1\times 20\times 10$

$=20N$

Friction between the plank and ground:

$f_2=\mu (m_{man}+m_{plank})g$

$=0.1\times (20+20)\times 10$

$=40N$

Now, we need to check whether friction on the man's feet is greater than the applied force ($15N$).

$T=15N$

$f_1=20N$

Since friction is greater than the applied force, then there will not be any motion and the value of friction on the man's feet will be equal to the applied force ($15N$).

$f_{man}=15N$

And we know, $T = 15\ N$

Then, net force on man

$F_{man}=f_{man}-T$

$=15-15$

$=0N$