Question

If a man is walking on a rough horizontal ground without slipping, then identify the correct statement(s) from the options given below:

• A. Frictional force on the man will act in backward direction.
• B. Frictional force on the man will act in forward direction.
• C. Work done by frictional force on the man will be negative.
• D. Work done by frictional force on the man will be $\text{zero}$.

Answer 1

Answer: (B), (D)

Work done by frictional force on the man will be $\text{zero}$.

When the man is walking on the ground, he will be pushing the ground backwards with a force $F_x$, hence the tendency for slipping is in the backward direction for man.

$\therefore$ To prevent the slipping between the man and ground, static friction $f_s$ will act in forward direction on the man.


If the man has moved by a distance $x\text{m}$ along forward direction, then work done by friction
$W=\overrightarrow{f_s}.\overrightarrow{x}=f_s\times x\times \cos 0^{\circ }=f_s\times x$

$\because$ When $f_s$ is acting at point $P$, then it is at rest or it is not moving, because the man is not slipping on the surface.
$\Rightarrow$ Displacement of the point of contact of man's feet $P$ with the ground is $\text{zero}$
i.e $\overrightarrow{x}=0$
$\Rightarrow W=0$

Therefore, work done by frictional force on the man $=0$
Options ($b$) and ($d$) are correct.