Question

An 80 N box initially at rest is pulled by a horizontal rope on a horizontal table. The coefficients of kinetic and static friction between the box and the table are 0.25 and 0.5 respectively. What is the friction force on this box if the pull is 25N?

A. 20 N
B. 40 N
C. 25 N
D. 0 N

Answer 1

Answer:

C. 25 N

Explanation:

w = Weight of box = 80 N

$\mu_k$ = Coefficient of kinetic friction = 0.25

$\mu_s$ = Coefficient of static friction = 0.5

F = Pulling force = 25 N

Static friction

$f_s=\mu_s w\\\Rightarrow f_s=0.5\times 80=40\ \text{N}$

$F<f_s$

The force of pulling is less than the static frictional force so this makes the friction force equal to the force of pulling.

So, the friction force on the box is 25 N.