Question

Multiple correct answers type
Mark the correct statement(s) regarding friction.
a. Friction force can be zero, even though the contact surface is rough.
b. Even though there is no relative motion between surfaces, frictional force may exist between them.
c. The expressions $f_L={\mu }_{s}N$ or $f_k={\mu }_{k}N$ are approximate expressions.
d. The expression $f_L={\mu }_{s}N$ tells that the directions of $f_L$ and N are the same.

• A. a
  
• B. b
• C. c
• D. d

Answer 1

Answer: (A), (B), (C)

The correct options are
A a
B b
C c

(a.) If no external force acts on the body, then frictional force is zero since it is a self balancing force.

(b.) The friction force may exist between the surfaces which might be equal to the external force so as to oppose tendency of relative motion.

(c.) This statement is true because we deal with the approximate values of ${\mu }_k$ & ${\mu }_s$, actual values are determined after lot of calculations.

(d.) This statement is false because generally $N$ is perpendicular to the surface of contact and $f$ is along the surface.

Hence correct statements are a,b,c.