Question:
Whenever the surfaces in contact tend to move or move with respect to each other, the force of friction comes into play
(a) only if the objects are solid
(b) only if one of the two objects is liquid
(c) only if one of the two objects is gaseous
(d) irrespective of whether the objects are solids, liquids or gases
Solution:
(d) Force of friction acts in solids, liquids and gases and opposes the motion with respect to each other.