While discussing friction, a teacher gave a number of examples to explain the…
2025
While discussing friction, a teacher gave a number of examples to explain the concept. Some of the examples the teacher quoted to show that friction is useful in our day-to-day life are given below. I. We are able to write because of friction between the tip of the pen and the paper. II. We are able to walk because of friction between our feet and the ground. III. A ball thrown vertically upwards comes back to us due to friction. IV. We are able to stop a moving vehicle by applying brakes because of friction. The correct examples are:
- A.
I, II, III and IV
- B.
I, II and IV
- C.
. I, III and IV
- D.
II, III and IV
Attempted by 18 students.
Show answer & explanation
Correct answer: B
Friction is the resistive force that acts between two solid surfaces in contact when one moves, or tends to move, relative to the other; it opposes relative sliding at that point of contact. A body that has no solid surface in contact — such as an object in free flight through the air — is not subject to this surface friction; besides gravity, only a separate fluid effect such as air resistance can act on it in flight, and that is physically distinct from the surface friction being discussed here.
Writing (I): the pen's tip stays in contact with the paper, and friction between them lets the tip grip and drag ink onto the surface.
Walking (II): each footstep pushes back against the ground, and it is the frictional grip between foot and ground that pushes the body forward; without it, feet would slip, as on ice.
Ball thrown upward (III): once released, the ball travels through the air with no solid surface in contact, so the surface friction being discussed does not act on it there. Its rise and fall are governed by gravity pulling it back down; any air resistance in flight is a separate drag effect and is not what brings the ball back.
Braking a vehicle (IV): the brake pads press against the wheel or disc, and the friction generated there converts the vehicle's motion into heat, slowing it down.
I, II, III and IV — folds the ball claim in alongside the other three examples, but the ball's return is governed by gravity, not friction.
I, III and IV — swaps the walking example out for the false ball claim.
II, III and IV — swaps the writing example out for the false ball claim.
So only I, II and IV genuinely describe friction; III describes gravity instead.