The statement "friction force is a contact force while magnetic force is a non-contact force" is
- (a)always true.
- (b)true only at 0°C.
- (c)a false statement.
- (d)either true or false depending upon the temperature of the surroundings.
Answer
Why
Correct — A, always true. Friction acts only when two surfaces are physically in contact and slide, or tend to slide, over each other, so it is a contact force. A magnet, by contrast, attracts or repels iron or another magnet across a gap without touching it, so magnetic force is a non-contact (field) force. This classification does not depend on temperature, so the statement is simply always true.
Why the others are wrong
- (b)true only at 0°C. — The contact or non-contact nature of these forces has nothing to do with temperature, so restricting the statement to 0 °C is baseless.
- (c)a false statement. — It is a correct classification — friction needs contact and magnetism acts at a distance — so calling the statement false is wrong.
- (d)either true or false depending upon the temperature of the surroundings. — Temperature does not decide whether a force needs contact; the classification holds at all temperatures.
Concept
Forces fall into two groups: contact forces (friction, normal reaction, tension, an applied push or pull), which act only when objects touch, and non-contact or field forces (gravitational, electrostatic, magnetic), which act across empty space through a field. Friction is a contact force; magnetic force is a non-contact force.
The distractors try to attach an irrelevant condition (temperature) to a definition-based fact. Once you recall the two categories, the classification is unconditional.
Key facts
- Contact forces such as friction, normal reaction and tension act only when bodies are in physical contact.
- Non-contact or field forces — gravitational, electrostatic and magnetic — act at a distance through a field.
- Friction opposes the relative motion between two surfaces in contact.
- A magnet can attract iron without touching it, showing magnetic force is non-contact.
Friction needs contact while magnetism acts at a distance, so the statement is always true — option (a).
Study next
Common traps
- Being lured by 'temperature-dependent' options into over-thinking a definition.
- Confusing 'non-contact' with 'weak' — a magnetic force can be strong yet still act at a distance.
Given as a true/false judgement of a classification statement, padded with irrelevant temperature conditions — treat it as a straight definition.
Related PYQs
Which one of the following about different frictional forces is correct?
- (a) Kinetic friction > Static friction > Rolling friction
- (b) Static friction > Rolling friction > Kinetic friction
- (c) Static friction > Kinetic friction > Rolling friction
- (d) Static friction > Kinetic friction = Rolling friction
Answer(c) Static friction > Kinetic friction > Rolling friction
Same area — the force of friction. That NDA item ranks static, kinetic and rolling friction; this one classifies friction as a contact force.
Which one of the following statements regarding magnetic field is NOT correct ?
- (a) Magnetic field is a quantity that has direction and magnitude
- (b) Magnetic field lines are closed curves
- (c) Magnetic field lines are open curves
- (d) No two magnetic field lines are found to cross each other
Answer(c) Magnetic field lines are open curves
Same area — magnetic force and fields. That NDA item tests properties of magnetic field lines; this one classifies magnetic force as a non-contact (field) force.
Practice
- practice — not a real PYQ
Which one of the following is a non-contact force?
- (a)Friction
- (b)Tension
- (c)Magnetic force
- (d)Normal reaction
Answer(c) Magnetic force — it acts across a distance. - practice — not a real PYQ
Friction is best described as a
- (a)non-contact force
- (b)contact force
- (c)field force
- (d)nuclear force
Answer(b) contact force — it acts only between surfaces in contact.