Which of the following is a non-contact force ?

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Correct — A, Magnetic force. A force is classified as non-contact when the two bodies influence each other across empty space, with no physical touching and no intervening medium required. Magnetic force is the textbook case: a bar magnet lifts an iron nail before the nail reaches it, deflects a compass needle from across a table, and does both equally well inside a vacuum, because the interaction is carried by the magnetic field surrounding the magnet rather than by any surface pressed against another. The strength falls off steeply with separation — the force between two magnetic poles follows an inverse-square law, so doubling the gap cuts the force to a quarter — which is exactly the behaviour of a field force and not of a push or a pull transmitted through touching surfaces. The other two named forces both fail this test by definition. Friction arises only where two surfaces are already in contact and are sliding, rolling or tending to slide over one another, and its magnitude f = μN depends on the normal force pressing those surfaces together; remove the contact and the friction vanishes instantly. An impact force is the very large, very short-lived force exerted during a collision — a bat on a ball, a hammer on a nail — and a collision is contact, by the meaning of the word. Since option (a) is a genuine non-contact force, option (d) 'None of these' cannot stand.

Why the other options are wrong — (b) Friction is the standard example of a contact force, not a non-contact one. It acts along the surface of contact, opposes relative motion, and is proportional to the normal reaction (f = μN). A body with no surface touching it — a stone in free fall through vacuum — experiences no friction at all.  (c) An impact force is the large force generated during a collision, related to the change of momentum by impulse = F × Δt. A collision requires the bodies to meet, so impact force is contact by definition. Candidates sometimes pick it because impacts feel instantaneous and 'invisible', but the bodies do touch.  (d) This would only be right if all three named forces were contact forces. Magnetic force plainly is not — it acts through a field across a gap, as a magnet attracting iron filings without touching them demonstrates — so at least one option qualifies and 'None of these' is false.

Concept: Forces are grouped in school physics into contact forces and non-contact (field, or action-at-a-distance) forces. Contact forces include muscular or applied force, friction, the normal reaction, tension in a string, air resistance, the spring force and impact force during a collision — every one of them needs the two bodies to be touching. Non-contact forces are gravitational, electrostatic and magnetic force; each is described by a field that fills the space around the source, so the second body responds to the field at its own location rather than to any direct touch. At the deeper level all interactions in nature reduce to four fundamental forces — gravitational, electromagnetic, strong nuclear and weak nuclear — and every so-called contact force, friction included, is ultimately electromagnetic repulsion between the electron clouds of atoms in the two surfaces.

State PCS 2024 Q122 — Science & Technology PYQ with Answer

State PCS 2024 Q122 — Science & Technology PYQ with Answer