Chapter 6: How Forces Affect Motion

Why this chapter matters for UPSC: Newton's three laws are among the most examinable ideas in physics, and general-science Prelims regularly tests inertia, F = ma, momentum, and the action-reaction principle. The chapter's headline application is directly current-affairs relevant: rocket propulsion and the Chandrayaan-3 Vikram lander's retro-firing for a soft landing near the Moon's south pole (August 2023) is Newton's third law in action — a ready-made GS3 Science & Technology anchor. Friction, airbags, and safe-driving physics add GS3 road-safety relevance.

🧠 First Principles — Read This First

A force is a push or pull (a vector) that can change an object's state of motion or shape, and the chapter's core idea is Newton's three laws: (1) an object keeps its state of rest or uniform motion unless a net force acts (inertia); (2) that net force produces an acceleration F = ma in its own direction; (3) every force comes with an equal and opposite force on another object. Force has magnitude and direction (SI unit newton, N) and is measured with a spring balance. When several forces act, only the net force matters: balanced forces (equal, opposite) give zero net force and no change in motion; unbalanced forces give a non-zero net force and a change in motion. Friction always opposes relative motion and depends on the surfaces in contact — it is why a pushed object stops, and (as the coin-and-rubber-band activity shows) smoother surfaces mean less friction and longer travel. Newton's first law (law of inertia): a body stays at rest or in uniform motion unless a net force acts — so no force is needed to keep an object moving at constant velocity (only to change it). Newton's second law: a net force produces an acceleration in its direction, proportional to the force and inversely proportional to mass — F = ma (1 N = force giving 1 kg an acceleration of 1 m s⁻²); expressed more generally, force = rate of change of momentum (mass × velocity). Newton's third law: forces occur in equal and opposite pairs acting on two different objects — the basis of walking, rowing, recoil, and rocket propulsion. Grasping that force changes motion via three laws — inertia, F = ma, and action-reaction — with friction the ever-present opposing force is the foundational insight of the chapter.

Why this matters: inertia, F = ma, momentum and action-reaction are core Prelims physics, and rocket propulsion (third law) is a live GS3 space-technology theme.

PART 1 — Quick Reference

PART 2 — Concepts & Narrative

Force, and measuring it

A force is a push or pull that can start an object moving, change its speed or direction, or change its shape. Like velocity and acceleration, it is a vector (magnitude + direction); its SI unit is the newton (N), measured with a spring balance. (A 100 g mass resting in your palm presses down with a force of about 1 N.)

Balanced and unbalanced forces

Usually more than one force acts on an object (e.g. a pushed box also feels friction; a floating ball feels gravity down and buoyancy up). Only the net force decides the motion:

• Balanced forces — equal in magnitude, opposite in direction → net force zero → no change in motion (tug-of-war with equal teams; the rope doesn't move).
• Unbalanced forces → non-zero net force → the object accelerates in the direction of the larger force. Forces in the same direction add; opposite forces subtract.

Friction: overlooked but always present

Friction opposes relative motion and depends on the surfaces in contact. It is why a box needs a larger push to start moving, and why a coasting object eventually stops. The coin-stack-and-rubber-band activity shows that on smoother surfaces (marble, tile) the object travels farther and slows more gradually — less friction. A thought experiment with zero friction reveals the deep truth Galileo grasped: with no friction, a moving object would never stop — leading directly to Newton's first law.

Newton's first law (inertia)

An object at rest remains at rest, and an object in motion continues to move with constant velocity, unless a net force acts upon it.

If the net force is zero, the object cannot start moving or change its velocity — its acceleration is zero. The key insight (contradicting ancient belief) is that no force is needed to keep an object moving at constant velocity — only to change its speed or direction, or to stop it. Inertia is this tendency to resist a change in state, and it increases with mass. Galileo argued this via thought experiments in the 17th century; Newton named it inertia and built his first law on it.

Newton's second law (F = ma)

When a net force acts on an object, it accelerates in the direction of the net force; the acceleration is proportional to the force and inversely proportional to the mass.

Mathematically F = ma. This defines the newton: 1 N is the force that gives a 1 kg mass an acceleration of 1 m s⁻². The gravitational force on a mass (its weight) is F = mg, with g ≈ 9.8 m s⁻² (independent of the object's mass — which is why all objects fall with the same acceleration). The "more complete" form uses momentum (p = mv): force = the rate of change of momentum — valid even when mass changes (as in a rocket).

Newton's third law (action-reaction)

Whenever one object exerts a force on a second object, the second simultaneously exerts an equal and opposite force on the first.

The two forces are equal in magnitude, opposite in direction, and act on different objects — so they do not cancel. This explains walking (push ground back, ground pushes you forward), rowing (paddle pushes water back, water pushes canoe forward), gun recoil, and climbing a tree. Because the paired forces act on different masses, they produce different accelerations (a fired bullet accelerates far more than the heavy gun; the Earth barely moves toward a falling fruit).

Systems of connected objects

Newton's laws extend to systems. Two boxes joined by a string and pulled by force F accelerate as a single object of mass (m₁ + m₂): a = F / (m₁ + m₂). Treating connected objects as one system means internal forces (the string tension) can be ignored — only external forces matter. This "look at the whole, not the parts" move is a powerful problem-solving idea (even a walking human body, with its complex limb motion, can be analysed as one object).

[Additional] 6a. Where Newton's laws hold — and where they don't

[Additional] 6b. Friction, safety and technology

PART 3 — UPSC Integration

This chapter is core general-science: Newton's three laws, inertia, F = ma, momentum, weight vs mass, and action-reaction are all directly examinable. It connects strongly to GS3 Science & Technology — rocket propulsion and Chandrayaan-3's soft landing (third law) is a live space-technology anchor — and to GS3 road safety (friction, airbags, the momentum principle). GS1 history of science is served by Galileo's inertia thought experiments and Newton's Principia (1687).

Exam Strategy

Prelims pointers:

• First law = inertia (no net force → no change in motion); a body at constant velocity has zero net force.
• F = ma; 1 N = 1 kg·m·s⁻²; weight = mg; g is independent of mass.
• Third-law pairs act on DIFFERENT objects → they don't cancel; equal forces can give unequal accelerations (different masses).
• Rocket propulsion = Newton's third law; works in vacuum. Momentum = mv.
• Friction opposes motion but enables walking (backward push → forward reaction).

Mains / Essay angles:

• Newton's laws behind India's space achievements (GS3 S&T).
• Physics of vehicle safety — airbags, seatbelts, friction — and road-safety policy (GS3).

Practice Questions

Prelims:

1. A rocket accelerates upward primarily because of:
   (a) Air pushing against the exhaust gases
   (b) The reaction force of the expelled gases (Newton's third law)
   (c) The reduced weight after fuel burns
   (d) Buoyancy in the atmosphere

2. An object moving with constant velocity on a straight road has:
   (a) A net force in the direction of motion
   (b) Zero net force acting on it
   (c) An acceleration equal to g
   (d) Increasing momentum

Mains:

1. Explain how Newton's third law governs rocket propulsion, and illustrate with the soft-landing manoeuvre of an Indian lunar mission. (GS3, 10 marks)
2. "The physics of momentum underlies modern vehicle-safety design." Discuss with reference to airbags, seatbelts and friction. (GS3, 10 marks)

Sources: NCERT, Exploration — Textbook of Science for Grade 9 (First Edition, April 2026; ISBN 978-93-5729-567-3), Chapter 6 "How Forces Affect Motion"; Isaac Newton, Philosophiæ Naturalis Principia Mathematica (1687); Galileo Galilei on inertia; ISRO Chandrayaan-3 soft landing near the lunar south pole (23 August 2023).

📦 Revision Capsule