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How Forces Affect Motion – Class 9 Science notes, important questions and MCQs

NCERT Class 9 Science, chapter 6. Notes, key terms, important questions with answers and MCQs.

Chapter overview

A force is a push or pull. Newton's three laws explain how forces change motion, and momentum is conserved when no outside force acts.

Newton's laws

First law (inertia): a body stays at rest or in uniform motion unless an unbalanced force acts.
Second law: F = ma; force equals the rate of change of momentum.
Third law: every action has an equal and opposite reaction (acting on different bodies).

Momentum

Momentum p = mv (kg m/s).
Conservation: total momentum before = total momentum after a collision or explosion (no external force).
Recoil of a gun, a rocket's motion and swimming use the third law and momentum conservation.

Balanced and unbalanced forces

Balanced forces do not change motion; unbalanced forces cause acceleration.
Friction opposes motion.
Mass is a measure of inertia.

Key terms

Inertia: tendency of a body to resist change in its state of motion
Momentum: product of mass and velocity
Newton: force that gives 1 kg an acceleration of 1 m/s²

Formulas

F = ma
p = mv
m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
Impulse = F × t = change in momentum

Important questions with answers

Why does a person fall forward when a moving bus stops suddenly?2 marks

The bus and the person's feet stop, but the upper body tends to keep moving forward because of inertia of motion.

A force of 20 N acts on a 4 kg body at rest for 3 s. Find the acceleration, the final velocity and the momentum gained.3 marks

a = F/m = 20/4 = 5 m/s². v = u + at = 0 + 15 = 15 m/s. Momentum = mv = 4 × 15 = 60 kg m/s.

State the law of conservation of momentum and use it to find the recoil velocity of a 4 kg gun firing a 20 g bullet at 400 m/s. Why does a cricketer move his hands back while catching?5 marks

When no external force acts, the total momentum of a system stays constant. Before firing total momentum = 0, so 0.02 × 400 + 4 × v = 0, giving v = −2 m/s; the gun recoils backward at 2 m/s. A cricketer pulls his hands back to increase the time over which the ball's momentum changes; since force = rate of change of momentum, a longer time means a smaller force on the hands.

Quick revision

First law = inertia.
F = ma.
Momentum is conserved in collisions.

Exam tips

Action and reaction act on different bodies, so they do not cancel.
Catching a ball by pulling the hands back increases time and reduces force.

Practice MCQs

SI unit of force is
1. joule
2. watt
3. newton
4. pascal
Show answer
Answer: (c) newton. Newton.
Passengers fall forward when a bus stops suddenly due to
1. friction
2. reaction
3. gravity
4. inertia of motion
Show answer
Answer: (d) inertia of motion. Their body tends to keep moving.
Momentum is
1. m × v
2. m × a
3. F × t only
4. v / m
Show answer
Answer: (a) m × v. p = mv.
A force of 10 N acts on a 2 kg mass. Acceleration is
1. 5 m/s²
2. 20 m/s²
3. 12 m/s²
4. 0.2 m/s²
Show answer
Answer: (a) 5 m/s². a = F/m = 5.
Recoil of a gun is explained by Newton's
1. first law
2. second law
3. third law
4. law of gravitation
Show answer
Answer: (c) third law. Action–reaction.
Momentum of a 5 kg ball moving at 4 m/s is
1. 9 kg m/s
2. 80 kg m/s
3. 1.25 kg m/s
4. 20 kg m/s
Show answer
Answer: (d) 20 kg m/s. 5 × 4.
A 10 g bullet leaves a 2 kg gun at 200 m/s. The gun's recoil speed is
1. 10 m/s
2. 2 m/s
3. 0.5 m/s
4. 1 m/s
Show answer
Answer: (d) 1 m/s. 0.01 × 200 = 2 × v ⇒ v = 1 m/s.
A cricketer pulls his hands back while catching to
1. increase the force
2. reduce the force by increasing time
3. increase momentum
4. stop the ball faster
Show answer
Answer: (b) reduce the force by increasing time. Same change in momentum over more time.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.