PhysicsICSE Class 9 · Newton's Laws of Motion

Inertia, Newton's First Law and the Third Law: Action and Reaction (ICSE Class 9)

Updated 17 October 2026 · Fact-checked

The first law says a body stays at rest or in uniform straight-line motion unless an unbalanced force acts; this property is inertia, which grows with mass. The third law says the force A exerts on B equals the force B exerts on A, opposite in direction; the two forces act on different bodies and never cancel.

Feet move with the bus but the body lags (inertia of rest); action and reaction are equal, opposite, and act on different bodies, so they never cancel.

Illustration drawn for QuestionBuddy.

Understand Inertia and Newton's First and Third Laws: Reasoning Questions

First law. A body continues in its state of rest or of uniform motion in a straight line unless an external, unbalanced force acts on it. The law gives two ideas:

Force is whatever changes, or tends to change, the state of rest or motion.
Inertia is the body's unwillingness to change that state. Its measure is the mass: a loaded truck is harder to start and harder to stop than an empty one.

Inertia of rest. A body at rest stays at rest until pushed. Examples: a coin on a card over a glass drops into the glass when the card is flicked sharply; a passenger is jerked backward when a bus starts; dust leaves a carpet when it is beaten.

Inertia of motion. A moving body keeps moving. Examples: a passenger lurches forward when the brakes are applied; a runner cannot stop at once; a person stepping out of a moving bus must run a few steps in the direction of travel because the feet stop but the body keeps moving.

How to answer a "why" question. (1) Say which part of the body is acted on by a force and which part is not, (2) name the kind of inertia, (3) state the result. For the bus starting suddenly: the feet move with the floor, the upper body tends to stay at rest (inertia of rest), so the passenger falls backward.

Third law. To every action there is an equal and opposite reaction. If A pushes on B with $F_{A B}$ , then B pushes on A with $F_{B A} = - F_{A B}$ .

Properties of the pair:

equal in magnitude, opposite in direction, along the same line;
act at the same instant, and are of the same kind (both contact, or both gravitational);
act on different bodies, so they cannot cancel; each affects only the body it acts on.

Which pair? A book on a table. The book's weight (Earth pulls book down) pairs with the book pulling the Earth up. The normal reaction (table pushes book up) pairs with the book pushing the table down. The weight and the normal reaction are *not* an action–reaction pair; they are two forces on the same book that happen to balance.

Equal forces, unequal motion. Because $a = \frac{F}{m}$ , equal and opposite forces on different masses give accelerations inversely proportional to the masses. This explains the recoil of a gun (small acceleration for the heavy gun, large for the light bullet) and why the Earth does not seem to move when you jump.

Walking, swimming, rockets. The foot pushes the ground backward; the ground pushes the foot forward and we move. A rocket pushes hot gas down; the gas pushes the rocket up.

Inertia and Newton's First and Third Laws: Reasoning Questions: formulas and key facts

Third law: $F_{B A} = - F_{A B}$; On different bodies.
Accelerations of the pair: $\frac{a _{A}}{a _{B}} = \frac{m _{B}}{m _{A}}$; Equal forces, so a is inverse to mass.
Second law link: $F = ma$; Used to compare motions of the two bodies.

How to solve Inertia and Newton's First and Third Laws: Reasoning Questions questions

Separate the bodies, then the forces on each.

1Choose one body and list only the forces acting on it.
2Match each force to its partner on the other body for the third-law pair.
3For numericals use F = ma on each body separately; the pair has equal magnitude.
4For reason questions use the three-step pattern: which part, which inertia, what result.

Inertia and Newton's First and Third Laws: Reasoning Questions in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Skateboard push

Suppose a 40 kg child on a skateboard pushes a wall with a force of 80 N.

What happens, and what is the child’s acceleration?

Try it first, then show the solutionSolution

The wall pushes back on the child with 80 N (third law).
$a = \frac{80}{40} = 2$ m s⁻² away from the wall; the wall, fixed to the Earth, does not appear to move.

Answer: The child moves away from the wall at 2 m s⁻².

Memory hook: You push, it pushes: same size, different bodies.

Tricks to remember Inertia and Newton's First and Third Laws: Reasoning Questions

Short memory aids. Say each one aloud once, then use it on a question the same day.

Different bodies, no cancelling
Action on B, reaction on A. Forces cancel only when they act on the same body.
Use it when: Horse-and-cart style questions.
Mass is inertia
More mass, more inertia, less acceleration for the same force.
Use it when: Comparisons.
Feet stop, body goes
Inertia of motion: lower body halts, upper body continues.
Use it when: Bus and stepping-off questions.

Inertia and Newton's First and Third Laws: Reasoning Questions: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Coin and card · 1 mark · MCQBoard level

A coin rests on a card placed over a glass. When the card is flicked sharply the coin drops into the glass. This is because of:
(a) inertia of motion
(b) inertia of rest
(c) the third law
(d) friction of the glass

Try it first, then show the solutionSolution

The coin was at rest and tends to remain at rest when the card is pulled away.

Answer: (b) inertia of rest

Example 2: Stepping off a bus · 2 marks · Short answerBoard level

Why should a person who steps out of a moving bus run a few steps in the direction of the bus?

Try it first, then show the solutionSolution

The feet touch the ground and are brought to rest, but the upper body is still moving with the speed of the bus (inertia of motion). (1)
Running reduces the speed of the body gradually and the person does not fall forward. (1)

Answer: Inertia of motion; the body keeps moving forward when the feet stop.

Example 3: Gun and bullet · 3 marks · Short answerHarder board level

A rifle of mass 4 kg fires a bullet of mass 20 g. During the firing the force on the bullet is 1200 N for 0.005 s. Find (a) the acceleration of the bullet and of the rifle, (b) the speeds they acquire.

Try it first, then show the solutionSolution

(a) By the third law the force on the rifle is also 1200 N. $a_{b} = \frac{1200}{0.02} = 60000$ m s⁻²; $a_{r} = \frac{1200}{4} = 300$ m s⁻². (1½)
(b) $v_{b} = 60000 \times 0.005 = 300$ m s⁻¹; $v_{r} = 300 \times 0.005 = 1.5$ m s⁻¹. (1½)

Answer: (a) 60 000 and 300 m s⁻² (b) 300 and 1.5 m s⁻¹

Example 4: Weight and normal reaction · 3 marks · Short answerHarder board level

A 2 kg book rests on a table ( $g = 10$ m s⁻²). (a) Find its weight and the normal reaction. (b) A student says these two are an action–reaction pair. Is she right? Name the true third-law partner of each.

Try it first, then show the solutionSolution

(a) $W = 2 \times 10 = 20$ N downward; the book is in equilibrium so $N = 20$ N upward. (1)
(b) No: both act on the same body (the book). (1)
Partner of the weight: the book pulls the Earth up with 20 N. Partner of $N$ : the book presses the table down with 20 N. (1)

Answer: (a) 20 N and 20 N (b) No; partners are the book’s pull on the Earth and its push on the table

Example 5: Horse and cart · 4 marks · Case-basedHOTS / competency

A horse pulls a cart of mass 100 kg through a rope with 600 N. The ground resists the cart with a friction force of 200 N. The horse has mass 400 kg and moves with the cart.
(a) Find the acceleration of the cart.
(b) What force does the cart (through the rope) exert on the horse?
(c) Find the forward force the ground must exert on the horse’s hooves.
(d) A student says the pair of 600 N forces cancel, so nothing can move. Reply.

Try it first, then show the solutionSolution

(a) Net force on cart $= 600 - 200 = 400$ N; $a = \frac{400}{100} = 4$ m s⁻². (1)
(b) 600 N backward on the horse (third law). (1)
(c) The horse also accelerates at 4 m s⁻²: $F_{g r o u n d} - 600 = 400 \times 4$ , so $F_{g r o u n d} = 2200$ N forward. (1)
(d) The two 600 N forces act on different bodies (cart and horse), so they cannot cancel; the cart moves because the net force on the cart is non-zero. (1)

Answer: (a) 4 m s⁻² (b) 600 N backward (c) 2200 N (d) forces on different bodies

Where students lose marks in Inertia and Newton's First and Third Laws: Reasoning Questions

The slips examiners see most often, and what to write instead.

Mistake: Action and reaction said to cancel
They act on different bodies.
Wrong working
The forces are equal and opposite, so they cancel.
Right working
They act on different bodies, so they cannot cancel.
Do this: Always name the two bodies.
Usually costs: The reasoning marks.
Mistake: Weight and normal reaction called a pair
Both act on the same body.
Wrong working
Weight is the action and the normal reaction is the reaction.
Right working
The pair of the weight is the pull of the book on the Earth.
Do this: Check "on which body does each force act?"
Usually costs: 1 to 2 marks.
Mistake: Inertia explained without naming the part of the body
Examiners want both parts: the part forced and the part tending to stay.
Wrong working
Because of inertia the passenger falls.
Right working
The feet move with the bus but the upper body tends to remain at rest (inertia of rest), so the passenger falls backward.
Do this: Use the three-step pattern.
Usually costs: 1 mark.

Exam tips

Name the kind of inertia (rest or motion) every time.
For third-law questions, state the two bodies on which the forces act.
Write the first law statement in full when it is asked.
When forces are equal and the masses differ, compare accelerations with a = F/m.

Practice questions on Inertia and Newton's First and Third Laws: Reasoning Questions

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
A rocket rises because:
(a) the exhaust gases push against the air
(b) the gases pushed down exert an equal upward force on the rocket
(c) the Earth attracts it upward
(d) its inertia pushes it up
Hint
Third law.
Answer
(b)
Q2 · 1 mark · Very short answerBoard level
Name the kind of inertia involved when dust falls off a carpet that is beaten.
Hint
The carpet moves, the dust does not.
Answer
Inertia of rest.
Q3 · 2 marks · Short answerHarder board level
Two skaters, A of mass 50 kg and B of mass 75 kg, push each other apart on smooth ice. A gets an acceleration of 3 m s⁻². Find the force and the acceleration of B.
Hint
Same force on both.
Answer
$F = 50 \times 3 = 150$ N; $a_{B} = \frac{150}{75} = 2$ m s⁻² in the opposite direction.
Q4 · 3 marks · Short answerHarder board level
A 60 kg boy on roller skates throws a 3 kg ball horizontally, exerting a force of 30 N on it for 0.2 s. Find the speed of the ball and the recoil speed of the boy.
Hint
Use F t = m v for each.
Answer
Ball: $v = \frac{30 \times 0.2}{3} = 2$ m s⁻¹. Boy (reaction 30 N for 0.2 s): $v = \frac{30 \times 0.2}{60} = 0.1$ m s⁻¹ backward.
Q5 · 2 marks · Assertion-reasonHarder board level
Assertion (A): A person standing in a bus falls backward when the bus suddenly starts.
Reason (R): The upper part of the body tends to remain at rest while the feet move forward.
(a) Both true, R explains A.
(b) Both true, R does not explain A.
(c) A true, R false.
(d) A false, R true.
Hint
Inertia of rest.
Answer
(a)
Q6 · 3 marks · Short answerHOTS / competency
A sailor fixes a fan on a boat to blow air against a sail fixed on the same boat, hoping to move the boat. Will it move? Explain using the third law.
Hint
Look at all the forces and their partners.
Answer
No. The fan pushes the air forward and the air pushes the fan backward with an equal force. The air then pushes the sail forward with an (ideally) equal force. These are internal forces of the boat and cancel in their effect on the boat, so there is no net force and no motion.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is inertia?Show answer
The property of a body to resist a change in its state of rest or of uniform motion.
2.What measures inertia?Show answer
Mass.
3.Do action and reaction cancel?Show answer
No; they act on different bodies.
4.Pair of the weight of a book?Show answer
The book’s pull on the Earth.
5.Why does a gun recoil?Show answer
The bullet is pushed forward and, by the third law, the gun is pushed backward with an equal force.

Inertia and Newton's First and Third Laws: Reasoning Questions: frequently asked questions

What is Newton’s first law of motion?

A body remains at rest or in uniform motion in a straight line unless an external unbalanced force acts on it.

Why do action and reaction not cancel each other?

They act on different bodies, so each one affects only the body on which it acts.

Is the normal reaction the reaction to the weight?

No. Both act on the same body. The true partner of the weight is the body’s gravitational pull on the Earth.