Newton's Three Laws of Motion: KS3 Physics Guide

Newton's three laws of motion describe the relationship between force and the movement of objects. First stated in 1687 in his Principia Mathematica, they underpin everything from why seatbelts save lives to how spacecraft leave Earth's orbit, and remain the cornerstone of KS3 and GCSE mechanics.

What is Newton's First Law of Motion?

Before I explain what the First Law says, here is a prediction question: if you pulled a tablecloth sharply from beneath a set of plates and glasses, what would you expect to happen to the crockery?

Most people guess the plates fly off with the cloth — but they stay put. That is Newton's First Law in action.

Newton's First Law (Law of Inertia): An object will remain at rest, or continue moving at a constant velocity in a straight line, unless acted upon by a resultant (unbalanced) force.

Objects do not change their motion on their own — they need a net force to start, stop, speed up, slow down, or change direction. The property that causes this resistance to change is called inertia. Greater mass means greater inertia: a bowling ball is much harder to get rolling than a tennis ball, and much harder to stop once it is moving.

Everyday examples of the First Law:

A book on a table stays still — gravity acts downward and the table's normal force acts upward; these balance, so there is no resultant force and no change in motion.

A hockey puck slides a long distance on ice because friction is very small, so barely any unbalanced force acts to slow it.

A passenger lurches forward when a bus brakes suddenly — the passenger's body continues moving forward (inertia) while the bus decelerates around them.

Seatbelts save lives by providing the force needed to decelerate a passenger alongside the car, rather than the dashboard providing it violently.

In the tablecloth trick, the cloth is yanked so quickly that friction between cloth and plates acts for only a tiny time interval — the impulse (force × time) is too small to significantly change the plates' motion. They stay put.

What is Newton's Second Law of Motion?

Predict first: if you push a shopping trolley with twice the force, what do you expect to happen to its acceleration? And if you load the trolley with twice as much shopping, what happens to the acceleration for the same push?

Newton's Second Law: The resultant force on an object equals the object's mass multiplied by its acceleration.

F = m × a

F = resultant force, measured in newtons (N)

m = mass, measured in kilograms (kg)

a = acceleration, measured in metres per second squared (m/s²)

One newton is defined as the force needed to accelerate a mass of 1 kg at 1 m/s². The law tells us:

Double the force → double the acceleration (for the same mass)

Double the mass → halve the acceleration (for the same force)

The formula rearranges to: a = F ÷ m and m = F ÷ a

Worked examples:

A 1,200 kg car accelerates at 3 m/s². What is the resultant force? F = m × a = 1,200 × 3 = 3,600 N

A resultant force of 500 N acts on a 50 kg object. What is the acceleration? a = F ÷ m = 500 ÷ 50 = 10 m/s²

A 300 N force produces an acceleration of 6 m/s². What is the mass of the object? m = F ÷ a = 300 ÷ 6 = 50 kg

Always check units before substituting values — mass must be in kg and force in N.

What is Newton's Third Law of Motion?

Before I explain the Third Law, consider this: when a rocket engine fires, hot gas is blasted downward. What do you predict happens to the rocket itself?

Newton's Third Law: When object A exerts a force on object B, object B exerts an equal and opposite force on object A.

This is often stated as "every action has an equal and opposite reaction." The two forces are called an action–reaction pair.

The most important point — and the most common source of confusion — is that the two forces act on different objects. They never cancel each other out, because cancellation only happens when equal and opposite forces act on the same object.

Examples of action–reaction pairs:

Walking: you push your foot backward against the ground (action on ground); the ground pushes you forward (reaction on you) — that reaction is what propels you.

Rocket launch: the engine expels gas downward at high speed (action on gas); the gas pushes the rocket upward with equal force (reaction on rocket).

Swimming: your arms push water backward (action on water); water pushes you forward (reaction on swimmer).

Gun recoil: the gun propels the bullet forward; the bullet pushes the gun backward with equal force — which is why guns kick when fired.

How do the three laws work together?

The three laws are not isolated rules — they form a single framework for understanding all motion.

Law | Key idea | Everyday example
First Law (Inertia) | No resultant force → no change in velocity | Passenger lurches when bus brakes; tablecloth trick
Second Law (F = ma) | Resultant force = mass × acceleration | Car accelerating; braking distance depends on mass and deceleration force
Third Law (Action–Reaction) | Forces always come in equal, opposite pairs on different objects | Rocket propulsion; swimming; gun recoil

When a car accelerates from rest: the engine provides a driving force (Second Law → acceleration); the car's inertia resists that acceleration (First Law); the tyres push backward on the road and the road pushes the car forward (Third Law). All three laws are operating simultaneously.

What are the common mistakes students make about Newton's Laws?

Understanding the laws is one thing — applying them correctly is another. Here are the four errors that appear most often in KS3 and GCSE answers:

Mistake 1 — Third Law confusion: Students write that action and reaction forces "cancel out." They only cancel if they act on the same object. The rocket's engine pushes gas down; gas pushes rocket up — these act on different objects, so both objects move.

Mistake 2 — First Law and the "natural" tendency to stop: In real life, moving objects slow down and stop (because friction acts on them). Students then assume that "staying still" is the natural state. Newton's First Law says the natural state is constant velocity — friction is simply an unbalanced force that causes deceleration.

Mistake 3 — Confusing weight and mass: Mass is measured in kilograms (kg) and is constant wherever you are. Weight is a force (the pull of gravity on a mass) measured in newtons (N). Weight = mass × g, where g ≈ 10 N/kg on Earth. In F = ma calculations, always use mass in kg — do not substitute weight in newtons as "m."

Mistake 4 — Balanced forces mean no forces at all: Balanced forces mean no change in motion, not that no forces exist. A book sitting on a desk has gravity (downward) and normal force (upward) both acting on it — they balance, so the book stays still.

How are Newton's Laws applied in real-world situations?

Seatbelts and airbags (First + Second Law): In a crash, the car decelerates suddenly. Without a seatbelt, a passenger continues forward at the original speed (inertia — First Law). The seatbelt provides the decelerating force (Second Law). Airbags extend stopping time, reducing peak force (same momentum change over more time → less force).

Rocket launches (Second + Third Law): Engines burn fuel and expel gas at high velocity, exerting a large downward force on the gas. By the Third Law, the gas exerts an equal upward force on the rocket. As the rocket burns fuel, its mass decreases — so by F = ma, the same thrust produces increasing acceleration.

Sports: In cricket, a faster ball hits a bat with greater force (F = ma applies to the bat decelerating the ball). A heavier bat needs more force to swing at the same speed. When bat meets ball, the ball exerts an equal and opposite force on the bat — both players can feel Newton's Third Law in their hands.

Frequently asked questions

Why does a heavy lorry need a longer stopping distance than a car at the same speed?

Newton's Second Law tells us that for the same braking force, a larger mass produces a smaller deceleration (a = F/m). A loaded lorry can have ten times the mass of a car, so the same braking force produces one-tenth the deceleration. First Law inertia further resists the change in motion. Combined, these effects mean a lorry at 60 mph needs a considerably greater stopping distance — hence the Highway Code's larger safe following distances for lorries.

If action and reaction forces are equal, why does a rocket move?

The engine pushes gas downward (action on the gas); the gas pushes the rocket upward with equal force (reaction on the rocket). These forces act on different objects — one on the gas, one on the rocket. Only the rocket is affected by the upward force, so it accelerates (Newton's Second Law). Cancellation requires equal and opposite forces on the same object — not on two different objects.

What is the difference between mass and weight?

Mass is the amount of matter in an object, measured in kilograms (kg) — constant everywhere in the universe. Weight is the gravitational force acting on a mass, measured in newtons (N), calculated as W = mg. On Earth (g ≈ 10 N/kg), a 70 kg person weighs 700 N. On the Moon (g ≈ 1.6 N/kg), they weigh only 112 N, but their mass remains 70 kg. In F = ma calculations, always use mass in kg.

How does F = ma explain why it hurts more to fall on concrete than on grass?

When you fall, your momentum must change to zero. Force = change in momentum ÷ time (Newton's Second Law rearranged). On concrete, you stop almost instantly, so the force is enormous. On grass or a mat, the surface extends the stopping time — the same momentum change spread over longer time means a smaller peak force. This principle underlies cycle helmets, crumple zones, and gymnastics floor padding.

Explore Newton's laws and all KS3 physics with Professor Newton, your Socratic AI physics tutor — visit aitutors.me.