Home / IGCSE 0625 / Motion, forces, energy / Newton’s second law
IGCSE 0625 / Section 1.5 / Extended
Supplement (Extended) content

Force sets the acceleration.

Push two trolleys with the same force and the lighter one speeds up faster. Push the same trolley harder and it speeds up more. Newton’s second law ties those observations into one equation, as long as you remember it is the resultant force that counts.

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The key idea

The resultant force on an object equals its mass times its acceleration: F = ma. The acceleration is in the same direction as the resultant force, larger for a bigger force and smaller for a bigger mass.

Definition · learn the exact words

The resultant force on an object is equal to its mass multiplied by its acceleration.

F = m a

F must be the resultant force in newtons (N), m the mass in kg, and a the acceleration in m/s².

Section 01

More force, or less mass.

Set the resultant force and the mass, then launch the trolley and read its acceleration. Increase the force and the acceleration rises in proportion; increase the mass and the same force produces less acceleration.

Stage 1 · Learn

Check F = ma

Four quick checks on the law and how acceleration depends on force and mass. Each correct answer earns XP and lights this skill on your star map.

Quick check+10 XP

Newton’s second law states that the resultant force on an object equals...

Quick check+10 XP

For a fixed mass, if the resultant force is doubled, the acceleration...

Quick check+10 XP

For the same resultant force, an object with a larger mass has...

Quick check+10 XP

The acceleration produced by a resultant force is always in the direction of...

Section 02

From forces to acceleration.

Worked example

A 4.0 kg box is pulled along the floor by a force of 30 N. Friction of 6 N acts against it. Find the acceleration of the box.

Step 1 · Find the resultant forceresultant = 30 − 6 = 24 N
Step 2 · Apply F = maa = F / m = 24 / 4.0 = 6.0 m/s²
Step 3 · AnswerThe acceleration is 6.0 m/s², in the direction of the resultant force (forward).
Examiner trap

The most costly error is putting the driving or pushing force straight into F = ma and ignoring friction and drag. Always work out the resultant force first by combining all the forces. And if an object moves at constant velocity, the resultant force is zero, so the acceleration is zero, even though it is still moving.

Stage 2 · Exam

Exam-style questions

Unlocks once the four checks above are done. Worth more XP, written in the style of Paper 2.

Finish the four checks above to unlock the exam questions
Exam style+20 XP

A trolley of mass 3.0 kg accelerates at 2.0 m/s². The resultant force on it is...

Exam style+20 XP

A 2.0 kg block is pushed with a force of 12 N while friction of 4 N opposes it. Its acceleration is...

Exam style+20 XP

A resultant force of 20 N gives an object an acceleration of 5.0 m/s². The mass of the object is...

Skill unlocked

Newton’s second law, mastered.

This skill is now lit gold on your star map. That completes the forces topic.

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Go deeper · practice
Six original Cambridge-style questions
The F = ma relationship, how acceleration depends on force and mass, finding a resultant before applying the law, and calculations for force, mass, and acceleration. Attempt each, then reveal the worked solution.
Stage 3 · master the unit
Motion, forces and energy challenge
Mixed questions across the whole unit, each one worth XP. Start this only when you feel confident across every topic in the unit, not just Newton’s second law.
Start the unit challenge →