A 1500 kg car experiences a resultant forward force of 3000 N. Find its acceleration. [1 mark]

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How do forces determine the way objects move?

Newton's three laws of motion, the equation F = ma for constant mass, the meaning of inertia and inertial mass, and applying the laws to connected bodies and everyday situations.

A focused answer to AQA A-Level Physics 3.4.1.5, covering Newton's three laws of motion, the equation F = ma for an object of constant mass, the meaning of inertia and inertial mass, and how the laws are applied to connected and everyday systems.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
Newton's first law
Newton's second law
Newton's third law
Inertia and inertial mass
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What this dot point is asking

AQA specification point 3.4.1.5 wants you to state and apply Newton's three laws, use $F = ma$ for an object of constant mass, explain inertia and inertial mass, and apply the laws to single objects, connected bodies and everyday situations.

Newton's first law

This defines what a force does: it changes motion. With no resultant force, velocity (speed and direction) stays constant. The first law overturned the everyday intuition that motion needs a continuous push: a puck slides forever on frictionless ice, and it is friction, not the absence of a force, that brings real objects to rest.

Newton's second law

The resultant force on a body equals the rate of change of its momentum. For an object of constant mass this reduces to the familiar form.

Acceleration is directly proportional to the resultant force and inversely proportional to the mass, and it acts in the same direction as the resultant force. The key word is resultant: you must combine all the forces acting before applying the law. The weight of an object, $W = mg$ , is the gravitational force; here $g$ acts as the gravitational field strength.

Newton's third law

A rocket pushes exhaust gas backwards and the gas pushes the rocket forwards; a swimmer pushes water back and the water pushes the swimmer forwards. The two paired forces are always the same type (both gravitational, both contact, and so on).

Inertia and inertial mass

Inertia is the property by which a body resists a change in its motion. Inertial mass measures that resistance: a larger inertial mass needs a larger force to produce the same acceleration. This is the mass that appears in $F = ma$ , and experiments show it is equal to the gravitational mass that appears in $W = mg$ .

Acceleration of a lift and the reading on a scale

A person of mass $60 \text{ kg}$ stands on a bathroom scale in a lift that accelerates upwards at $2.0 \text{ m s}^{-2}$ . Find the reading (the normal contact force). Take $g = 9.8 \text{ m s}^{-2}$ .

step 1: Identify the forces

Two vertical forces act: the upward normal force $N$ from the scale and the downward weight $W = mg = 60 \times 9.8 = 588 \text{ N}$ .

step 2: Apply Newton's second law

Taking upwards as positive, the resultant force is $N - W = ma$ .

step 3: Rearrange for the normal force

$N = m(g + a) = 60(9.8 + 2.0)$ .

step 4: Evaluate

$N = 60 \times 11.8 = 708 \text{ N}$ , larger than the person's true weight because the lift is accelerating upwards.

Try this

Q1. State Newton's second law of motion. [2 marks]

Cue. The resultant force equals the rate of change of momentum; for constant mass, $F = ma$ .

Q2. A $1500 \text{ kg}$ car experiences a resultant forward force of $3000 \text{ N}$ . Find its acceleration. [1 mark]

Cue. $a = \dfrac{F}{m} = \dfrac{3000}{1500} = 2.0 \text{ m s}^{-2}$ .

Q3. State one example of a Newton's third law force pair. [1 mark]

Cue. A rocket pushes gas backwards; the gas pushes the rocket forwards (or similar).

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20183 marksA resultant force of

12 \text{ N}

acts on a trolley of mass

2.0 \text{ kg}

. Calculate the acceleration, and state what happens to the acceleration if the same force acts on a

4.0 \text{ kg}

trolley.

Show worked answer →

Apply Newton's second law $F = ma$ , rearranged to $a = \dfrac{F}{m}$ .

For the $2.0 \text{ kg}$ trolley, $a = \dfrac{12}{2.0} = 6.0 \text{ m s}^{-2}$ .

For the $4.0 \text{ kg}$ trolley the mass has doubled, so for the same force the acceleration halves to $3.0 \text{ m s}^{-2}$ .

Markers reward correct use of $F = ma$ and recognising the inverse relationship between mass and acceleration.

AQA 20214 marksTwo boxes, of mass

3.0 \text{ kg}

and

2.0 \text{ kg}

, are connected by a light inextensible string and pulled along a frictionless surface by a horizontal force of

10 \text{ N}

applied to the

3.0 \text{ kg}

box. Calculate the acceleration of the system and the tension in the string.

Show worked answer →

Treat the two boxes as one system: the total mass is $5.0 \text{ kg}$ , so $a = \dfrac{F}{m} = \dfrac{10}{5.0} = 2.0 \text{ m s}^{-2}$ .

To find the tension, consider the $2.0 \text{ kg}$ box alone. The only horizontal force on it is the tension, so $T = ma = (2.0)(2.0) = 4.0 \text{ N}$ .

Markers reward finding the system acceleration from the total mass, then isolating one box to find the tension, with consistent use of $F = ma$ .

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Sources & how we know this

AQA A-level Physics (7408) specification — AQA (2017)