AQA GCSE Physics 4.5 Forces: a complete overview of scalars, work, motion, Newton's laws, stopping distances and momentum

What topic 4.5 actually demands

Forces is the largest and most equation-heavy topic in AQA GCSE Physics. It ties together the energy work from topic 4.1 with the mathematics of motion. Examiners test precise definitions, fluent calculation, graph interpretation, and clear application to real situations such as driving and collisions.

This guide walks through all six dot points of the topic, then sets out the exam patterns AQA repeats. Each dot point has a matching page with practice questions; this overview ties them together.

Scalars, vectors and forces

A scalar has size only (mass, speed, energy); a vector has size and direction (force, velocity, weight). Forces are contact (friction, tension, normal contact) or non-contact (gravity, magnetism, electrostatic). Weight is $W = mg$, acting at the centre of mass. The resultant force is the single force with the same effect as all the forces combined.

Work done and elasticity

Work is done when a force moves an object, $W = Fs$, and equals the energy transferred. Hooke's law, $F = ke$, says extension is proportional to force up to the limit of proportionality, and a stretched spring stores elastic potential energy $E_e = \frac{1}{2}ke^2$. The required practical investigates force and extension.

Motion: distance, speed and velocity

Distance (scalar) differs from displacement (vector), and speed (scalar) from velocity (vector). Speed is $v = \frac{s}{t}$. On a distance-time graph the gradient is the speed.

Acceleration and Newton's laws

Acceleration is $a = \frac{\Delta v}{t}$, with the uniform acceleration equation $v^2 - u^2 = 2as$. On a velocity-time graph the gradient is the acceleration and the area is the distance. Newton's laws: first (constant velocity unless a resultant force acts), second ($F = ma$), third (equal and opposite forces on interacting objects).

Stopping distances

The stopping distance is the thinking distance plus the braking distance. Thinking distance grows with speed, tiredness, alcohol, drugs and distraction; braking distance grows with speed, wet or icy roads, and worn tyres or brakes. Braking does work, transferring kinetic energy to heat in the brakes.

Momentum

Momentum is $p = mv$, a vector. In a closed system, momentum is conserved in collisions and explosions. Force is the rate of change of momentum, $F = \frac{m\Delta v}{t}$, so extending a collision time reduces the force, which is how crumple zones, air bags and crash mats reduce injury.

How topic 4.5 is examined

A typical AQA profile for Forces:

• Calculations. Weight, work, Hooke's law, speed, acceleration, $F = ma$ and momentum, often in multi-step problems.
• Graph work. Reading speed from distance-time graphs and acceleration and distance from velocity-time graphs.
• Definitions and laws. Scalar versus vector, Newton's three laws, and the parts of stopping distance.
• Extended answers. Explaining stopping-distance factors, energy transfer in braking, and how safety features reduce force using momentum.

Check your knowledge

A mix of recall and calculation questions covering topic 4.5. Attempt them under timed conditions, then check against the solutions.

1. State the difference between a scalar and a vector quantity. (2 marks)
2. Calculate the weight of a $12\,kg$ object on Earth ($g = 9.8\,N/kg$). (2 marks)
3. State Hooke's law. (2 marks)
4. A car travels $250\,m$ in $10\,s$. Calculate its speed. (2 marks)
5. State Newton's second law as an equation. (1 mark)
6. A resultant force of $30\,N$ acts on a $6\,kg$ mass. Calculate its acceleration. (2 marks)
7. State the two parts that make up the stopping distance and one factor affecting each. (3 marks)
8. A $1000\,kg$ car moves at $15\,m/s$. Calculate its momentum. (2 marks)