GCSE Physics forces

By The STEM-Tutor team · Updated October 2026

Forces explain how and why things move. Here are the GCSE Physics ideas, equations and exam favourites.

Scalars and vectors

Scalars have size only, such as speed and mass. Vectors have size and direction, such as velocity, force and displacement.

Resultant forces

The resultant force is the single force that has the same effect as all the forces acting together. Balanced forces give a resultant of zero.

Newton's laws

1. With no resultant force, an object stays at rest or keeps moving at a steady velocity.
2. Resultant force = mass × acceleration (F = ma).
3. When two objects interact, they exert equal and opposite forces on each other.

Weight and terminal velocity

• Weight = mass × gravitational field strength (W = mg).
• A falling object speeds up until air resistance balances its weight. It then falls at terminal velocity.

Stopping distance

Stopping distance = thinking distance + braking distance. Tiredness, alcohol and distractions increase thinking distance. Speed, wet or icy roads and worn tyres or brakes increase braking distance.

Momentum (Higher)

Momentum = mass × velocity (p = mv). In a closed system, total momentum before an event equals total momentum after.

Exam favourites

• Explaining terminal velocity using forces.
• Calculating acceleration from a velocity-time graph.
• Describing the acceleration required practical.

Thinking about forces

A force is a push or a pull. Forces can be contact forces, like friction, air resistance and tension, or non-contact forces, like gravity, magnetism and electrostatic forces. When several forces act on an object, you add them as vectors to find the resultant force. If the resultant is zero, the object stays still or keeps moving at a steady speed in a straight line. If the resultant is not zero, the object accelerates in the direction of the resultant.

Worked examples

Example 1: resultant force

A car's engine gives a forward force of 3000 N, and friction and air resistance total 1200 N. Find the resultant force.

• Resultant = 3000 − 1200 = 1800 N forwards.

Example 2: Newton's second law

That car has a mass of 1200 kg. Find its acceleration.

• a = F/m = 1800/1200 = 1.5 m/s².

Example 3: weight

Find the weight of a 65 kg person on Earth (g = 9.8 N/kg) and on the Moon (g = 1.6 N/kg).

• Earth: W = mg = 65 × 9.8 = 637 N.
• Moon: 65 × 1.6 = 104 N. Mass stays 65 kg in both places.

Example 4: acceleration

A cyclist speeds up from 2 m/s to 8 m/s in 4 s. Find the acceleration.

• a = (v − u)/t = (8 − 2)/4 = 1.5 m/s².

Example 5: using v² − u² = 2as

A car travelling at 20 m/s brakes to a stop with a deceleration of 5 m/s². Find the braking distance.

• 0 − 400 = 2 × (−5) × s, so s = 40 m.

Example 6: momentum (Higher)

A 0.5 kg ball travels at 10 m/s. Find its momentum.

• p = mv = 0.5 × 10 = 5 kg m/s.

Example 7: work done

A 50 N force pushes a box 4 m along the floor. Find the work done.

• W = Fs = 50 × 4 = 200 J.

Example 8: springs

A spring with spring constant 40 N/m is stretched 0.05 m. Find the force and the energy stored.

• F = ke = 40 × 0.05 = 2 N.
• E = ½ke² = ½ × 40 × 0.0025 = 0.05 J.

Reading velocity-time graphs

On a velocity-time graph, the gradient gives the acceleration and the area under the line gives the distance travelled. A horizontal line means constant velocity, a line sloping upwards means acceleration, and a line sloping down means deceleration. On a distance-time graph, the gradient gives the speed, and a horizontal line means the object is stationary. Mixing up these two types of graph is one of the most common mistakes.

Stopping distance in detail

Stopping distance is the thinking distance plus the braking distance. Thinking distance increases if the driver is tired, distracted, or has taken alcohol or drugs, because their reaction time is longer. Braking distance increases with worn brakes or tyres, and with wet or icy roads. Both increase with speed; braking distance increases a lot, because the kinetic energy to remove depends on speed squared. Doubling the speed makes the braking distance about four times longer.

Common mistakes

• Confusing mass (kg) and weight (N).
• Thinking a moving object needs a resultant force to keep moving at a steady speed.
• Reading the gradient of a distance-time graph as acceleration.
• Forgetting that deceleration is a negative acceleration in equations.

Practice questions

Try each one before opening the answer.

Frequently asked questions

What is Newton's third law?

When two objects interact, they exert equal and opposite forces on each other. The forces act on different objects, so they do not cancel.

What is inertia?

Inertia is the tendency of an object to stay at rest or keep moving at a steady velocity. Objects with more mass have more inertia.

Key takeaways

• The resultant force decides whether an object accelerates.
• F = ma links force, mass and acceleration.
• Weight = mass × g, measured in newtons.
• Gradient of a v-t graph is acceleration; area is distance.
• Stopping distance = thinking distance + braking distance.

Where to go next

Forces link to energy and to the equations you need.

• GCSE Physics energy
• GCSE Physics equations
• SUVAT equations

How to revise forces

Start with free body diagrams: for any object, draw every force as an arrow with a label. Then practise finding resultant forces and using F = ma. Learn the shapes of distance-time and velocity-time graphs and what the gradient and area mean on each. Finally, practise six-mark questions on stopping distances and terminal velocity, which come up often. Explaining each step in full sentences is as important as the calculations.

Momentum in collisions (Higher)

In a collision, total momentum before equals total momentum after, as long as no external forces act. Car safety features such as crumple zones, airbags and seatbelts increase the time taken for a person to stop. Since force equals the rate of change of momentum, a longer stopping time means a smaller force on the body, which reduces injuries.

Forces is a topic where clear diagrams and careful wording earn most of the marks, so practise both.