Key Stage 4 science · Physics – Forces

GCSE forces: fields, vectors, work done and pressure

This GCSE physics topic builds a fuller picture of forces. Students study the three kinds of field that exert forces at a distance (electrostatic, magnetic and gravitational), treat forces as vectors with both size and direction, calculate work done as force multiplied by distance, compare elastic and inelastic stretching, and explain how pressure in liquids and gases varies with depth and height, including the upward push called upthrust.

Forces, fields, work done and pressure, explained

A field is a region where an object feels a force without touching anything. Gravity pulls any two masses together, and near Earth it gives every kilogram a weight of about 10 newtons. Magnetic fields act on magnets and on magnetic materials such as iron, with poles that repel when alike and attract when different. Electrostatic fields act between charged objects: like charges repel and unlike charges attract. In each case the force weakens as the distance grows, and students draw field lines to show the direction of the force. Contact forces, such as friction and tension, are the other group.

A vector has a size and a direction, unlike a scalar such as mass or temperature, which has size only. Forces are vectors, so a 5 N push to the right and a 3 N push to the left combine to give a resultant of 2 N to the right, not 8 N. Students add forces in the same line, and in many courses use scale diagrams to find the resultant of two forces at an angle. How far this goes depends on the course and the tier of paper.

Work done is the energy transferred when a force moves something through a distance, calculated as force multiplied by distance moved in the direction of the force, in joules. Pushing a heavy box across the floor does work against friction, and that energy is transferred to the thermal store of the box and floor. Stretching a spring does work, too, and the energy is stored in its elastic store. If the spring returns to its original length when the force is removed, the stretching was elastic; if it stays permanently stretched, it was inelastic. Students also learn that, up to a point called the limit of proportionality, a spring’s extension is proportional to the force on it, so doubling the force doubles the extension. Beyond that point the force-extension graph curves, and a spring stretched much further will not return to its original shape.

The last part concerns fluids, meaning liquids and gases. Pressure in a fluid acts in all directions and at right angles to any surface it touches. The pressure in a liquid grows with depth because there is more liquid above pushing down, which is why a diver feels more pressure at the bottom of a deep pool. Atmospheric pressure falls with height because there is less air above. Upthrust is the upward force on an object in a fluid, caused by the pressure being greater on the bottom surface than on the top. An object floats when the upthrust equals its weight. At this stage the ideas are largely qualitative, so students explain them in words rather than calculating.

Words your child will hear

Field: A region in which a force acts on an object without contact, such as a gravitational or magnetic field.
Vector: A quantity with both size and direction, like force or velocity.
Resultant force: The single force that has the same effect as all the forces acting on an object.
Work done: Energy transferred when a force moves an object. Force × distance, in joules.
Elastic: Returns to its original shape when the force is removed.
Pressure: Force pressing on a surface. In a fluid it acts in all directions.
Upthrust: The upward force a fluid exerts on an object placed in it.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about forces and fields: electrostatic, magnetic, gravity
Students should be taught about forces as vectors
Students should be taught about calculating work done as force x distance; elastic and inelastic stretching
Students should be taught that pressure in fluids acts in all directions, including its variation in Earth's atmosphere with height and with depth for liquids, and the up-thrust force (qualitative)

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

A student pulls a sledge 12 m along level ground. The rope pulls forwards with a force of 50 N in the direction of travel, and friction pushes backwards with 30 N. Find the resultant force, the work done by the student, and the work done against friction.

Forces are vectors, so direction matters. Take forwards as positive. The pull is +50 N and friction is -30 N.
Resultant force is the sum of the forces along the line.
```
resultant = 50 - 30
          = 20 N forwards
```
Work done by the student uses the pulling force and the distance moved along the direction of the force.
```
W = F × s
  = 50 × 12
  = 600 J
```
Work done against friction uses the friction force over the same distance.
```
W = 30 × 12
  = 360 J
```
Interpret. Of the 600 J transferred by the student, 360 J is dissipated as thermal energy at the surfaces, and the remaining 240 J goes to the kinetic store of the sledge, which is why it speeds up. Check: 20 N × 12 m = 240 J.

The resultant force is 20 N forwards. The student does 600 J of work; 360 J is done against friction and 240 J increases the sledge’s kinetic energy.

Common mistakes and how to help

Thinking pressure in a liquid acts only downwards

Weight acts downwards, and children picture the liquid pressing on the bottom of the container.

What helps: Over a sink, make small holes at different heights in the side of a plastic bottle (an adult should do this) and fill it with water. Water squirts out sideways, and the jet from the lowest hole comes out hardest. Pressure at a point acts in all directions, and it is the depth that sets how big it is.

Believing upthrust only acts on objects that float

Students link upthrust with boats and bobbing corks, and assume a sinking stone has none.

What helps: Lift a heavy object in a bath and notice that it feels lighter. Upthrust acts on anything in a fluid. A stone sinks because its weight is bigger than the upthrust, not because there is none.

Adding forces as plain numbers and ignoring direction

In maths, 5 + 3 = 8, and forces look like numbers on a page.

What helps: Draw arrows. A tug of war with 500 N one way and 450 N the other has a resultant of 50 N, not 950 N. Always ask which way each force points before combining them.

Assuming every stretched spring will return to its shape, or that extension is always proportional to force

Everyday springs and bands are used well within their limits, so students rarely see them break the pattern.

What helps: Show a graph of force against extension that is a straight line and then curves. The straight part is where the extension is proportional to the force. Beyond the limit of proportionality the line curves, and if the spring is stretched far enough it stays permanently stretched, which is inelastic.

How to help at home

Map a magnetic field with a compass. Place a bar or fridge magnet on a table and move a small compass around it, drawing arrows for the needle’s direction. Your child will see the pattern of field lines emerge. Keep magnets away from phones, cards and screens.
Static with a balloon. Rub a balloon on hair or a woollen jumper and see it stick to a wall or lift small pieces of paper. Ask your child to explain it using electrostatic forces between charges, and why the effect fades.
Feel upthrust in the bath. Push a ball under water and feel it push back, then let go. Compare a hollow ball with something heavy. Ask your child to say which force is bigger, weight or upthrust, in each case and what that makes happen.
Stretch an elastic band with coins. Hang a small bag from a thick elastic band and add coins one at a time, measuring the length each time. Plot the points and see whether they form a straight line. Keep the band away from faces and stop well before it is close to breaking.
Notice pressure changes. A sealed crisp packet looks puffier after a drive up a high hill because the air pressure outside is lower. Ask your child to explain it using the idea of less air above pushing down.

Builds on

Key Stage 3: Speed and distance-time graphs
Key Stage 3: Forces, force diagrams and moments
Key Stage 3: Deformation, friction and Hooke’s Law
Key Stage 3: Pressure in fluids, floating and sinking
Key Stage 3: Balanced forces, equilibrium and motion

Everything in Key Stage 4 science

Updated 20 October 2026.