Key Stage 3 science · Physics – Motion and forces

KS3 balanced forces and motion: equilibrium explained

In Key Stage 3, children learn why things move and why they don’t. Opposing forces that are equal and opposite put an object in equilibrium, for example a book supported by a table or a bag hanging from a spring. Unbalanced forces are needed to start something moving, stop it, or change its speed or direction. The change depends on both the direction of the force and how big it is, and at this stage the ideas are qualitative, not calculated.

Balanced forces, equilibrium and motion, explained

An object is in equilibrium when the forces on it cancel out. A book resting on a table is pulled down by gravity, but the table pushes up with a force of the same size, so the book stays still. It is easy to forget that the table is doing anything, but at the microscopic level the surface is slightly squashed, and the squashed surface pushes back, rather like a very stiff spring. In the same way, a bag hanging from a stretched spring is in equilibrium: the spring’s upward force balances the bag’s weight.

Balanced forces don’t mean no forces, and they don’t mean nothing is moving. A car on a straight motorway at a steady speed also has balanced forces: the driving force from the engine is matched by air resistance and friction. This is the big idea of the topic. Forces are not needed to keep something moving at a steady speed, but they are needed to change the motion. An unbalanced force is required to make an object start moving, stop it, speed it up, slow it down or change its direction.

The effect depends on the direction of the force. A force acting in the same direction as the motion speeds the object up; a force against the motion slows it down; and a sideways force makes it change direction, as when a goalkeeper’s hand pushes a ball round the post. It also depends on the size of the force. A larger unbalanced force gives a bigger change in motion, while the same force changes the motion of a heavy object less than a light one. Children describe these effects in words, and at this stage they are not asked to calculate them.

A good way to think about it is to ask two questions of any moving object: which forces act, and are they balanced? If there are only balanced forces, the motion stays the same, whether that is standing still or moving steadily. If there is an unbalanced force, ask which way it points compared with the motion. Everyday explanations such as why a bike rolls to a stop or why a lorry takes longer to stop than a car then follow from the same two questions, because stopping needs a force in the opposite direction to the motion.

Words your child will hear

Equilibrium: A state where the forces on an object are balanced, so its motion does not change.
Balanced forces: Forces that are equal in size and opposite in direction, so they cancel out.
Unbalanced force: A resultant force that is not zero, which changes the motion of an object.
Support force: The upward push from a surface, such as a table, on an object resting on it.
Driving force: The forward force that moves a vehicle or a person, such as from an engine or leg muscles.
Resistance: Forces such as friction and air resistance that oppose motion.
Steady speed: Moving at a constant speed in a straight line, which happens when forces are balanced.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about opposing forces and equilibrium: weight held by a stretched spring or supported on a compressed surface
Pupils should be taught about forces being needed to cause objects to stop or start moving, or to change their speed or direction of motion (qualitative only)
Pupils should be taught about how the change in motion depends on the direction of the force and its size

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

Try it: explaining a cycle ride with forces

A cyclist rides along a flat, straight road. At first the driving force forward is 60 N and the resistance (friction and air resistance) is 60 N. What happens when she (a) pedals harder, (b) stops pedalling, and (c) is blown by a strong side wind?

Start with the balanced case. 60 N forward and 60 N backward cancel out, so the resultant is zero. She is in equilibrium and keeps moving at a steady speed. She doesn’t speed up or slow down, because no unbalanced force is acting.
(a) She pedals harder and the driving force rises to 90 N while the resistance is still 60 N.
```
90 N forward - 60 N back
= 30 N forward (unbalanced)
```
The unbalanced force acts in the direction of motion, so she speeds up. As she speeds up the air resistance rises, until it matches the driving force again. Then the forces balance, and she travels at a faster steady speed.
(b) She stops pedalling. The driving force falls to nothing, so only the backward resistance remains. This unbalanced force is against the motion, so she slows down. The bike does not stop at once, because it takes time for a force to change the motion.
(c) A strong side wind pushes at right angles to her direction of travel. The sideways force is unbalanced, so her path changes direction. To stay on course she must lean or steer to produce an opposite sideways force from the tyres.

(a) She speeds up, until the resistance grows to match the new driving force. (b) She slows down, because the only force acting along her path is the backward resistance. (c) She is pushed off course, because a sideways force changes direction. The change always depends on which way the unbalanced force points and how big it is.

Common mistakes and how to help

Believing a force is needed to keep an object moving

In daily life, things do slow down and stop, so it looks as if a push is needed to keep them going. The hidden force is friction.

What helps: Ask what is acting backwards. A ball rolling on a smooth floor goes further than on a carpet because there is less friction. A slide on ice is the same idea.

Thinking a resting object has no forces acting on it

Nothing is moving, so it seems nothing is happening.

What helps: Draw the book on the table with two arrows, weight down and support force up. Push a book into a soft sponge and watch the sponge squash, as evidence that a surface pushes back.

Thinking balanced forces mean the object must be stationary

The word “balanced” suggests stillness, and the examples used first are books on tables.

What helps: Use a car on a motorway or a skydiver at a steady speed. Balanced forces mean no change in motion, which includes staying still or moving at a steady speed.

Thinking a bigger force gives a higher speed rather than a bigger change in speed

Strong pushes seem linked to fast movement, so children match the size of the force to the speed.

What helps: A constant unbalanced force keeps changing the speed. Ask what happens to a shopping trolley pushed with a steady force that is bigger than the friction on it: it speeds up, and keeps speeding up for as long as that unbalanced push continues.

How to help at home

Sponge test. Place a heavy book on a thick kitchen sponge and look at how it squashes. Add a second book. Ask your child what is pushing the books up and why the sponge pushes harder when the weight rises.
Hang it and watch. Hang a bag from a rubber band or a spring and add coins one at a time. The band stretches until its upward force matches the weight. Ask what would happen if you added more, and what forces are balanced when the bag is still.
Roll and compare. Give a ball or toy car the same push across a smooth floor, a rug and a doormat. Measure how far each goes. Ask your child to explain the differences using the backward force of friction.
Change the direction. Roll a ball in a straight line along a hall and tap it from the side as it passes. Notice that it veers sideways while still travelling forward. Then try tapping it from behind and from in front, and describe each change.
Trolley thinking. Next time you push a trolley along a quiet shop aisle, ask which forces act on it when it is pushed, when you let go and when you slow it down. Go gently and keep it away from other shoppers.

Leads on to

Key Stage 4: Forces, fields, work done and pressure
Key Stage 4: Motion, acceleration and road safety