Key Stage 3 science · Physics – Motion and forces

KS3 forces: force arrows, balanced forces and moments

In Key Stage 3, children refine their idea of a force as a push or a pull that arises when two objects interact. They learn to draw force arrows on diagrams, add forces acting along one line to decide whether they are balanced or unbalanced, and meet the moment, the turning effect of a force. They also study non-contact forces: gravity, magnetism and static electricity, which act across a gap.

Forces, force diagrams and moments, explained

A force is a push or a pull, and it always comes from an interaction between two objects. When your hand pushes a door, the door pushes back on your hand with a force of the same size, in the opposite direction. This is why a force is never a property of a single object. Children learn to name forces by the pair of objects involved, such as “the table pushes up on the book” or “the Earth pulls down on the book”, instead of vaguely saying “the force”.

Force diagrams are the main tool. Each force is drawn as an arrow starting on the object, pointing in the direction the force acts, with a length that shows its size and a label giving the size in newtons (N). Forces along one line can be added: those in the same direction are added, those in opposite directions are subtracted, and the result is the resultant force. If it is zero, the forces are balanced and the motion does not change; if not, they are unbalanced, and the object’s motion changes in the direction of the resultant.

Non-contact forces act without touching. Gravity is a force of attraction between any two objects with mass, and it is only noticeable when one of them, such as the Earth, is huge. It acts on Earth and in space, holding the Moon in orbit around the Earth and the planets in orbit around the Sun. The gravitational pull on an object is its weight, measured in newtons, which differs from its mass, measured in kilograms. Magnets exert forces on magnetic materials and on each other, and objects charged by rubbing can attract or repel because of static electricity.

The moment of a force is its turning effect about a point called the pivot. A bigger force gives a bigger moment, and so does applying the same force further from the pivot, which is why a door is easy to push open at the handle and hard near the hinges. The statutory requirement is the idea of the turning effect. Many schools go further and calculate moment = force × distance from the pivot, and use it to see why a seesaw balances, but this is a choice that varies from school to school.

Words your child will hear

Force: A push or a pull that arises from an interaction between two objects, measured in newtons (N).
Resultant force: The single force that has the same effect as all the forces acting on an object added together.
Balanced forces: Forces that cancel out, so the resultant is zero and the motion does not change.
Weight: The pull of gravity on an object, measured in newtons. On Earth about 10 N for each kilogram of mass.
Non-contact force: A force that acts across a gap, such as gravity, magnetic force or the force between charged objects.
Moment: The turning effect of a force about a pivot.
Pivot: The point about which something turns, such as the hinge of a door.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about forces as pushes or pulls, arising from the interaction between two objects
Pupils should be taught about using force arrows in diagrams, adding forces in one dimension, balanced and unbalanced forces
Pupils should be taught about moment as the turning effect of a force
Pupils should be taught about non-contact forces: gravity forces acting at a distance on Earth and in space, forces between magnets and forces due to static electricity

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

Try it: drawing and adding force arrows

A box weighing 20 N rests on the floor. A child pushes it to the right with a force of 40 N, and friction between the box and the floor pulls back with 15 N. Draw the forces, find the resultant and say what happens to the box.

Name each force by the objects involved. The Earth pulls the box down with 20 N (its weight). The floor pushes the box up with a support force. The child pushes the box right with 40 N. The floor drags the box left with 15 N of friction.
Draw an arrow for each force starting on the box: one down, one up, one right and one left. Make the lengths match the sizes, so the push arrow is the longest of the horizontal pair. Label each in newtons.
Up and down first. The box isn’t rising or sinking, so the support force must match the weight: 20 N up and 20 N down. These balance, so the resultant is zero in the vertical direction.
Now left and right. The forces point in opposite directions, so subtract.
```
40 N right - 15 N left
= 25 N to the right
```
The forces along the line of motion are unbalanced, so the box does not stay at a steady speed. A 25 N resultant to the right means that it speeds up towards the right.

The resultant force is 25 N to the right (the up and down forces cancel out). The box speeds up in the direction of the push. If the child pushed with only 15 N, the forces would balance and the box would not speed up.

Common mistakes and how to help

Confusing mass and weight

In everyday speech we say “it weighs 2 kilograms”, so the two words are used for the same thing.

What helps: Mass is the amount of matter, in kg, and stays the same everywhere. Weight is the pull of gravity, in N. On the Moon, where gravity is about one sixth of Earth’s, the same astronaut has the same mass but a much smaller weight.

Believing there is no gravity in space

Pictures of astronauts floating suggest that gravity has switched off.

What helps: Gravity is still very strong at the height of the space station, nearly as strong as at the surface. The astronauts float because they and the station are falling together while moving sideways fast enough to keep missing the ground.

Thinking a bigger object pushes harder in a collision

A lorry hitting a car seems to be the one “doing” the pushing, so the lorry must exert the larger force.

What helps: Forces come in pairs between the two objects, and the two forces are the same size. The car is damaged more because the same force makes a lighter, flimsier object change its motion more.

Thinking you can only turn something by pushing harder

Children focus on the size of the force and overlook where it is applied.

What helps: Try pushing a door open with one finger at the handle, and then near the hinge. The same effort turns it easily in one place and hardly at all in the other. The distance from the pivot matters as much as the force.

How to help at home

Force detectives. Pick objects around the home, such as a book on a table, a hanging coat and a pushed chair, and have your child draw all the force arrows with labels. Ask for both sides of each pair: the floor on the chair, and the chair on the floor.
Kitchen scale conversion. Weigh a few items on kitchen scales in grams or kilograms and convert to weights in newtons, using roughly 10 N for each kilogram. A 500 g bag of flour weighs about 5 N. Ask what the same bag would weigh on the Moon.
Balloon static. Rub an inflated balloon on dry hair or a woolly jumper and hold it near tiny pieces of tissue paper, or put it against a wall. Ask your child to explain what is happening in terms of forces acting across a gap.
Ruler seesaw. Balance a 30 cm ruler on a pencil at its centre. Put one coin on one side and find where two stacked coins on the other side balance it. Notice that the heavier stack is nearer the pivot.
Door experiment. Compare how easy it is to open a door by pushing at the handle with one finger against pushing close to the hinge. Ask which position gives the bigger turning effect, and why a spanner or wheel brace has a long handle.

Leads on to

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