Forces and Motion
A force is a push or a pull. The hard part is that balanced forces do not mean stopped, they mean not changing, and almost every wrong answer in this topic comes from that one confusion.
What a force is
A force is a push or a pull, and it is measured in newtons.
You cannot see a force. You can only see what it does, which is why the whole topic is taught with arrows: the arrow is standing in for something invisible. Every force has a direction and a size, and a diagram that shows both is doing the job that a sentence cannot.
Forces do three things, and it is worth being able to name all three rather than only the first. They can make something start moving or speed up, they can slow it down or stop it, and they can change its shape or its direction. Squashing a ball is as much a force at work as throwing it.
Reading a force diagram
The arrow carries two things at once
Which way it points is the direction the force acts.
How long it is drawn is how big the force is.
So two arrows of the same length pointing opposite ways are two equal forces cancelling out, and you can see that without reading a single number.
Balanced forces
Forces are balanced when they cancel out. The book above is the easy case, because it is sitting still and looks like it. Here is the case that catches people.
The parachutist is going down at a steady speed and the forces on them are balanced. Both statements are true at once, and a child who believes balanced means stopped will get this question wrong every time it appears.
What balanced actually means
Balanced forces mean the motion is not changing.
Something still stays still. Something moving carries on at the same speed in the same direction. Neither of those is more balanced than the other.
A car on a motorway at a steady 100 kilometres an hour has balanced forces. The engine is pushing it forward exactly as hard as air resistance and friction are pushing it back. Take your foot off the accelerator and the forces become unbalanced, which is why it slows.
Unbalanced forces
When one direction wins, the motion changes. The object speeds up, slows down, or changes direction, and it does so in the direction of the bigger force.
A sledge is pulled forwards with 40 N. Friction pushes back with 25 N. What happens?
- The two forces act along the same line in opposite directions, so take one from the other.
- 40 - 25 = 15.
- There are 15 N left over, and they act forwards, because forwards was the bigger one.
- Unbalanced forces mean the motion changes.
A resultant force of 15 N forwards, so the sledge speeds up
Common mistakes
1. Thinking balanced means not moving
The single biggest one, and it is an entirely reasonable guess from the word. Balanced means the motion is not changing, which includes a lorry doing a steady seventy. If a question says "constant speed", it is telling you the forces are balanced.
2. Believing a moving thing needs a forward force to keep going
It feels obviously true, because everything on Earth does slow down when you stop pushing. That is friction doing the slowing, not the absence of a push. In space, with nothing to rub against, a thrown object keeps going forever.
3. Drawing every arrow the same length
If the arrows are all one size, the diagram has thrown away the information it exists to carry. A 30 N arrow must be visibly longer than a 10 N one, and marks are given for that in exams.
Practice questions
Seven questions. One attempt each, and the explanation appears once the answer is in.
Check yourself
No sign up. Nothing is sent anywhere, the score stays on this device.
What to learn next
Friction turns up in almost every force question as the thing pushing back, so it is worth meeting properly rather than as a word that appears on arrows.
For parents
In the car, ask what pushes it forward and what pushes it back, then ask what happens to those two when you stop accelerating. It is a two-minute conversation that covers most of what this topic is assessed on, and it uses something your child is already looking at.
Still stuck on this?
Send us the question and a teacher will answer it. No charge, and you do not have to be a student here.
What to learn next
Everything in the Learning Hub
Every topic is free and every one ends with practice questions. Start anywhere.
Fractions
Math · 9 topics
What Is a Fraction?Equivalent FractionsSimplifying FractionsComparing FractionsAdding and Subtracting Fractions With the Same DenominatorAdding Fractions With Different DenominatorsFinding a Fraction of a NumberMixed Numbers and Improper FractionsFractions to DecimalsDecimals and percentages
Math · 7 topics
What Are Decimals?Comparing and Ordering DecimalsAdding and Subtracting DecimalsRounding DecimalsWhat Are Percentages?Finding a Percentage of a NumberFractions, Decimals and PercentagesScience: matter and the water cycle
Science · 5 topics
What Is Matter?Solids, Liquids and GasesChanging StateThe Water CycleMixtures and SolutionsWhole numbers
Math · 10 topics
Place Value to 1,000,000Rounding Whole NumbersAdding and Subtracting With RegroupingTimes Tables That Actually StickMultiplying by 10, 100 and 1000Long Multiplication, Step by StepShort DivisionLong Division, Step by StepFactors, Multiples and PrimesOrder of OperationsForces and energy
Science · 6 topics
Forces and MotionFrictionGravity, Mass and WeightLevers and Simple MachinesEnergy and Its FormsSimple CircuitsStuck on this topic? Get one class, free.
A tutor works through it live with your child, one on one, and sends you a short written report afterwards. No card needed.