Forces & Motion is a core part of GCSE Physics. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
ForceA push or a pull, measured in newtons (N)
NewtonThe SI unit of force, symbol N
WeightThe pull of gravity on a mass, in newtons
MassThe amount of matter in an object, in kilograms
Resultant forceThe single force equal to all forces combined
FrictionA contact force that opposes motion
Hooke's lawF = k × e — force is proportional to extension
Spring constant (k)Stiffness of a spring, in N/m
ExtensionHow much a spring stretches from natural length
Elastic PEEnergy stored in a stretched spring: ½ k e²
Terminal velocitySteady falling speed when forces are balanced
Limit of proportionalityPoint where Hooke's law stops working
W = mgWeight equals mass times gravitational field strength
F = maResultant force equals mass times acceleration
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
If an object is moving at constant velocity in a straight line, what is the net force on it?✗ There must be a forward force on the object to keep it moving. ✓ Zero — by Newton's first law, constant velocity requires zero net force.
What is the difference between mass and weight?✗ Mass and weight are the same thing, just expressed in different units. ✓ Mass is the amount of matter (kg); weight is the gravitational force on that mass (N) and depends on g.
If air resistance is ignored, which falls faster: a heavy ball or a light ball dropped from the same height?✗ Heavier objects fall faster than lighter objects because gravity pulls more on them. ✓ Both fall at the same rate because gravitational acceleration is independent of mass.
When you push a wall, what does Newton's third law say about the wall?✗ The wall only pushes back if your force is strong enough to make it react. ✓ The wall exerts an equal and opposite force on you at the same instant.
A car is slowing down as it brakes. What is true about its acceleration?✗ It has no acceleration because it is not speeding up. ✓ It is accelerating — acceleration is any change in velocity, here negative.
Practise Forces & Motion — free games
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