Forces is the biggest topic on the course. This mini-lesson walks you through the whole of AQA Topic 4.5 — Forces: scalars and vectors, contact and non-contact forces, work done, elasticity, moments, pressure, motion and momentum.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
A scalar has size only (mass, distance, speed, energy). A vector has size and direction (force, weight, velocity, displacement, acceleration, momentum).
A force is a push or a pull on an object that results from its interaction with another object. Because it has a direction, a force is always a vector — we draw it as an arrow whose length shows the size.
Tip: if a quantity needs the word "north", "up", "to the right" to make sense, it's a vector. If just a number-and-unit is enough, it's a scalar.
Tap whether each quantity is a scalar (size only) or a vector (size + direction).
Some forces only act when objects touch; others act at a distance through a field:
Tap a force, then tap the box it belongs in.
Weight is the force of gravity on an object. It depends on the object's mass and the gravitational field strength, g:
On Earth g ≈ 9.8 N/kg. Weight is a vector that always acts straight down, as though all the object's mass were concentrated at one point — its centre of mass.
Don't confuse mass with weight. Mass (kg) is the amount of matter in an object and never changes. Weight (N) is the force of gravity on that mass, so it changes with location: an astronaut's mass is the same on the Moon, but their weight is about 1/6 of its Earth value because the Moon's g is smaller.
A 5 kg bag of flour (g = 9.8 N/kg).
W = 5 × 9.8 = 49 N
Usually several forces act on an object at once. A free-body diagram shows the object on its own with every force drawn as an arrow from it — the longer the arrow, the bigger the force. The single force with the same effect as all of them together is the resultant force.
Higher tier: a single force can be resolved into two forces (components) acting at right angles — and forces that act at an angle can be added tip-to-tail using a scale drawing to find the resultant.
When a force moves an object along the line of the force, it does work — transferring energy:
One joule is the work done when a force of 1 newton moves an object 1 metre (1 J = 1 N·m). Work done against friction is transferred to the thermal store — the surfaces get warmer.
A 40 N force drags a crate 6 m.
W = 40 × 6 = 240 J
For a spring, the extension is proportional to the force applied, up to the limit of proportionality:
The spring constant k measures stiffness. Bending the spring back into shape afterwards means the deformation was elastic; if it stays bent, it was inelastic.
Elastic potential energy stored in a stretched spring is Ee = ½ k e². Required practical: hang masses on a spring and plot force against extension.
A force can turn an object about a pivot. The turning effect is the moment, and it depends on the force and how far from the pivot it acts:
This is the principle of moments: when balanced, the total clockwise moment equals the total anticlockwise moment. Levers and gears use a long distance to multiply the effect of a force.
Think of a spanner. A long-handled spanner loosens a stiff bolt far more easily than a short one, even with the same push — because the moment is force × distance, so a longer handle (bigger d) gives a bigger turning effect for the same force.
A fluid (liquid or gas) pushes on every surface it touches. The pressure is the force spread over the area:
In a column of liquid the pressure increases with depth, density and gravity:
Floating & sinking: a submerged object feels greater pressure on its bottom than its top, giving an upthrust. If upthrust ≥ weight it floats. Atmospheric pressure falls as you go higher, because there is less air above you pressing down.
Acceleration is how quickly velocity changes:
Higher tier: for uniform acceleration, v² − u² = 2 a s (final² − initial² = 2 × acceleration × distance).
On a distance–time graph the gradient is the speed. On a velocity–time graph the gradient is the acceleration and the area under the line is the distance travelled.
Biggest misconception: a moving object does not need a resultant force to keep moving. By Newton's first law, with zero resultant force it carries on at a constant velocity forever. A force is only needed to change motion (speed up, slow down or turn) — not to maintain it. Things on Earth slow down only because friction and drag provide a resultant force.
Inertia is like a loaded shopping trolley. A full trolley (large mass) is much harder to get moving — and once moving, much harder to stop or steer — than an empty one. More mass means more inertia: a greater resistance to any change in motion.
Action–reaction pairs act on different objects, so they never cancel. Push off a wall on a skateboard: you push the wall (action) and the wall pushes you back (reaction). The two forces are equal and opposite but act on different bodies — the force on you is what sends you rolling away.
A resultant force of 60 N acts on a 4 kg trolley.
a = F ÷ m = 60 ÷ 4 = 15 m/s²
Stopping distance = thinking distance + braking distance. Thinking distance is travelled during the driver's reaction time; braking distance is travelled once the brakes are applied.
A falling object speeds up until the upward air resistance grows to balance its downward weight. With the forces balanced the resultant force is zero, so it stops accelerating and falls at a steady maximum speed — its terminal velocity.
Terminal velocity ≠ no forces. At terminal velocity the resultant force is zero (so acceleration is zero), but weight and air resistance are both still acting — they simply balance. Opening a parachute hugely increases air resistance, so it briefly exceeds weight, the skydiver decelerates, and they settle to a new, slower terminal velocity.
A moving object has momentum — a vector that depends on mass and velocity:
In a closed system (no outside forces), total momentum is conserved: the total momentum before an event equals the total momentum after it.
Higher tier: force equals the rate of change of momentum, F = Δp ÷ Δt. Spreading a collision over a longer time (crumple zones, air bags) reduces the force.
A 0.5 kg ball moves at 4 m/s.
p = 0.5 × 4 = 2 kg·m/s
Weight: W = m g
Work done: W = F s
Hooke's law: F = k e (elastic p.e. Ee = ½ k e²)
Moment: M = F d
Pressure: p = F ÷ A (column: p = h ρ g)
Acceleration: a = Δv ÷ t (HT: v² − u² = 2 a s)
Newton's 2nd law: F = m a
Momentum: p = m v (HT: F = Δp ÷ Δt)
You've covered the whole of AQA 4.5 — scalars & vectors, contact & non-contact forces, work, elasticity, moments, pressure, motion, Newton's laws, stopping distance and momentum. Press Finish to see your score.
You've worked through Forces for AQA GCSE Physics. 🎉
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