AQA GCSE Physics (8463) · Paper 2

🏎️ Forces

Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.

Forces, weight & springs

A scalar quantity has size only, such as speed, distance, mass, energy or time. A vector quantity has both size and direction, such as velocity, displacement, force or acceleration. A force is a push or pull. Contact forces need the objects to be touching, such as friction, air resistance and tension, while non-contact forces act at a distance, such as gravity, magnetic forces and electrostatic forces.

Mass vs weightMass is the amount of matter in an object, measured in kilograms, and it is the same wherever the object is. Weight is the force of gravity acting on that mass, given by W = m g, measured in newtons. On the Moon an object's weight would be smaller, because gravity there is weaker, but its mass would stay the same.

Centre of massThe centre of mass is the single point in an object where its whole weight can be thought of as acting; it is also the object's balance point.

Resultant forceA resultant force is the single force that has the same effect as all the forces acting on an object combined; it is found by adding the forces together, taking their directions into account.

work done W = F s (force × distance moved along the force) — 1 joule = 1 newton metre

Hooke's law states that a spring's extension is proportional to the force applied to it, following the equation F = k e, up to the limit of proportionality. Beyond that point the spring deforms inelastically, meaning the deformation is permanent. The energy stored in a stretched or compressed spring is given by Ee = ½ k e².

🧪 Required practical: In the required practical on springs, a spring is hung up and masses are added one at a time, with the extension read against a ruler using a fiducial marker at eye level. Plotting force against extension gives a straight line through the origin, and the gradient equals k; the line curves once the spring is stretched past its limit of proportionality.

Motion & graphs

speed v = distance ÷ time; acceleration a = Δv ÷ t; v² − u² = 2 a s

Some typical speeds are worth knowing. Walking is around 1.5 m/s, running is around 3 m/s, cycling is around 6 m/s, cars travel between 13 and 30 m/s, and sound travels at about 330 m/s in air.

On a distance–time graph, the gradient shows the speed: a flat section means the object is stationary, and a curve means the speed is changing. On a velocity–time graph, the gradient shows the acceleration, and the area under the line gives the distance travelled. These two graphs are easy to confuse, and mixing them up is one of the most common errors in physics.

As a skydiver falls, their speed increases, so air resistance grows with it. Eventually air resistance becomes equal to their weight, the resultant force falls to zero, the acceleration stops, and the skydiver reaches a constant terminal velocity. Opening the parachute suddenly increases air resistance by a large amount, causing a rapid slow-down to a new, lower terminal velocity.

Newton's laws

First lawIf the resultant force on an object is zero, a stationary object stays stationary, and a moving object continues at a constant velocity.

Second lawF = m a: the resultant force on an object equals its mass multiplied by its acceleration. For the same force, doubling the mass halves the acceleration.

Third lawWhen two objects interact, they exert equal and opposite forces on each other. Because these forces act on two different objects, they never cancel out.

Higher tier only

Higher content: the tendency of an object to keep doing what it is already doing is called inertia. Inertial mass measures how hard it is to change an object's velocity and is defined by m = F ÷ a.

🧪 Required practical: In the required practical on acceleration, a trolley on a track is pulled by hanging masses over a pulley, and its motion is timed using light gates. Moving masses from the trolley to the hanger changes the force while keeping the total mass constant, which shows that a ∝ F. Piling mass onto the trolley while keeping the force constant shows that a ∝ 1/m.

Stopping & momentum

stopping distance = thinking distance + braking distance

Thinking distance is the distance travelled during the driver's reaction time, typically 0.2–0.9 seconds, and it increases with speed, tiredness, alcohol and distraction. Braking distance increases with speed, wet or icy roads, and worn tyres or brakes. Braking converts kinetic energy into heat in the brakes, and very large decelerations can overheat the brakes or cause the car to skid.

Higher tier only

At Higher tier, momentum (p = m v) is conserved in a closed system, meaning the total momentum before a collision or explosion equals the total momentum after it. Safety gear such as crumple zones, seat belts and airbags works by stretching the time over which the momentum changes, following F = Δp ÷ t. A longer time therefore means a smaller force is needed to bring about the same change in momentum, reducing the force on the people inside the car.

Moments & pressure (Triple) Separate science only

moment M = F d (force × perpendicular distance from the pivot)

For a balanced object, the total clockwise moment equals the total anticlockwise moment, which is the principle of moments. Levers and gears change the balance between force and distance: a long spanner increases the turning effect produced by a given push, and a large gear turning a smaller one increases speed rather than force.

pressure p = F ÷ A, measured in pascals: for the same force, a smaller area gives a bigger pressure, as with drawing pins and knife edges, and a larger area gives a smaller pressure, as with snowshoes

In a liquid, pressure rises with depth, because there is more liquid above pushing down. Liquid pressure acts in all directions. The atmosphere works the same way: at higher altitude there is less air above, so the pressure is lower.

Higher tier only

Higher content: the pressure at depth h in a liquid of density ρ is p = h ρ g. A submerged object experiences more pressure on its lower surface than on its upper surface, and this difference produces an upward force called upthrust, equal to the weight of fluid displaced. An object floats if upthrust can equal its weight: an object less dense than the fluid floats, and one denser than the fluid sinks.

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