Forces and motion · GCSE Physics
Forces
Contact and non-contact forces, resultant force, Hooke’s law, weight, Newton’s laws, moments and stopping distance.
A resultant force changes speed or direction. No resultant force means stationary or steady speed in a straight line — that is Newton’s first law, not “no forces”.
The important bits
What you need to know
- 1
Scalar quantities have magnitude only (mass, speed, time, energy). Vector quantities have magnitude and direction (force, velocity, acceleration, displacement, momentum).
- 2
Contact forces need touching (friction, tension, normal contact, air resistance). Non-contact forces act at a distance (gravity, electrostatic, magnetic).
- 3
Weight W = mg is the gravitational force on a mass. It acts at the centre of mass, towards the Earth. Mass is not weight; mass is in kilograms, weight in newtons.
- 4
A free-body diagram shows forces as arrows on one object. The resultant is the vector sum. If the resultant is zero, the object is in equilibrium.
- 5
Hooke’s law: F = ke, up to the limit of proportionality. The gradient of a force–extension graph is k, the spring constant. Beyond the limit the spring is permanently deformed.
- 6
Newton 1: resultant force zero → constant velocity (which may be zero). Newton 2: F = ma (resultant force). Newton 3: equal and opposite forces on two different objects.
- 7
Stopping distance = thinking distance + braking distance. Thinking distance rises with speed and with distraction, alcohol, drugs or tiredness. Braking distance rises with speed, poor brakes, worn tyres and icy or wet roads. Kinetic energy is transferred thermally by friction in the brakes.
Go deeper
Newton’s first law is about resultant force, not about “no forces”
A skydiver at terminal velocity has large forces: weight down and drag up. They cancel, so velocity is constant. A car at 70 mph on a motorway is the same story: driving force matches drag and friction. Students write “no forces act” and lose the mark. Draw the arrows, compare lengths, then state the motion. Newton’s third law is the other trap: the pair act on different objects. Your feet push the Earth down; the Earth pushes you up. They do not cancel on the same diagram because they do not both act on you. The upward force that can cancel your weight is the normal contact force from the floor, which is a different pair.
Go deeper
Stopping distance is two different physics stories
Thinking distance is how far the car travels during reaction time. At roughly constant speed, it is speed × time, so it doubles if speed doubles (for a fixed reaction time). Alcohol and phones stretch the time, not the brakes. Braking distance is about work done: the brakes must transfer the kinetic store, ½mv², so if speed doubles, kinetic energy quadruples, and braking distance roughly quadruples on the same dry road. That is why speed is so expensive in metres. Wet roads cut friction, so the same energy is transferred over a longer distance. In an answer, split thinking and braking, name a factor for each, and if you have numbers, use v².
See the idea in action
A 1200 kg car travels at 20 m/s. The driver has a reaction time of 0.50 s. Thinking distance = 20 × 0.50 = 10 m. Kinetic energy = ½mv² = 0.5 × 1200 × 400 = 240 000 J. If the braking force is 8000 N, braking distance from W = Fs is s = 240 000 / 8000 = 30 m. Stopping distance = 10 + 30 = 40 m. At 40 m/s, thinking distance would be 20 m and kinetic energy four times larger, so braking distance about 120 m if the force stayed 8000 N — a much bigger jump.
Exam technique
Turn knowledge into marks
Label every force arrow and say which object it acts on. Use W = mg with g = 9.8 N/kg (or 10 N/kg if the paper says so). For F = ma, F must be the resultant. Split stopping distance into thinking and braking and match each factor to the correct part.
Common mistakes
Do not give these marks away
- 01
Using mass in grams in F = ma or W = mg, or calling mass “weight”.
- 02
Saying a car at constant speed has no forces, or putting Newton’s third-law pair on the same object.
- 03
Claiming alcohol increases braking distance — it increases thinking distance (reaction time).
A car travels at constant 30 m/s in a straight line. What is the resultant force on it?
AEqual to its weight
BZero
CEqual to the driving force
DImpossible to tell
Show the answer
Zero. Newton’s first law: constant velocity means the resultant force is zero. Driving force and resistive forces still exist; they cancel.
Quick questions
If this is the bit you searched
What is the difference between mass and weight?
Mass is the amount of matter in kilograms. Weight is the gravitational force on that mass, W = mg, in newtons.
What factors affect stopping distance?
Thinking distance: speed and reaction time (alcohol, drugs, distraction, tiredness). Braking distance: speed, condition of brakes and tyres, and road surface.