Newton's laws, velocity-time graphs, stopping distances, momentum and resultant forces — all with worked examples.
Forces and Motion is the backbone of GCSE Physics. It covers the largest number of equations of any topic and generates the widest variety of question types — from drawing free body diagrams to calculating momentum changes to interpreting motion graphs. This guide works through every skill systematically, starting from the fundamental concepts and building to the harder Higher tier material.
This distinction comes up in exam questions regularly and students often get it backwards. A scalar quantity has magnitude (size) only. A vector quantity has both magnitude and direction.
Speed and velocity are often confused. Speed is scalar — it tells you how fast something is moving. Velocity is vector — it tells you how fast and in which direction. An object moving in a circle at constant speed has changing velocity (because direction is constantly changing) and therefore is accelerating.
On Earth, g = 9.8 N/kg (sometimes rounded to 10 N/kg in calculations). The gravitational field strength is the force per unit mass — so a 70 kg person has a weight of 70 × 9.8 = 686 N.
An object remains at rest or moves with constant velocity unless acted on by a resultant force. This means: if the resultant force on an object is zero, it either stays still or continues moving at the same speed in the same direction. A car on a motorway at constant speed has zero resultant force — driving force equals friction and air resistance.
The resultant force on an object equals its mass times its acceleration: F = ma. A larger force produces a larger acceleration. A larger mass produces a smaller acceleration for the same force. The acceleration is always in the same direction as the resultant force.
For every action there is an equal and opposite reaction. When object A exerts a force on object B, object B exerts an equal and opposite force on object A. These forces act on different objects — they are not a balanced pair acting on the same object. A book on a table: the book pushes down on the table (gravity) and the table pushes up on the book (normal reaction force). These are a Newton's Third Law pair.
❌ Newton's Third Law pairs must act on different objects. "Weight and normal reaction force on a stationary book" is NOT a Newton's Third Law pair — they both act on the book. The Newton's Third Law pair to weight is the gravitational pull the book exerts on the Earth.
A resultant force is the single force that has the same effect as all the forces acting on an object combined. To find the resultant, add forces in the same direction and subtract opposing forces.
Free body diagrams show all forces acting on a single object as arrows. Arrow length represents magnitude, arrow direction represents direction. For equilibrium (zero resultant), all arrows balance — forces in each direction cancel out.
At Higher tier, you may need to resolve forces or find a resultant using a scale drawing when forces are not along the same line.
Motion graphs are tested on almost every GCSE Physics paper. See the dedicated article on GCSE Motion Graphs for full detail. Key points:
Stopping distance = thinking distance + braking distance.
Thinking distance is the distance travelled during the driver's reaction time — before brakes are applied. It depends on: reaction time (affected by tiredness, alcohol, drugs, distractions) and speed.
Braking distance is the distance travelled once brakes are applied. It depends on: speed (doubling speed quadruples braking distance, because KE = ½mv²), road conditions (wet/icy increases braking distance), tyre condition, brake condition, mass of vehicle.
Braking distance is proportional to speed squared — not speed. This is because kinetic energy (which the brakes must dissipate) is proportional to v². At twice the speed, braking distance is four times as long. This non-linear relationship is a common exam question.
Momentum (p) = mass (m) × velocity (v). Units: kg m/s. Momentum is a vector — it has direction.
The law of conservation of momentum states that in a closed system (no external forces), total momentum before a collision equals total momentum after.
Force is also related to momentum: F = Δp/Δt (force = rate of change of momentum). This is actually the more precise statement of Newton's Second Law. It explains why crumple zones in cars reduce injury — by increasing the time over which momentum changes, they reduce the force experienced by passengers.
When an object falls through a fluid (air, water), drag forces increase with speed. Initially, weight exceeds drag — the object accelerates. As speed increases, drag increases until it equals weight — resultant force is zero and the object reaches terminal velocity, falling at constant speed.
On a velocity-time graph, terminal velocity appears as the curve levelling off to a horizontal line. The acceleration at any point on the graph equals the gradient at that point — so as the curve flattens, acceleration decreases toward zero.
The AQA Forces specification is at the AQA GCSE Physics specification page.
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