Forces

What Forces covers

Forces is examined in Paper 11 (Physics). It covers measuring distance and time, and calculating speed; converting units, and rates; vectors and scalars in motion; distance-time and velocity-time graphs; average speed, and the equations of uniform motion; how objects interact, and forces as vectors; Newton's first law; resolving forces, and free body diagrams; resultant forces, and balanced forces; Newton's second law, and inertia; momentum and its conservation; work done, and stored energy; power, and Newton's third law; circular motion and changing velocity; stretching, bending and compressing: elastic and plastic; force and extension, linear and non-linear; the spring constant, and work done in stretching; gravitational fields, weight and free fall; and calculations with weight and gravitational field strength.

The 19 subtopics

Each one has its own page.

• Measuring distance and time, and calculating speedInstruments for measuring length and time, why light gates beat a hand-held stopwatch, how reaction time is reduced, and finding speed from distance, time and the gradient of a distance-time graph.Open →
• Converting units, and ratesThe standard units of distance and time, converting kilometres per hour to metres per second, minutes to seconds and centimetres to metres, and why units must match in an equation.Open →
• Vectors and scalars in motionThe difference between a scalar and a vector, why distance is a scalar and displacement a vector, how velocity differs from speed, and journeys where the two disagree.Open →
• Distance-time and velocity-time graphsWhat horizontal, sloping and curved lines mean on a distance-time graph, what the gradient and area of a velocity-time graph give, and calculations of acceleration and distance from them.Open →
• Average speed, and the equations of uniform motionHow average speed is found for a changing journey, distance, speed and time calculations, acceleration from a change in velocity, and the equation linking final velocity, starting velocity, acceleration and distance.Open →
• How objects interact, and forces as vectorsThe four ways objects interact and which act at a distance, normal contact force and friction, the equal and opposite pair of forces produced by every interaction, free body force diagrams, and finding the resultant of forces acting along one line.Open →
• Newton's first lawWhat happens to an object with no resultant force on it, what a resultant force can change, and why spacecraft, books, cyclists and sliding loads behave as they do.Open →
• Resolving forces, and free body diagramsResolving a force into two components at right angles, scale drawings and finding the size and direction of a resultant, forces in equilibrium, terminal velocity for a skydiver before and after the parachute opens, and why a car has a top speed that streamlining raises.Open →
• Resultant forces, and balanced forcesFinding the resultant of forces along one line, worked examples for rockets, trolleys, swimmers and lifts, what balanced forces mean, and why a zero resultant does not always mean rest.Open →
• Newton's second law, and inertiaThe relationship between resultant force, mass and acceleration, what one newton means, calculations of force, mass and acceleration, and how inertial mass explains why loaded objects are harder to move.Open →
• Momentum and its conservationHow momentum is calculated from mass and velocity, its unit and why it is a vector, the law of conservation of momentum, and using it for trolleys that join after a collision, a snooker ball struck by another and two skaters pushing apart.Open →
• Work done, and stored energyHow work done is calculated and its unit, why holding a bag still does no work, where energy goes when work is done against friction, and the energy in a moving object's kinetic store.Open →
• Power, and Newton's third lawThe definition and unit of power with calculations for motors, lamps and stairs, the law describing interaction pairs, and why the two forces of a pair never cancel each other.Open →
• Circular motion and changing velocityWhy an object moving in a circle at steady speed has a changing velocity and is accelerating, the direction of the resultant force, and what supplies it for cars, strings and satellites.Open →
• Stretching, bending and compressing; elastic and plasticThe three ways forces change shape and why at least two forces are needed, the forces that stretch a spring, bend a diving board and compress a pillar, elastic and plastic deformation and how to tell them apart, and why an overstretched spring stays longer.Open →
• Force and extension, linear and non-linearExtension and how to calculate it, direct proportion up to the limit of proportionality, the elastic limit, linear and non-linear force-extension graphs, and the spring-stretching investigation with its variables, precautions, results table, hypothesis, length against force graph and conclusion.Open →
• The spring constant, and work done in stretchingWhat the spring constant measures and its unit, force = spring constant × extension with calculations and stiffness comparisons, work done in stretching as stored energy, work done = ½ × spring constant × extension², the area under a force-extension graph, and the effect of doubling the extension.Open →
• Gravitational fields, weight and free fallGravitational fields around every mass and why they only attract, field strength on the Earth and the Moon, weight as a force measured in newtons with a newtonmeter, how mass differs from weight, the centre of mass, and free fall at about 10 m/s² whatever the mass.Open →
• Calculations with weight and gravitational field strengthThe relationship between weight, mass and gravitational field strength, its rearrangements, worked examples on the Earth and a moon, and how weight and mass behave when the field strength changes.Open →

What you need to know

Answer each one before you reveal it.

Take Forces further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

• Distance vs Displacement

  Distance: the total length of the path travelled, with no direction. Displacement: the straight-line separation from start to finish, together with its direction.

• Speed vs Velocity

  Speed: how fast an object moves, with size only. Velocity: how fast an object moves in a stated direction.

• Mass vs Weight

  Mass: the quantity of matter in an object, in kilograms, the same everywhere. Weight: the gravitational force on an object, measured in newtons.

Questions from this topic

Each one with the model answer underneath.

1. 1

   Which instrument would you choose to measure the length of a laboratory bench, and to what precision can it be read?

   Model answer

   A metre rule or a tape measure, read to the nearest millimetre.

2. 2

   How could you measure the distance around a running track without laying a rule end to end along it?

   Model answer

   Use a trundle wheel, which counts complete turns of a wheel of known circumference.

3. 3

   Why is a hand-operated stopwatch unreliable for timing a very short event?

   Model answer

   Human reaction time adds an uncertainty of roughly 0.2 s at the start and again at the stop, which is a large fraction of a short interval.

4. 4

   How do light gates give a more trustworthy time than a stopwatch pressed by hand?

   Model answer

   The timer starts and stops electronically as the object breaks each beam, so no human reaction time is involved.

5. 5

   How would you reduce the effect of reaction time when timing a swinging pendulum?

   Model answer

   Time twenty complete swings and divide the total time by twenty.

6. 6

   Describe how you would measure the distance travelled by a toy car along a bench and the time it takes, using simple equipment.

   Model answer

   Mark a start line and a finish line, measure between them with a metre rule, and time the run between the marks with a stopwatch.

What is inside Levo

Six ways to work the same material.

Forces revision questions

What does Forces cover in OCR GCSE Combined Science Physics?

19 subtopics: measuring distance and time, and calculating speed, converting units, and rates, vectors and scalars in motion, distance-time and velocity-time graphs, average speed, and the equations of uniform motion, how objects interact, and forces as vectors, newton's first law, resolving forces, and free body diagrams, resultant forces, and balanced forces, newton's second law, and inertia, momentum and its conservation, work done, and stored energy, power, and newton's third law, circular motion and changing velocity, stretching, bending and compressing; elastic and plastic, force and extension, linear and non-linear, the spring constant, and work done in stretching, gravitational fields, weight and free fall and calculations with weight and gravitational field strength. Each one has its own page with questions and model answers.

How is Forces organised?

By the OCR J250 specification, subtopic by subtopic, so it matches the order you are taught it in.