OCR GCSE Physics: Forces (J249)

This guide covers Topic P2 Forces, the second of eight content topics in OCR GCSE (9-1) Physics A – Gateway Science (J249), for first assessment 2018. It follows directly from Topic P1 Matter and is assessed on Paper 1 (alongside Topics P1, P3, P4 and P9) at Foundation Tier, and on Paper 3 (alongside the same topics) at Higher Tier – statements marked in bold in the specification are Higher-tier-only, with everything else assessed on both tiers.

Syllabus coverage

OCR GCSE (9-1) PHYSICS A (GATEWAY SCIENCE) J249 – TOPIC P2 FORCES

• P2.1 Motion – measuring distance and time and using them to calculate speed, including from graphs; converting units using ratios and proportional reasoning; the vector-scalar distinction as it applies to displacement/distance and velocity/speed; relating changes in motion to distance-time and velocity-time graphs, including interpreting slopes and the area enclosed under a velocity-time graph; calculating average speed for non-uniform motion; and applying the formulae distance = speed × time, acceleration = change in velocity ÷ time, (final velocity)² − (initial velocity)² = 2 × acceleration × distance, and kinetic energy = ½ × mass × speed².
• P2.2 Newton’s laws – recalling examples of how objects interact (electrostatics, gravity, magnetism, and contact forces including friction); representing forces as vectors using free body diagrams; applying Newton’s first law to explain motion at constant velocity and when speed or direction changes; using vector diagrams to find a resultant force and to show equilibrium; describing examples of balanced and unbalanced forces, including objects reaching terminal velocity; applying Newton’s second law (force = mass × acceleration) in calculations; explaining inertia and inertial mass; defining momentum (mass × velocity) and describing its conservation in collisions; using the relationship between work done, force and distance moved, and describing the resulting energy transfer; calculating stored energy and converting between newton-metres and joules; explaining power as the rate of energy transfer; recalling and applying Newton’s third law; and explaining why an object moving in a circle at constant speed still has a changing velocity.
• P2.3 Forces in action – explaining that more than one force must act to stretch, bend or compress an object; describing the difference between elastic and plastic deformation; describing the relationship between force and extension for a spring, including the difference between linear and non-linear behaviour; calculating a spring constant and the work done in stretching a spring (force = spring constant × extension; energy transferred = ½ × spring constant × extension²); explaining that all matter has a gravitational field and that field strength is far greater for massive objects; defining weight and its relationship to gravitational field strength g (weight = mass × g); recalling and applying the relationship between gravitational potential energy, mass, gravitational field strength and height (gravitational potential energy = mass × g × height); recalling the acceleration in free fall; describing examples where forces cause rotation; defining and calculating the moment of a force, including the principle of moments for balanced objects; explaining how levers and gears transmit rotational effects as force multipliers; and recalling that pressure in fluids causes a net force at right angles to a surface, including the relationship between force, pressure and area used in simple hydraulic systems.

Where Forces sits in the course

Topic P1 Matter established the particle model, changes of state and pressure – ideas revisited directly in P2.3, where gas and fluid pressure reappears in the context of hydraulics. Forces itself introduces the vector/scalar distinction that recurs throughout the rest of the physics course, particularly in Topic P4 (Magnetism) and Topic P5 (Waves), where direction matters just as much as size. The specification flags a common misconception directly: learners often assume velocity must be positive and struggle to associate a reversal in direction with a change in sign, which is exactly why the vector-scalar distinction is introduced formally here rather than assumed from Key Stage 3.

Worked example: Newton’s second law

A trolley of mass 2 kg is pushed with a resultant force of 6 N. Calculate its acceleration.

Using force = mass × acceleration, rearranged to acceleration = force ÷ mass:

acceleration = 6 ÷ 2 = 3 m/s²

This is the calculation underlying the Practical Activity Group (PAG) investigation of the link between force and acceleration using light gates, weights and trolleys – a common context for exam questions that ask you to both calculate a result and evaluate the experimental method used to obtain it.

Common mistakes

• Treating a stationary object as having no forces acting on it. A parked car still has weight acting downward and a normal contact force acting upward; these are balanced, not absent, which is why the resultant force – and therefore the acceleration – is zero.
• Confusing mass and weight. Mass (kg) is constant everywhere; weight (N) depends on gravitational field strength and changes between planets, which is a favourite exam context for testing this distinction.
• Forgetting gravitational potential energy when a height changes. Any object raised through a height h gains gravitational potential energy = mass × g × height; a common exam context links this directly to weight (mass × g), since both share the same mg term.
• Mixing up momentum and kinetic energy. Momentum (mass × velocity, a vector) and kinetic energy (½mv², a scalar) both involve mass and velocity, but they behave completely differently in collisions, since momentum is always conserved and kinetic energy often is not.
• Forgetting that Newton’s third law pairs act on different objects. The two forces in a third-law pair are equal, opposite, and of the same type, but they never act on the same object, so they can never cancel each other out or produce equilibrium on their own.
• Losing marks on moments calculations by ignoring the “normal distance” requirement. The moment of a force is force × distance measured perpendicular to the line of action, not simply the straight-line distance to the pivot.

How to approach it

Because P2.1 and P2.2 build a shared vocabulary of vectors, resultant forces and free body diagrams that P2.3 then applies to springs, gravity and moments, revise them in the order they are taught rather than jumping to the most calculation-heavy parts first. Practise drawing free body diagrams for simple scenarios (a falling object at terminal velocity, a car accelerating, a book resting on a table) until identifying every force acting – and whether the resultant is zero – becomes automatic, since this single skill underlies most of Newton’s-laws exam questions. For the equation-heavy parts of P2.3 (Hooke’s law, moments, hydraulics), practise rearranging each formula for every variable rather than only the form given in the specification, since exam questions frequently ask for the “wrong” variable. Finally, treat the Practical Activity Group (PAG) investigations referenced throughout this topic (investigating the force-acceleration link, and investigating Hooke’s law) as genuine exam content, not just classroom activities – OCR routinely examines the method, the variables controlled, and the reliability of results from practicals like these.

Related resources

• Forces revision notes
• Forces practice questions

Official syllabus

OCR, GCSE (9-1) Physics A (Gateway Science) J249 Specification, version 5.0 (July 2026), Topic P2 Forces, https://www.ocr.org.uk/Images/234600-specification-accredited-gcse-gateway-science-suite-physics-a-j249.pdf, fetched and verified in full 2026-09-02.