OCR GCSE Physics: Forces — Revision Notes

Condensed for the final weeks. For the full explanation, use the Forces study guide.

Motion (P2.1)

Equation	Use
distance = speed × time	Basic motion
acceleration = change in velocity ÷ time	Rate of change of speed
v² − u² = 2 × acceleration × distance	No time given
kinetic energy = ½ × mass × speed²	Energy of a moving object

Vector vs. scalar: displacement/velocity are vectors (direction matters); distance/speed are scalars (direction doesn’t matter). The specification flags a common misconception directly: assuming velocity must be positive, and struggling to associate a reversal in direction with a change in sign.

Graphs: distance-time graph slope = speed; velocity-time graph slope = acceleration, and the area under a velocity-time graph = distance travelled.

Newton’s laws (P2.2)

Law	States
First	An object continues at constant velocity unless a resultant force acts on it
Second	Force = mass × acceleration (F = ma)
Third	Every action has an equal and opposite reaction

Balanced vs. unbalanced forces: balanced → constant velocity (including zero); unbalanced → acceleration. Terminal velocity — reached when driving force = resistive force (e.g. air resistance), so resultant force = 0.

Momentum = mass × velocity — conserved in collisions (unlike kinetic energy, which often is not). Don’t confuse the two: momentum is a vector, KE is a scalar.

Work, energy and power:

Work done = force x distance moved
Power = rate of energy transfer (work done / time)

Circular motion: an object moving in a circle at constant speed still has changing velocity, because direction is constantly changing — this is why it has acceleration (centripetal) despite constant speed.

Forces in action (P2.3)

Concept	Key point
Elastic deformation	Object returns to original shape once force removed
Plastic deformation	Permanent change of shape
Spring force-extension	Linear within limit of proportionality; F = ke
Elastic PE stored	E = ½ke²
Weight	W = mg (depends on gravitational field strength g)
Moment of a force	M = force × perpendicular distance from pivot
Pressure in fluids	Force = pressure × area (hydraulics)

Mass ≠ weight. Mass (kg) is constant everywhere; weight (N) depends on gravitational field strength and changes between planets — a favourite exam context.

Worked example: Newton’s second law

A 2 kg trolley is pushed with a resultant force of 6 N.

a = F / m = 6 / 2 = 3 m/s^2

This underlies the Practical Activity Group (PAG) investigation of the force-acceleration link using light gates, weights and trolleys.

Worked example: moments

A 5 N force is applied 0.4 m from a pivot, perpendicular to the lever.

Moment = force x perpendicular distance = 5 x 0.4 = 2 N.m

If the force is NOT perpendicular to the lever, only the perpendicular component counts — a common source of lost marks is using the straight-line distance to the pivot instead of the perpendicular distance.

Worked example: interpreting a velocity-time graph

A velocity-time graph shows an object accelerating uniformly from 0 to 20 m/s over 4 seconds, then travelling at constant velocity for 6 seconds.

Phase 1 (0-4 s): acceleration = gradient = (20 - 0) / 4 = 5 m/s^2
                 distance = area of triangle = 1/2 x 4 x 20 = 40 m

Phase 2 (4-10 s): acceleration = 0 (flat line, constant velocity)
                  distance = area of rectangle = 6 x 20 = 120 m

Total distance travelled = 40 + 120 = 160 m

Splitting the graph into geometric shapes (triangle, rectangle, trapezium) and calculating the area of each separately, rather than trying to estimate the total area in one step, is the reliable technique for any velocity-time graph question – and works whether the graph shows constant acceleration, constant velocity, or a combination of both.

How P2 connects to the rest of the course

Topic P1 (Matter) established the particle model, changes of state and pressure – ideas that reappear directly in P2.3, where gas and fluid pressure resurfaces in the context of hydraulic systems. Forces itself introduces the vector/scalar distinction that recurs throughout the rest of the course, particularly in Topic P4 (Magnetism) and Topic P5 (Waves), where direction matters just as much as magnitude – treat the vector/scalar distinction learned here as a tool you will keep reusing, not content specific to Forces alone.

Key terms

Resultant force — the single overall force equivalent to all forces acting on an object. Terminal velocity — constant velocity reached when driving and resistive forces balance. Momentum — mass × velocity; conserved in collisions. Moment — force × perpendicular distance from the pivot. Inertia/inertial mass — an object’s resistance to a change in its motion.

Common mistakes

• Treating a stationary object as having no forces acting on it — weight and normal contact force are balanced, not absent, giving zero resultant force.
• Confusing mass (constant, kg) with weight (varies with g, N).
• Confusing momentum (vector, always conserved) with kinetic energy (scalar, often not conserved).
• Forgetting Newton’s third law pairs act on different objects — they can never cancel each other out.
• Using straight-line distance instead of perpendicular distance in a moments calculation.

Quick self-test

• Calculate the acceleration of a 5 kg object under a resultant force of 15 N.
• Explain why an object moving in a circle at constant speed still accelerates.
• State Newton’s third law and explain why its two forces never produce equilibrium on their own.
• Calculate the elastic potential energy stored in a spring with k = 40 N/m extended by 0.2 m.
• Explain why a parked car has zero resultant force despite gravity acting on it.

Related resources

• Forces study guide
• 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.