Forces and motion is the most-examined topic in GCSE and A-Level Physics. You need confident use of Newton's three laws, the equation F = ma, momentum conservation, and (at A-Level) the four SUVAT equations.
Examiners reward students who write the equation, substitute with units, and quote answers to the correct number of significant figures. This topic also overlaps with required practicals on motion, acceleration and Newton's second law.
At GCSE you apply F = ma, weight = mg, and use distance-time and velocity-time graphs (gradient = velocity or acceleration, area = displacement). Stopping distance and the factors that affect thinking and braking distance are routinely tested.
At A-Level you use the four SUVAT equations for constant acceleration, resolve forces on inclined planes, and apply conservation of linear momentum to elastic and inelastic collisions. Questions on impulse (FΔt = Δp) and projectile motion are common.
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Q: State Newton's second law as an equation.
A: F = m × a (resultant force = mass × acceleration).
Q: Define momentum and state its unit.
A: Momentum = mass × velocity; unit kg·m/s.
Q: Why is braking distance proportional to speed squared?
A: Work done by brakes = ½mv²; for fixed braking force, distance ∝ v².
Q: Calculate the force needed to accelerate a 1500 kg car at 3 m/s².
A: F = 1500 × 3 = 4500 N.
Mass (kg) is the amount of matter in an object and stays the same everywhere. Weight (N) is the force of gravity on that mass and varies with gravitational field strength.
Newton's first law — the passenger continues at the original velocity until a force (the seatbelt) acts to decelerate them.
Crumple zones, airbags and seatbelts all extend the time over which momentum changes, reducing the force on occupants (F = Δp/Δt).
No — action-reaction pairs always act on different objects, so they cannot cancel out. Balanced forces act on the same object.
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