State Newton's second law of motion and show that it leads to F = ma. Define the SI unit of force.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · State the law. The rate of change of momentum of a body is directly proportional to the net external force, and takes place in the direction of the force.
2 · Write it as an equation. F ∝ Δp/Δt, so F = k Δp/Δt, where k is a constant.
3 · Use constant mass. Δp = m Δv, so F = k m (Δv/Δt) = k m a.
4 · Fix the unit. Choose the unit so that F = 1 when m = 1 kg and a = 1 m s⁻². Then k = 1 and F = ma.
5 · Define the newton. 1 N is the force that gives a 1 kg mass an acceleration of 1 m s⁻²; 1 N = 1 kg m s⁻².
6 · Real-life link. A heavier or faster ball needs a larger force to stop it in the same time.
7 · Close in one line. So the second law measures a force by the acceleration it produces.
Scoring tip: Write F = k Δp/Δt before you reach F = ma. Showing why k = 1 is where most of the marks lie.
