State Newton's second law of motion. Derive F = ma from it and define one newton. Use the law to explain one daily-life example, such as a fielder catching a fast ball.
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 force on it and takes place in the direction of the force.
2 · Set up. A force F changes the velocity of a mass m from u to v in time t. Change in momentum = mv − mu.
3 · Derive.
F ∝ (mv − mu)/t = m(v − u)/t, so F = k m(v − u)/t
(v − u)/t = a, so F = kma
Choose the unit so that k = 1: F = ma
4 · Define the unit. One newton is the force that gives a 1 kg mass an acceleration of 1 m s⁻².
5 · Daily-life use. Pulling the hands back makes the ball stop over a longer time. The rate of change of momentum falls, so the force on the hands is smaller.
6 · Close in one line. So force equals mass times acceleration, and a slower stop means a smaller force.
Scoring tip: Show the step (v − u)/t = a clearly, and say why k = 1. Give the newton with its definition.
