Newton's laws and force as rate of change of momentum
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 1 worked example, 4 practice questions, 3 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Catching a cricket ball
- Newton's second law
- Using F = ma
Read the transcript of the teaching part
Last time
Before we start: last time, in Momentum and its conservation, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Catching a cricket ball
A fielder catches a fast cricket ball, and pulls his hands back as it lands. Why? The ball must lose all its momentum either way, but pulling back takes longer. A longer time means a smaller force. Force is linked to how fast momentum changes.
Newton's second law
Newton's second law: the resultant force on an object is proportional to the rate of change of its momentum, and acts in the direction of the change. With the unit newton defined properly, the constant is one, so F equals Δp over Δt. For constant mass, Δp is m Δv, so F equals m a. One newton is the force that gives a mass of one kilogram an acceleration of one metre per second squared.
Using F = ma
F in F equals m a is always the resultant force: add up all the forces, with signs. A twelve hundred kilogram car has engine thrust of three thousand newtons and drag of six hundred newtons. The resultant is three thousand minus six hundred, two thousand four hundred newtons. The acceleration is two thousand four hundred over twelve hundred: two metres per second squared.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Force as rate of change of momentum · Newton's second law · Newton's third law · Force as rate of change of momentum.
Chapter 3.1: Dynamics
- Momentum and its conservation 5 min
- Newton's laws and force as rate of change of momentum 5 min
Also for A Level Physics 9702
Which topics come up most in A Level Physics 9702 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency.