Moments and equilibrium
Watch the whole lecture free
Make a free account to watch every lecture to the end: 2 worked examples, 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.
That was the teaching part. The rest of this lecture is free with an account: 2 worked examples, 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.
No password needed now: we email you a link to set one. Already have an account? Sign in, then come back to this page.
The teaching part (1 min) is free to watch. A free account opens the whole lecture: 2 worked examples, 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
- Why a long spanner helps
- The moment of a force
- Principle of moments
Read the transcript of the teaching part
Last time
Before we start: last time, in Density and pressure in fluids, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Why a long spanner helps
A wheel nut will not budge. Slip a longer spanner on, and it turns. The force is the same. What changed is the distance from the pivot, and the turning effect of a force depends on both.
The moment of a force
The moment of a force about a point is the force multiplied by the perpendicular distance from the point to the line of action of the force. The unit is newton metre. It is a vector with a sense: clockwise or anticlockwise. The distance must be perpendicular to the line of the force. If the force is at an angle, resolve it, or find the perpendicular distance. A force whose line passes through the pivot has zero moment about it.
Principle of moments
For a body in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about that point. Choose the point that removes unknown forces. A force acting through that point has no moment. The weight of a uniform object acts at its centre. For a uniform rod, that is the middle.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Conditions for equilibrium · Moments · Principle of moments · Principle of moments.
The whole chapter: Forces, Density and Pressure, Forces, Density and Pressure.
Chapter 4.1: Forces
- Moments and equilibrium 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.