CCEA A Level Physics
CCEA A Level Physics 1.3 Principle of Moments Exam Questions
Free CCEA A Level Physics revision for 1.3 Principle of Moments: what you need to know, common mistakes and exam-style questions with answers. Covers moment of a force, centre of gravity, the principle of moments and calculations for beams and levers in equilibrium.
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About CCEA A Level Physics 1.3 Principle of Moments
The principle of moments is the rotational partner to resolving forces in Unit AS 1: Forces, Energy and Electricity. You need to define the moment of a force, locate the centre of gravity of an object and apply the principle of moments to bodies in equilibrium, such as beams on supports, levers, see-saws and booms held by cables. Exam questions usually combine two conditions for equilibrium: no resultant force and no resultant moment. Expect a calculation of an unknown force or distance, often with the weight of a uniform beam acting at its centre, and a definition worth one or two marks.
What you need to know
- Moment of a force Moment = force x perpendicular distance from the pivot to the line of action of the force. Unit N m. If the force is at an angle, use the perpendicular component or the perpendicular distance.
- Centre of gravity The point through which the whole weight of an object appears to act. For a uniform beam it is at the midpoint. It can be found for a flat lamina by hanging it from two points and marking where plumb lines cross.
- Principle of moments For a body in equilibrium, the sum of clockwise moments about any point equals the sum of anticlockwise moments about the same point.
- Conditions for equilibrium A rigid body is in equilibrium when the resultant force is zero and the resultant moment about any point is zero.
- Choosing the pivot Take moments about a point where an unknown force acts, so that force has zero moment and drops out of the equation. Then use the vertical force balance to find the other unknown.
Common mistakes in 1.3 Principle of Moments
- Using the distance along a slanting beam instead of the perpendicular distance to the line of action; resolve or use the perpendicular distance.
- Forgetting the weight of the beam itself; a uniform beam's weight acts at its centre and must be included.
- Stating the principle of moments without saying the body is in equilibrium or that moments are taken about the same point.
- Giving the unit of moment as N/m or J; the unit is N m (although dimensionally the same as the joule, it is not called a joule).
Try three 1.3 Principle of Moments exam-style questions
1. State the principle of moments. (2 marks)
Show the mark scheme answer
For a body in equilibrium; the sum of clockwise moments about a point equals the sum of anticlockwise moments about the same point.
2. A uniform beam 2.0 m long weighing 50 N rests on supports at each end. A load of 200 N is placed 0.50 m from the left end. Calculate the force from each support. (4 marks)
Show the mark scheme answer
Moments about left support: R(right) x 2.0 = 50 x 1.0 + 200 x 0.50; R(right) = 150/2.0 = 75 N; total upward force = 250 N; R(left) = 250 - 75 = 175 N.
3. Explain why a crane carries a large concrete counterweight. (2 marks)
Show the mark scheme answer
The load produces a large moment about the tower; the counterweight provides an opposing moment so the resultant moment is near zero and the crane does not topple.
1.3 Principle of Moments FAQs
What does CCEA A Level Physics 1.3 Principle of Moments cover?
The moment of a force, centre of gravity, the principle of moments and the conditions for a rigid body to be in equilibrium.
Which unit is 1.3 examined in?
It is in Unit AS 1: Forces, Energy and Electricity.
How do I approach a moments calculation?
Draw all forces with their positions, take moments about the point where an unknown force acts, set clockwise equal to anticlockwise, then use upward forces equal downward forces for the remaining unknown.
