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Using Moments - Equilibrium (AQA A Level Maths: Mechanics)
Revision Note
Using Moments - Equilibrium
What happens with moments if a rigid body is in equilibrium?
- If a rigid body is in equilibrium then:
- The resultant force is 0 N
- The resultant moment about any point is 0 N m
- The sum of the magnitudes of the clockwise moments equals the sum of the magnitudes of the anti-clockwise moments
How do I solve problems involving moments and equilibrium?
- Step 1: Identify all forces and perpendicular distances from the pivot
- Step 2: Use the fact that the resultant force is 0 N to form an equation
- Step 3: Use the fact that the resultant moment about any point is 0 N m to form equations
- If there is an unknown force you can choose a pivot that is on the line of action of that force – this will mean that the moment of that force is 0 N m
- Step 4: Solve the equations
Worked example
Examiner Tip
- If you see a force diagram that involves distances then you can be fairly sure that moments will be involved! A clear diagram showing all the distances is key.
- If you pick a pivot and form a moment equation which involves more than one unknown, try choosing another pivot to form simultaneous equations.
- Pick your pivot wisely as unknown and unwanted forces can be eliminated from the equation by choosing the point to be on the line of action of a force.
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