Moments & LeversOxford AQA IGCSE Physics: Revision notes
Section 1
What is a moment?
A moment is the turning effect of a force about a pivot. It is calculated using:
M = F x d
where M is the moment (Nm), F is the turning force (N), and d is the perpendicular distance from the pivot to the line of action of the force (m).
The further the force is applied from the pivot, or the greater the force, the larger the turning effect.
The distance used must be the perpendicular distance from the pivot to the line of action of the force, not simply the length along the object, if the force is not applied at a right angle.
Section 2
What is the principle of moments?
The principle of moments states that for an object in balance (equilibrium), the total clockwise moment about any pivot equals the total anticlockwise moment about that same pivot.
This principle can be used to calculate an unknown force or an unknown distance from the pivot, given that the object is balanced and the other moments are known.
A 4 N weight sits 3 m to the left of a pivot on a seesaw (clockwise moment about the pivot, if we take left as producing a clockwise turn on that side, is 4 x 3 = 12 Nm). To balance it, a 6 N weight must sit at a distance d on the other side such that 6 x d = 12 Nm, so d = 2 m.
Section 3
What happens when an object's weight acts outside its base?
If the line of action of an object's weight falls outside its base, there is a resultant (unbalanced) moment about the edge of the base, and the object topples over.
This is why the position of the centre of mass relative to the base is critical for an object's stability.
Section 4
How do levers act as force multipliers?
A simple lever is a rigid object that turns about a pivot. Levers act as force multipliers: by applying a force (effort) at a greater distance from the pivot than the load, a smaller effort force can move a larger load force, because the moments on each side must balance.
A crowbar lets a small hand-applied force at the long end lift a heavy object at the short end near the pivot — the longer distance from the pivot 'multiplies' the effect of the applied force.
Must Know
- M = F x d, where d is the perpendicular distance from the pivot
- Principle of moments: for a balanced object, total clockwise moment = total anticlockwise moment about any pivot
- The principle of moments can be used to find an unknown force or distance
- If the line of action of an object's weight falls outside its base, there is a resultant moment and the object topples
- Simple levers act as force multipliers, using distance from the pivot to amplify an applied force
That's the notes covered.
Carry on to the next subtopic.