Moments and the Turning Effect of Forces
Moments for Cambridge IGCSE Physics 0625. The principle of moments, balanced beams, centre of gravity and stability.
What you need to know
- A moment is the turning effect of a force about a pivot.
- The distance you use is the perpendicular distance from the pivot to the line of action of the force.
- A body in equilibrium has no resultant force and no resultant moment.
- The centre of gravity is the single point where the whole weight of the object appears to act.
- An object is more stable if it has a wide base and a low centre of gravity. It topples once the line of action of the weight falls outside the base.
Equations
- moment = force x perpendicular distance from the pivot
- sum of clockwise moments = sum of anticlockwise moments
- moment is measured in newton metres
Moments and the Turning Effect of Forces questions students ask
What is a moment?
The turning effect of a force about a pivot. It equals the force multiplied by the perpendicular distance from the pivot to the line of action of the force.
What is the 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 same point.
Which distance do you use in a moment calculation?
The perpendicular distance from the pivot to the line of action of the force, not the slanted distance along the beam.
What is the centre of gravity?
The single point at which the entire weight of an object appears to act. For a uniform beam it is at the centre.
How do you find the centre of gravity of an irregular shape?
Hang it from a point, hang a plumb line from the same point and draw the vertical line. Repeat from a different point. The centre of gravity is where the lines cross.
What makes an object stable?
A wide base and a low centre of gravity. Both make it harder for the line of action of the weight to fall outside the base.
When does an object topple?
As soon as the vertical line through its centre of gravity falls outside its base of support.
What are the units of a moment?
Newton metres. Giving the answer in newtons is a marked error.
Do you include the weight of the beam?
Only if the question says the beam is not light. If it has mass, its weight acts at its centre of gravity and gives its own moment.
Mistakes that cost marks
- Using the slanted distance instead of the perpendicular distance.
- Forgetting the weight of the beam itself when it is not uniform or not light.
- Taking moments about a point and then forgetting to include a force that also acts there. Its moment is zero, which is exactly why you pick that point.
- Giving the answer in newtons instead of newton metres.
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