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Moments (Edexcel GCSE Physics)
Revision Note
Forces & Rotation
- As well as causing objects to speed up, slow down, change direction and deform, forces can also cause objects to rotate
- A system of forces can also do this
- An example of a rotation caused by a force is on one side of a pivot (a fixed point that the object can rotate around)
- This rotation can be clockwise or anticlockwise
The force will cause the object to rotate clockwise about the pivot
- More examples of rotation caused by a force are:
- A child on a see-saw
- Turning the handle of a spanner
- A door opening and closing
- If two forces act on an object without passing through the same point, then the object can still rotate
- If the forces are equal and opposite, this is known as a couple
The above forces are balanced, but will still cause the object to rotate clockwise as they don’t act through a common point
The Moment of a Force
- A moment is defined as:
The turning effect of a force about a pivot
- The size of a moment is defined by the equation:
M = F × d
- Where:
- M = moment in newton metres (Nm)
- F = force in newtons (N)
- d = perpendicular distance of the force to the pivot in metres (m)
The moment depends on the force and perpendicular distance to the pivot
- This is why, for example, the door handle is placed on the opposite side to the hinge
- This means for a given force, the perpendicular distance from the pivot (the hinge) is larger
- This creates a larger moment (turning effect) to make it easier to open the door
- Opening a door with a handle close to the pivot would be much harder, and would require a lot more force
Examiner Tip
The units of a moment is Newton metres (N m), but can also be Newton centimetres (N cm) ie. where the distance is measured in cm insteadIf the exam question doesn't ask for a specific unit, always convert the distance into metres
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