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Moments (Cambridge O Level Physics)
Revision Note
Moments
- As well as causing objects to speed up, slow down, change direction and deform, forces can also cause objects to rotate
- 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
Pivot Point on a Metal Bar
The force will cause the object to rotate clockwise about the pivot
- 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)
Moment of a Spanner
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
- Some other examples involving moments include:
- Using a crowbar to prize open something
- Turning a tap on or off
- A wheelbarrow
- Scissors
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