How do forces have a turning effect?

Moments

A moment is a turning force or torque
We may give the direction of a force. We may give the sense of a moment about a pivot. A moment may be clockwise or anti-clockwise.

The size of a moment depends on

  a) the size of the force producing it and

b) the perpendicular distance between the line of action of the force and the axis of rotation.

Moment (Nm) = Force (N) x perpendicular distance (m)

The moment produced by the spanner above = 60 x 0.2 = 12Nm anti-clockwise

Notice on the second diagram that if the force acts at an angle then the perpendicular distance must be used.

Centre of Mass

If you split an object up into lots of pieces then each piece has its own weight. When all the pieces are together to make a single object then all these little forces acting together make a single big force, the total weight. The centre of mass is the point from which this single force acts from.  If the object has a line of symmetry then the centre of mass is along this line, e.g. the centre of mass of a circle is at its centre.

If a body is suspended then its centre of mass is directly below the point of suspension. If it wasn’t then there would be a turning force and the object would not be balanced.

We can use this fact to find the centre of mass of an irregularly shaped object

Balance

The objects below are balanced. Why?

 It is because the clockwise and anti-clockwise moments about the pivot, in fact about any point, are balanced.

Stability

An object will topple over if the line of action of is weight lies outside its base.

The object on the left in the diagram above is stable but not very. If it were tilted by just a few degrees then it would topple over.

The bottle on the left in the diagram above is the most stable. Because its centre of mass is very low you would have to tilt it by quite a big angle before the line of action of its weight was outside its base.

The clown toy will never fall over because its centre of mass is very low (it has something heavy inside it very low down).

An object is more stable if

a)      It has a large base

b)      It has a low centre of mass

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Revision Questions

1. A man of weight 700N stands on the end of a diving board 4m long. What moment does this produce about where the board is supported?

2. What is meant by the centre of mass of an object?

3. Where is the centre of mass of a football?

4. Describe how you could find the centre of mass of an irregular thin piece of wood with uniform thickness

5. Jim's has a weight of 500N. He sits on one end of a see saw 2m from the middle
a) what moment would he produce about the middle?
b) how far from the middle should Mary sit on the other side, whose weight is 350N, so that the see saw balances?

6. If you lean forward without support eventually you will fall over. Explain why.

7. Why are Formula 1 racing cars very stable?