Forces that cause rotation

1. A force can make an object turn about a fixed point instead of moving the whole object along.
2. The fixed point that an object turns about is called the pivot, or the fulcrum.
3. A force produces a turning effect when its line of action does not pass through the pivot.
4. A force whose line of action passes directly through the pivot has no turning effect, however large it is.
   1. Pushing on a door right at its hinges does not open it, because the force acts through the pivot.
5. The turning effect of a force is larger when the force is larger or acts further from the pivot.
6. The turning effect is greatest when the force acts at right angles to the line joining the point of action to the pivot.

Examples of turning forces

1. A door turns about its hinges, which is why the handle is fitted at the edge furthest from them.
2. A spanner turns a nut about the centre of the nut, and a longer spanner makes a tight nut easier to loosen.
3. A seesaw turns about its central support when children sit on either end.
4. A bicycle pedal turns the crank about the axle, and pushing down when the crank is horizontal gives the largest turning effect.
5. A steering wheel turns about the steering column when you push up on one side and pull down on the other.
6. A tap handle, a screwdriver and a bottle cap are all turned by forces acting some distance from their central axis, which is the pivot.
7. A wheelbarrow turns about the wheel axle when you lift its handles.
8. Scissors and pliers turn about the rivet that joins the two blades or jaws.

Clockwise and anticlockwise turning

1. A turning effect acts in one of two directions about a pivot, clockwise or anticlockwise.
2. To find the direction, imagine the object is free to turn about the pivot and picture which way the force would swing it.
   1. Pushing down on the right-hand end of a horizontal seesaw with a central pivot turns it clockwise.
   2. Pushing down on the left-hand end turns it anticlockwise.
   3. Pushing up on the right-hand end turns it anticlockwise, because reversing the force reverses the direction of turning.
3. When two forces act on an object, one may try to turn it clockwise while the other tries to turn it anticlockwise.
4. The object turns in the direction of the larger turning effect, so the resultant turning effect decides which way it rotates.
5. If the clockwise and anticlockwise turning effects are equal, there is no resultant turning effect, and the object stays balanced.