## P15. Turning on a pivot

The moment of a force is the

The ability of a force to make an object turn depends on TWO factors:

**turning effect**that it has. It is measured in newton metres (Nm)The ability of a force to make an object turn depends on TWO factors:

- the
**size of the force**that acts at right-angles to a line through the turning point of the object you wish to turn - the
**perpendicular distance**the force is applied from the turning point.

This explains why less force is needed to open a door by pushing at the side furthest from the hinge than at the side closest to the hinge. To push at the hinge side of the door requires more force to be exerted because the distance is smaller.

A moment can be calculated using this equation:

where:

M = the moment of the force in newton-metres, Nm

F = the force in newtons, N

d = the perpendicular distance from the line of action of the force to the pivot in metres, m

A moment can be calculated using this equation:

**M = F × d**where:

M = the moment of the force in newton-metres, Nm

F = the force in newtons, N

d = the perpendicular distance from the line of action of the force to the pivot in metres, m

## Sample Calculations

## Balancing moments

Where an object is not turning around a pivot, the total clockwise moment must be exactly balanced by the total anti-clockwise moment. We say that the

**opposing**moments are balanced:**sum of the clockwise moments = sum of the anti-clockwise moments**## Levers

A

When someone uses a lever, they exert a force (the effort) around a pivot to move an object (the load).

**lever**is a simple machine that makes work easier to do. Examples of simple levers include cutting with scissors, or lifting the lid on a tin of paint with a screwdriver. Levers**reduce the force needed to**perform these tasks.When someone uses a lever, they exert a force (the effort) around a pivot to move an object (the load).

Levers rely on the principle of moments to act as

**‘force multipliers’**- they reduce the effort needed to move the load by increasing the distance over which it is acting. This means a relatively small effort force has a much greater effect.## Classes of Levers

First-class levers have the fulcrum placed between the load and the effort, as in the seesaw, crowbar, and balance scale. If the two arms of the lever are of equal length, as with the balance scale, the effort must be equal to the load. If the effort arm is longer than the load arm, as in the crowbar, the effort travels farther than the load and is less than the load.Second-class levers have the load between the effort and the fulcrum. A wheelbarrow is a second-class lever. The wheel’s axle is the fulcrum, the handles take the effort, and the load is placed between them. The effort always travels a greater distance and is less than the load.Third-class levers have the effort placed between the load and the fulcrum. The effort always travels a shorter distance and must be greater than the load. A hammer acts as a third-class lever when it is used to drive in a nail: the fulcrum is the wrist, the effort is applied through the hand, and the load is the resistance of the wood. Another example of a third-class lever is the human forearm: the fulcrum is the elbow, the effort is applied by the biceps muscle, and the load is in the hand. |