Moments, Gears and Levers

One or more forces can make an object rotate. A moment is the turning effect of a force. We can calculate moments using the equation:

  • M = Moment of a force in newton-metres (Nm)
  • F = Force in newtons (N)
  • d = Distance in metres (m)

Keep in mind that the distance refers to the perpendicular (90°) distance from the pivot to the line of action of the force.

When an object is balanced:

The total clockwise moment about the pivot = The total anti-clockwise moment about the pivot


Simple levers

There are various types of levers, but what they all have in common is that they transmit the rotational effects of a force. We apply an input force at one point, which creates an output force somewhere else.

Forces in levers

With all levers, the input and output forces are important.

Input and output forces

If the input and output forces are both on the same side of the pivot, they will act in the same direction. In the case of the wheelbarrow, both forces will be acting up.

Whereas, if the input and output forces are on different sides of the pivot, they will act in different directions. When scissors are used, one force will be acting up and the other one down.


With a lever, if the effort is closer to the pivot than the load, the exerted force will be greater. This is why levers are so useful, we can get a large output force with a relatively small input force.


Similar to a lever, gears also transmit turning effects and they are used to multiply a force.

When using gears, we exert force (effort) on the smaller wheel (load). As the small wheel turns, the rotational effect is transmitted to the larger wheel. This is because the teeth of each gear must move in the same direction. In the diagram above, the teeth of both gears move downwards, which means that the gears rotate in opposite directions.

  • The 16 teeth gear rotates clockwise
  • The 24 teeth gear rotates anti-clockwise

There is a mechanical advantage achieved by using a system with two gears of different sizes. When effort is applied to the smaller gear, it translates into a greater force (though over a shorter distance) on the larger gear.