moments, levers and gears

• A force or a system of forces may cause an object to rotate

  (eg, a child on a see-saw, a door opening and closing)
• The turning effect of a force is called the moment of the force.
• The size of the moment is defined by the equation:

  moment of a force = force × distance

  M = F d

  moment of a force, M, in newton-metres, Nm

  force, F, in newtons, N

  distance, d, in metres, m
• If an object is balanced

  the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.
• A lever consists of:

  • a pivot
  • an effort
  • a load
• Simple levers and rotation

  As effort is applied to rotate one end about the pivot. The opposite end is also rotated about the pivot in the same direction. This has the effect of rotating or lifting the load.

  Levers make use of moments to act as a force multiplier. They allow a larger force to act upon the load than is supplied by the effort, so it is easier to move large or heavy objects.
• Rotation and transmission of forces by gears

  • If the driven gear is made larger is will rotate more slowly but with a greater moment. For example, a low gear ratio on a bike or car.
  • If the driven gear is made smaller it will rotate more quickly but with a smaller moment. For example, a high gear ratio on a bike or car.