Wooden pulleys. Image credit wisegeek.com

Wooden pulleys. Image credit wisegeek.com

ZIMSEC O Level Combined Science Notes: Machines: Pulleys

  • A pulley is a basic device or machine made of a wheel with a rim that a chain/cord/rope fits around
  • A single fixed pulley as shown below is used to change the direction of effort
Single pulley

Single pulley

  • The pulley makes it easier to lift a load by changing the direction in which an effort is applied to a more convenient direction
  • In this case pulling instead of lifting
  • The single fixed pulley has a velocity ratio of 1
  • It would also have a mechanical advantage of 1
  • However because of friction the effort is always bigger than the load
  • A movable pulley can also be used instead of a fixed pulley
  • A single movable pulley has a velocity ratio of 2
  • This means for example the effort has to move 2m to lift the load 1m
  • However the pulley requires an effort to be applied in an inconvenient direction
Block and tackle pulley.

Block and tackle pulley.

  • If the two pulleys (the movable and fixed pulley) are combined
  • The required effort is reduced and the effort can be applied in a more convenient direction
  • This type of an arrangement is known as a block and tackle

Block and Tackle pulley

Block and Tackle pulley

  • More and more of these pulleys can be arranged in blocks of fixed and movable pulleys
  • Pulleys can also be arranged side by side
  • Each of the fixed pulleys in the system merely changes the direction of the applied force
  • Each of the movable pulleys increases the mechanical advantage of the system by two
  • This is when friction and the weight of the blocks below are not taken into account
  • Each movable pulley also increases the velocity ratio by 2.

To access more topics go to the Combined Science Notes page

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