A pulley that moves
Movable pulleys.
Cranes and elevators
Cranes and lifts raise very heavy loads, yet the effort that works the rope is much smaller than the load.
What do you think is different about the pulleys they use?
What this lesson covers
The idea
In a movable pulley system the load is attached to the movable pulley; movable pulleys or systems of pulleys can have a mechanical advantage greater than 1.
Cranes and elevators
Cranes and lifts raise very heavy loads, yet the effort that works the rope is much smaller than the load.
What do you think is different about the pulleys they use?
- Some of the pulleys move with the load
- The pulleys are fixed to the ceiling
- They have no rope
A movable pulley
Press Next step through the pictures.
Less effort
With the load on the movable pulley and one end of the rope fixed, the effort needed can be much smaller than the load. That is a mechanical advantage greater than 1. A fixed pulley cannot do this.
In a movable pulley system the load is attached to the movable pulley; one end of the rope is fixed and the other end is free to apply the effort. Movable pulleys or systems of pulleys can have a mechanical advantage greater than 1.
Notes
A movable pulley carries the load; with it the mechanical advantage can be greater than 1, so a smaller effort lifts a heavier load.
Check yourself
In a movable pulley system, the load is attached to…
The mechanical advantage of a movable pulley system can be…
A load of 100 N is lifted with an effort of 40 N using a pulley system (an example). What is the mechanical advantage?
Answer: 2.5
100 N ÷ 40 N = 2.5, which is greater than 1.
Where are pulleys widely used?
- the fixed end of the rope only. In a movable pulley system the load is attached to the movable pulley.
- the movable pulley — correct. Yes!
- the person's hand. The person applies the effort at the free end of the rope. The load is attached to the movable pulley.
- exactly 1, like a fixed pulley. A fixed pulley has a mechanical advantage of 1. Movable pulleys can have a mechanical advantage greater than 1.
- less than 0. A mechanical advantage is a ratio of two forces, so it is not negative. For movable pulleys it can be greater than 1.
- greater than 1 — correct. Yes!
- In elevators and cranes — correct. Yes!
- In thermometers. A thermometer does not use pulleys. Pulleys are used in elevators and cranes.
- In mirrors. Mirrors do not use pulleys. Pulleys are used in elevators and cranes.