Pulleys

Definition

A pulley is a grooved wheel with a rope, chain, or cable running along the groove.

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• A pulley is said to be a fixed pulley if it does not rise or fall with the load being moved. A fixed pulley changes the direction of a force; however, it does not create a mechanical advantage.

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Fixed Pulleys

A fixed pulley is similar to a 1st class lever!The wheel is the fulcrum and the rope on both

sides is the effort and resistance!A fixed pulley is attached to something that

doesn’t move, like the ceiling or wall– Ex: Elevator

Fixed pulleys do NOT increase force. They only change direction. Small IMA!

Fixed Pulleys

Pulleys

• A moveable pulley rises and falls with the load that is being moved. A single moveable pulley creates a mechanical advantage; however, it does not change the direction of a force.

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Movable Pulleys

A movable pulley is one in which one end of the rope is fixed and the wheel is free to move.

Movable pulleys can give you a MA!

Movable Pulleys

Block and Tackle Pulleys

Block and tackle pulleys are a combination of fixed and movable pulleys.

They can have a large MA!

Block and Tackle Pulleys

Pulleys• A pulley can be used to

simply change the direction of a force or to gain a mechanical advantage, depending on how the pulley is arranged.

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MA of Pulleys

Here are a couple of simple rules toremember for finding the MA of pulleys.

1) Count the rope segments.2) If force is pulling down, subtract 1.3) If force is pulling up, it stays the same.

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Pulleys

• If you are going to suspend the weight in the air then you have to apply an upward force of 100 pounds to the rope. If the rope is 100 feet (30.5 meters) long and you want to lift the weight up 100 feet, you have to pull in 100 feet of rope to do it.

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Pulleys• Does this change anything?

Not really. The only thing that changes is the direction of the force you have to apply to lift the weight.

• You still have to apply 100 pounds of force to keep the weight suspended, and you still have to reel in 100 feet of rope in order to lift the weight 100 feet.

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Pulleys• The weight is now suspended by two

pulleys rather than one. That means the weight is split equally between the two pulleys, so each one holds only half the weight, or 50 pounds (22.7 kilograms).

• If you want to lift the weight 100 feet higher, then you have to reel in twice as much rope 0- 200 feet of rope must be pulled in.

• This demonstrates a force-distance tradeoff. The force has been cut in half but the distance the rope must be pulled has doubled.

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Pulleys• This arrangement cuts the

force in half and doubles the distance from the previous arrangement.

• To hold the weight in the air you must apply only 25 pounds of force, but to lift the weight 100 feet higher in the air you must now reel in 400 feet of rope.