The fixed pulley

The idea

A pulley is a wheel with a groove that guides a rope; a fixed pulley changes only the direction of the effort, not its magnitude, so its mechanical advantage is 1.

The flagpole

To raise a flag, you pull the rope down, and the flag goes up. The wheel at the top of the pole turns the rope.

Does the wheel at the top make the pull smaller?

• No, the pull is as large as the weight
• Yes, it makes the pull much smaller
• It makes the pull larger

With or without a pulley

Choose how to raise the load.

Same size, new direction

With the pulley the effort was the same size as the load, but it pointed down instead of up. Pulling down is easier for us than lifting up. So the pulley helps by changing the direction of the effort, and its mechanical advantage is 1.

A pulley is a wheel with a groove that guides a rope. A fixed pulley changes only the direction of the effort, not its magnitude, so its mechanical advantage is 1.

Notes

A fixed pulley changes the direction of the effort, not its magnitude: mechanical advantage = 1.

Check yourself

A pulley is…

What does a fixed pulley change?

The mechanical advantage of a fixed pulley is…

Why is a fixed pulley convenient for raising a flag?

• a wheel with a groove that guides a rope — correct. Yes!
• a rigid bar that rotates about a fixed point. That describes a lever.
• a sloping plank used to raise loads. That describes an inclined plane.
• The magnitude of the effort. A fixed pulley does not change the magnitude of the effort.
• The direction of the effort — correct. Yes!
• The weight of the load. The load keeps its weight. The pulley changes the direction of the effort.
• 2. The effort and the load are equal in magnitude, so the ratio is 1.
• 0. The ratio of two equal forces is 1, not 0.
• 1 — correct. Yes!
• It is easier to pull downward than to lift upward — correct. Yes!
• It halves the weight of the flag. The effort has the same magnitude as the load. The pulley changes only the direction.
• It makes the flag lighter. The pulley does not change the weight of the flag.