Grade 9 · Mechanical Systems & Control · Term 2 · Lesson 2

Pulleys & Mechanical Control Systems

Pulleys let us redirect or multiply a pulling force. Control systems like ratchets, brakes and cleats let us control motion — allowing it, stopping it, or locking it in place.

The single fixed pulley

A single fixed pulley is a wheel with a grooved rim, mounted in a fixed position (for example, bolted to a beam), with a rope running over it. Pulling down on one end of the rope lifts the load on the other end.

A fixed pulley does not multiply force — the effort you apply equals the load you lift. Its only job is to change the direction of the pulling force, which is why a flagpole lets you raise a flag by pulling downward instead of climbing up.

Single fixed pulley: Mechanical Advantage (MA) = 1. Direction changes, force does not.
Single fixed pulley — one rope section supports the load

The single moveable pulley

A single moveable pulley is attached directly to the load, so the pulley itself moves as the load is lifted. One end of the rope is fixed (often to the same beam or frame), the rope loops under the moveable pulley, and you pull on the free end.

Because the load is now supported by two rope sections instead of one, you only need to pull with about half the force to lift it — giving MA > 1. The trade-off is that you must pull the rope twice as far to raise the load the same distance.

Single moveable pulley: MA = 2 (approximately). Effort ≈ half the load, but rope moves twice as far.
Single moveable pulley — two rope sections support the load

Pulley block systems (block and tackle)

A block and tackle combines several fixed and moveable pulleys (mounted in "blocks") with one continuous rope threaded through them. The more load-bearing rope sections supporting the moveable block, the greater the mechanical advantage — each extra rope section shares the load between more strands.

Rule of thumb: Mechanical Advantage ≈ the number of rope sections directly supporting the moveable pulley block. Two supporting ropes give MA ≈ 2; three supporting ropes give MA ≈ 3, and so on. This is how cranes and engine hoists lift loads far heavier than a person could pull directly.
2-pulley block and tackle — four rope sections support the moveable block
SystemLoad-bearing ropesApprox. MA
Single fixed pulley11 (direction change only)
Single moveable pulley22
2-pulley block and tackle33
3-pulley block and tackle44

Mechanical control systems

A control system doesn't create motion — it manages it: allowing movement in only one direction, slowing it down, or locking it in place.

Common thread: ratchets control direction, brakes and cleats control motion using friction — pressing one surface against another to convert movement into heat and hold or slow the system.

Pulley System Builder

Drag the slider to change the number of rope sections supporting the load, and see how more pulleys mean less effort — but more rope to pull.

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Review the explanations above.
Answer in your exercise book.
Question 1 · (2 marks)
Explain why a single fixed pulley has a mechanical advantage of 1, even though it makes lifting a flag feel easier.
Question 2 · (3 marks)
A single moveable pulley is used to lift a 60 N load. (a) What effort force is theoretically needed? (b) If the load must rise 1 m, how far must the rope be pulled? (c) Explain the trade-off shown by your answers.
Question 3 · (3 marks)
A block and tackle system has 4 rope sections supporting the moveable pulley block. (a) State the approximate mechanical advantage. (b) Calculate the effort force needed to lift a 400 N load. (c) Name one real machine that uses this type of system.
Question 4 · (4 marks)
Name the four mechanical control systems studied in this lesson and, for each, state whether its main purpose is to control direction or to control motion using friction.
Question 5 · (2 marks)
Describe how a cleat holds a rope in place on a sailboat, without tying a knot.