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Lesson 7 of 14

Three machines

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7 min

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A pulley, a lever and a ramp look different. Do they work the same way?

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Lever: a 400 N load on a 2 m arm, a 100 N effort on an 8 m arm.
Step 1 of 4
  1. Lever: a 400 N load on a 2 m arm, a 100 N effort on an 8 m arm.
  2. A movable pulley: a 400 N load, held up by a 200 N pull.
  3. Ramp with no friction: a 400 N load rising 2 m, pushed about 6 m.
  4. One trade, three machines.

Rule

Yes. The lever, movable pulley and ramp all trade force for distance. A lever turns about its fixed point, the fulcrum. An arm is the distance from the fulcrum to a force.

A fixed pulley only changes the direction of the force. A lever balances when effort × effort arm = load × load arm. With no friction, a ramp's advantage is its length ÷ its height.

  • Pulley — a grooved wheel that guides a rope. A groove is a channel round the rim.
  • Lever — a bar that turns about a fixed point.
  • Fulcrum — the fixed point of a lever.

A fixed pulley has advantage 1. The pull needed equals the load. A movable pulley shares the load between two ropes. Each rope holds half the load, so the pull is half. With no friction, its advantage is 2.

A 400 N load is 2 m from the fulcrum. So the load arm is 2 m. The effort arm is 8 m. The lever balances, so effort × 8 = 400 × 2 = 800. Then effort = 800 ÷ 8 = 100 N. Advantage = load ÷ effort = 400 ÷ 100 = 4. That equals 8 ÷ 2, the effort arm ÷ the load arm.

Try the lab to finish this lesson.