The big idea: Mechanical advantage (MA) = load ÷ effort. It is how many times a mechanism multiplies the force you put in.
MA above 1 multiplies force. MA below 1 multiplies movement instead. MA of exactly 1 changes only the direction.
One lever, stepped through: the ratio, the distance you pay for it, and the case where you want MA below 1.
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Nothing is free: Force multiplied means distance divided, in exactly the same ratio.
Example: a car jack: A car jack with an MA of 40 lifts a wheel a few centimetres for forty long strokes of the handle. You did not get energy from nowhere — you spread the same work over a longer movement.
| MA | What you get | What you pay | A product |
|---|---|---|---|
| Above 1 | More force at the load than you applied | The load moves less far than your hand did | Bolt cutters, a car jack, a nutcracker, a crowbar |
| Exactly 1 | The same force, in a different place or direction | Nothing but friction | A pulley redirecting a rope, a bicycle brake cable |
| Below 1 | More movement and more speed at the load | You must push harder than the load resists | Tweezers, a fishing rod, a broom, the human forearm |
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Efficiency = useful output ÷ input: The ratio a drawing predicts is the theoretical MA. The one a product delivers is always lower, because friction turns some of the input into heat.
Where the friction happens
- Pivots
- Bearings
- Gear teeth
- Sliding surfaces
Example: A mechanism with a theoretical MA of 4 that delivers 3.6 is 90% efficient.
How performance is improved in practice
Reduce friction
Bearings instead of plain holes, lubrication, smoother bearing surfaces, low-friction polymer bushes at the pivots.
Stiffen the members
A handle that flexes wastes your movement bending itself. Stiff members deliver the movement to the load instead.
Shorten the chain
Every extra joint adds a loss and some free play. Fewer parts between the effort and the load means more of the effort arrives.
Match the ratio to the user
A very high MA that makes the handle travel further than an arm can reach has made the product worse, not better.
How this is tested — outlining mechanical advantage and how a mechanism improves function and efficiency. It comes up two ways:
Paper 1 — multiple choice
- Calculate a mechanical advantage from a load and an effort.
- Identify whether a named product has an MA above or below 1.
Paper 2 — analysing a product
- Outline the mechanical advantage a named product gives its user.
- Explain how a designer could improve the efficiency of a mechanism.
The trap: Treating a high MA as automatically better. A mechanism whose handle has to travel a metre has made the product unusable, whatever the ratio says.
A pair of long-handled bolt cutters is used to cut a 6 mm steel padlock shackle. Outline the mechanical advantage the tool gives, and how its design improves performance.
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