Levers — effort, load and pivot
A lever is a bar that turns about a point. Three things matter and no others: where the load is, where your effort goes, and where the pivot sits between or beside them.
Turning force, not just force
What a lever actually balances is torque — a force multiplied by how far it is from the pivot. A small force a long way out does the same job as a large force close in, which is why a spanner with a longer handle undoes a tighter bolt.
effort × effort arm = load × load arm
Both distances are measured from the pivot. That is the part people get wrong: the arm is not the length of the bar, it is the distance from the pivot to where the force is applied.
Move the pivot and watch both sides of that equation change.
Drag the triangle, or use the arrow keys. The load never changes — only where you put the pivot.
The pivot is near the middle, so effort and load are close to equal. This is a seesaw: no force gained, no distance gained, and easy to reason about.
The same bargain again
The mechanical advantage — how much your force is multiplied — is just the ratio of the two arms. And exactly as with pulleys and gears, whatever you gain in force you pay for in distance: a lever that lets you lift with a third of the effort makes your end travel three times as far.
The three classes
| Class | Arrangement | Examples |
|---|---|---|
| First | pivot in the middle, effort and load on opposite sides | seesaw, crowbar, scissors, a pair of pliers |
| Second | load in the middle, pivot at one end | wheelbarrow, nutcracker, a bottle opener |
| Third | effort in the middle, pivot at one end | tweezers, a fishing rod, your own forearm |
A third-class lever always needs more effort than the load, which sounds useless until you notice what it buys: speed and reach. Your bicep pulls hard over a couple of centimetres and your hand travels half a metre quickly. A catapult arm and a robot’s throwing arm are the same idea.
Levers in a LEGO build
- A motor plus a lever arm is the cheapest way to turn a small rotation into a big movement — or a weak motor into a strong push. Which one you get depends entirely on which side of the pivot the motor is.
- The pivot has to be braced. A pivot pin in a single beam flexes under load, and the arm you carefully calculated stops being the arm you built.
- Long arms magnify play as well as force. A wobbly 15-stud arm has a very wobbly tip.
Why it matters
Every gripper, every digger arm and every set of scales is a lever. So is a light switch, a car’s brake pedal and the claw on a claw machine. The question to ask of any of them is always the same: where is the pivot, and how far is each force from it?
More mechanics tutorials
- Mechanisms without motors — How to investigate a build with no electronics in it, and where each idea lives.
- Gearing up and gearing down — Trade turns for force, or force for turns — you cannot have both.
- Changing the direction of a turn — Reverse a turn, restore it with an idler, or send it round a 90° corner.
- Pulley systems — Fixed, movable and combined — how rope pulled trades against force needed.
- Elastic and stored energy — Stretch a band, store energy, let it go — and find where more stops helping.
- Scissor mechanisms — Crossed links that extend and retract, and why the last bit is the hardest.
- Oscillatory motion — Turning a rotation into a back-and-forth, and what sets the rhythm.
- Centre of gravity — Why a robot tips, and how to build one that does not.
- Biomimetic mechanisms — Linkages that copy how animals move — turning a rotation into a step.