Levers and Pulleys: How Can a Small Push Lift a Big Load?

A crowbar, a seesaw, a flagpole rope: simple machines let us lift big loads with small pushes. They don’t give you something for nothing, though, and a good experiment shows the trade-off.

Part 1: the ruler lever

You need: a 30 cm ruler, a pencil as a pivot (the fulcrum) and about ten identical coins.

Balance the ruler on the pencil at the 15 cm mark.
Put 2 coins on the left at 5 cm from the pivot. How many coins on the right, at 5 cm, balance it? (2.)
Now put the coins on the right at 10 cm from the pivot. How many balance 2 coins at 5 cm on the left? (1.)
Try other combinations, and record them.

The rule

A lever balances when load × distance from the pivot is equal on both sides. Two coins at 5 cm (2 × 5 = 10) balance one coin at 10 cm (1 × 10 = 10).

That means a small force far from the pivot can balance a bigger force close to it. That’s why a long crowbar is easier to use than a short one, and why a door handle is far from the hinge.

Part 2: pulleys

You need: a spool or cup as a pulley wheel, string, a load of about 300 g, a force meter or a rubber band, and a ruler.

Lift the load by pulling the string directly. Measure the force and how far you pulled the string.
Use a fixed pulley: string over a pulley attached above. You pull down instead of up. The force is about the same, but the direction is more convenient.
Use a moving pulley: the load hangs from a pulley that moves with it, with the string anchored above. The force needed is about half, but you pull about twice as much string.

The big idea

Simple machines trade force for distance. You can use less force, but you pull farther. No machine gives you more work out than you put in. Friction means you get slightly less.

Everyday examples

Levers: seesaws, scissors, wheelbarrows, bottle openers
Pulleys: flagpoles, cranes, window blinds, exercise machines

Safety

Keep loads light, fingers clear and don’t stand under a hanging load.

The full lesson is How can a small push lift a big load? in Forces at Work. Sign up for three lessons free.