Unit 6, lesson 27. About 12 minutes.

Levers

How can a small child lift a big grown-up on a see-saw?

Play

Getting the experiment ready…

Explain

A long bar helps you lift.

Push far from the middle.

A lever is a bar that turns around a point called a pivot.

The further from the pivot you push, the easier it is to lift the load.

That's how a child can lift a grown-up on a see-saw. Sit further from the middle and it works.

A lever balances when effort × its distance = load × its distance.

The trade-off is that the effort moves further than the load, so no energy is saved.

Worked example: a 400 N load is 1 m from the pivot. Pushing at 2 m takes 400 × 1 / 2 = 200 N.

A bar turns on a pivot
Push far from the pivot
Small force lifts big load

Want the full story?

The sim leaves out the weight of the bar. Levers come in three classes, with the pivot, load and effort in different orders. Your forearm is a third-class lever, which gives speed rather than saving force.

The Formula

effort × its distance = load × its distance

F_e \times d_e = F_l \times d_l

$F_e$: the effort force you apply, measured in newtons (N)
$d_e$: the distance from the pivot to the effort, measured in metres (m)
$F_l$: the load force, measured in newtons (N)
$d_l$: the distance from the pivot to the load, measured in metres (m)

Try the numbers

Load: 400 NLoad distance: 1 mEffort distance: 2 m

Effort needed: 200 N

Try it at home

Ask a grown-up to help

You need a ruler, a pencil, some coins or small toys.

You need: a ruler, a pencil, some coins or small toys.

Balance a ruler on a pencil.

Rest a ruler across a pencil.
Put a coin on one end.
Press the other end and feel the force, then move the pencil and try again.

What's happening: pressing further from the pivot takes less force to lift the coin.

Physics is life

Your body

Your arm is a lever.

Your elbow is a pivot, and your muscles pull on your bones to move them.

Daily life

Scissors are two levers.

Scissors, bottle openers and wheelbarrows are all levers.

Nature

Beaks work like levers.

Birds' beaks and animals' jaws work as levers.

Space

Astronauts use tools too.

Long tools help astronauts turn things on a space station.

Quick check

Question 1 of 3

What is the pivot?

Words to know

Lever: A bar that turns on a pivot, used to move loads with less force.

Lever: A bar that turns on a pivot, used to move loads with less force.
Pivot: The point that something turns around.
Simple machine: A tool that changes the size or direction of a force.

Meet the scientist

Archimedes

Next up

Pulleys, ramps, wheels and gears