Unit 3, lesson 14. About 12 minutes.

Galileo's falling objects

Does a heavy hammer really fall faster than a feather?

Play

Getting the experiment ready…

Explain

Air slows the feather down.

With no air, they fall the same!

People used to think heavy things fall faster. Galileo tested this, and found it wasn't the mass that mattered.

A feather falls slowly because air pushes against it. Take away the air, and the two fall exactly together.

An astronaut tried this on the Moon in 1971, with a hammer and a feather. They landed at the same time!

With no air, every object has the same acceleration, g, whatever its mass.

A bigger mass has a bigger weight, but it is also harder to accelerate. With W = m × g and a = F / m, the mass cancels: a = (m × g) / m = g.

Worked example: a 10 kg hammer and a 0.01 kg feather both get a = 9.8 m/s² in a vacuum.

Drop two objects
Air slows the feather
No air: they land together

Want the full story?

The sim shows a vacuum on Earth, but Apollo 15's test was on the Moon, where g is only 1.6 m/s². The fact that gravity accelerates all masses equally led Einstein to his theory of gravity, general relativity.

The Formula

falling time = square root of (2 × height ÷ gravity)

t = \sqrt{\frac{2h}{g}}

$t$: the time to fall, measured in seconds (s)
$h$: the height it falls, measured in metres (m)
$g$: the acceleration due to gravity, 9.8 on Earth, measured in metres per second squared (m/s²)

Try the numbers

Height: 20 m

Time to fall on Earth: 2.02 s

Try it at home

You need a book, a sheet of paper.

You need: a book, a sheet of paper.

Put paper on a book and drop them. Together!

Drop a book and a sheet of paper side by side.
Now put the paper on top of the book.
Drop them together.

What's happening: on top of the book, the paper is sheltered from air resistance. Without it, the paper falls as fast as the book.

Physics is life

Your body

Your hair floats up as you fall.

When you jump, you and your hair fall together. That is why it floats for a moment.

Daily life

Marbles of all sizes fall together.

A big marble and a small marble land at about the same time, because air barely affects them.

Nature

Tiny bugs can fall safely.

Air resistance matters more for tiny animals, so a falling ant lands safely.

Space

A hammer and feather fell together on the Moon.

Astronaut David Scott dropped a hammer and a feather on the Moon. There is no air there, so they landed together.

Quick check

Question 1 of 3

Why does a feather fall slowly on Earth?

Words to know

Vacuum: A space with nothing in it, not even air.

Vacuum: A space with nothing in it, not even air.
Air resistance: The force from air pushing against something moving through it.
Gravity: The pull between all things that have mass.

Meet the scientist

Galileo Galilei

Next up

Orbits: why the Moon doesn't fall down