Gravity
Why do people in Australia not fall off the Earth?
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Explain
Things fall down.
Earth pulls everything towards it.
Gravity is a pull between all things that have mass. Earth is so huge that its pull is what we feel.
Down always means towards the centre of the Earth, wherever you stand on it. So nobody falls off.
When you drop something, it speeds up as it falls. It gains nearly 10 m/s every second.
Near the surface of the Earth, gravity gives everything an acceleration of g ≈ 9.8 m/s², if we ignore air resistance.
Starting from rest, height fallen is h = ½gt², and we can rearrange this to find the time.
Worked example: from 45 m, t = √(2 × 45 / 9.8) ≈ 3.0 s.
- Earth has a huge mass
- It pulls everything in
- Things fall towards its centre
Want the full story?
The value g changes slightly across Earth, from about 9.78 m/s² at the equator to 9.83 m/s² at the poles. The sim leaves out air resistance, which matters for light objects and long falls.
The Formula
falling time = square root of (2 × height ÷ gravity)
- the time to fall, measured in seconds (s)
- the height it falls, measured in metres (m)
- the acceleration due to gravity, 9.8 on Earth, measured in metres per second squared (m/s²)
Try the numbers
Time to fall on Earth: 2.86 s
Try it at home
You need a ball, a stopwatch.
You need: a ball, a stopwatch.
Drop a ball. Which way does it go?
- Hold a ball up high.
- Drop it and time the fall.
- Try again from a lower height.
What's happening: gravity speeds the ball up all the way down, so a longer drop takes more time.
Physics is life
Your body
Gravity keeps your feet on the ground.
Daily life
Water flows downhill.
Nature
Apples fall from trees.
Space
Gravity holds the Moon near Earth.
Quick check
Question 1 of 3
Words to know
Gravity: The pull between all things that have mass.
- Gravity
- The pull between all things that have mass.
- Mass
- How much stuff is in an object. It is measured in kilograms.
- Acceleration
- How quickly the velocity of something changes.