Science - Coming down to Earth
- There's no gravity in space.
- NASA has an anti-gravity room where gravity can be turned off.
- If the Earth stopped spinning we would float into space.
- There is no gravity on the International Space Station.
- Nottingham Forest won the European Cup in 1979 and 1980.
Believe it or not, most pupils - and teachers who are not physicists - think that the last statement is the only false one. In fact, it's the only one that's true.
To emphasise that gravity is not just a force between us and the earth but a force between all objects, explain your attraction to various objects in the room and theirs to you. The force between everyday objects is small - I illustrate this by calculating the force of attraction between my wife and myself standing 1m apart (around 0.0000004 newtons). But the size of the force of gravity depends on mass and we only notice the force when one or both of the objects are approximately the mass of a planet. The force goes both ways - so if a body exerts a force on us, we exert an equal but oppositely directed force on the body. This can be linked to a discussion of Newton's third law.
Weight can then be introduced as another name for the force of gravity. Discuss why the weight of an object is less on the Moon than on the Earth (because the Moon is smaller). Gravity also depends on distance apart; this is why, far from any other objects deep in interstellar space, you are almost "weightless".
Mass can then be introduced as a measure of the amount of material in an object (and also its resistance to motion). "Mr Porter is made up of 80kg of blood, bones, hair and poo. If he travels to the moon, he is still made up of 80kg of blood, bones, hair and poo so his mass does not change." The link between weight and mass can then be introduced and the concept of gravitational field strength can be used to calculate the weight of an object.
Test pupils with a couple of conceptual questions. What would happen if you drilled a hole directly through the centre of the Earth and dropped a ball down (assuming no friction)? What would gravity be like in a room at the centre of the Earth?
Simon Porter supports Nottingham Forest and teaches physics at the British School in Warsaw
Try Simon's lesson about gravity, mass and weight on TES Resources to help pupils separate fact from fiction.
For a clear summary of Newton's laws try SRWhitehouse's presentation.
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Find all links and resources at www.tes.co.ukresources041.