# Science - Back down to Earth

There's no gravity in space - at least that is what most pupils (and a fair number of teachers) think, writes Simon Porter.

In order to emphasise that gravity is not just a force between us and the Earth, but between all objects, explain your attraction to various objects around the classroom. The force between everyday objects is small, but the force of gravity depends on mass and we only notice it when at least one of the objects is approximately the mass of a planet.

The force goes both ways - so if a body exerts a force on us, we exert an equal force on it. This can be linked to Newton's third law.

Weight can then be introduced as another name for the force of gravity. Move on to discuss why the weight of an object is less on the Moon than on the Earth (this is because the Moon is smaller). Gravity also depends on distance apart, which is why you are almost "weightless" if you are far from any other objects deep in interstellar space.

Mass can then be introduced as a measure of the amount of material in an object. "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 used to calculate the weight of an object.

What else?

Try Simon's lesson about gravity, mass and weight on TES Resources. For a summary of Newton's laws try SRWhitehouse's presentation.

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