Newton's Law of Gravitation
Earth and Space

The gravitational force

Physics Narrative for 11-14 Supporting Physics Teaching

Introducing gravity

When was the last time you watched a young child playing that favourite game of throwing the toy out of the pram? You know what happens.

The child throws the soft toy out of the pram. The parent puts it back. The child throws it out… and so on. Sometimes the child pulls up on the side of the pram to see where the toy has gone.

Which way do they look? Downwards of course!

From a very early age, children become aware of the fact that things fall downwards when you let go of them. Much later on in school science lessons, they are introduced to the idea that things don't just fall but that they fall because of the force of gravity – but what is this gravity?

Gravity is a force that exists between all objects with mass in the universe. The soft toy falls to the ground because of the gravitational attraction of the Earth. Sometimes we refer to this force as the pull of the Earth.

The fascinating thing about the gravitational force is that it allows one object to pull on another without being in contact with it. If you think that this is a curious state of affairs, you'd be absolutely right!

For example: If you hold a bag of sugar in the palm of your hand, it is tempting to think that it is just the heaviness of the sugar that you can feel pushing down on your hand. Not so! In fact it is the gravitational force of the Earth that is acting on the sugar and pulling down through your hand.

Even though the bag of sugar and the surface of the Earth might be separated by a distance of more than one metre, the Earth is still able to exert its pulling force. This feature of the gravitational force is in contrast with those everyday pushes and pulls where the force acts through direct contact with the object.

For example: When you push on a door to open it, it doesn't start moving until contact is made and the force is exerted. We register this difference by saying that the gravitational force is not a contact force but acts at a distance.

Newton's Law of Gravitation
is expressed by the relation F=G(m_1)(m_2)/r^2
can be used to derive Kepler's First Law

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