Forces acting without contact

A non-local piece of the environment interacts with our object

Wrong Track: How can the Earth pull on the Moon to make it fall? It's not even touching it!

Right Lines: Some forces can act on objects without touching them. Gravity is one example of these non-contact forces.

Explaining action-at-a-distance

Thinking about the learning

If a student argues in this way, you might encourage them by recounting something of the history of the development of the idea of a field. It's possible to argue that this (very powerful) idea came into existence precisely to help scientists with their unease about this somewhat spooky action-at-a-distance. Certainly acting on something physical without touching it seems a very alien idea.

Thinking about the teaching

Gravitational, magnetic and electric effects happen at a distance: they are in a special class of non-contact forces. If students think that the action-at-a-distance effects of gravity are obvious, then you might well be worried. It's a strange way of thinking, rather different from the thinking in identifying all the other kinds of forces (normal and retarding).

However, the idea of a field is even more pervasive than the idea of a force in more recent physics, so it's something we may just gradually get used to. It's certainly an idea that's proved to have enduring cultural worth, and therefore worth investing some time in.

It's probable that it grew from worrying about how atoms interact – a line of thinking attributed to Boscovich (pictured here) – and was then picked up by Faraday to help him understand the non-local interactions of magnets and electromagnets. However, take care to keep magnetism and gravity separate as conflating the two is a well-known difficulty (see the SPT: Earth in space topic for more details).