Forces and Motion

Newton's first law

Physics Narrative for 14-16 Supporting Physics Teaching

Motion in a natural environment

We've talked a lot about putting the object into its natural environment – one where there is only the mass, and none of the messy interactions affecting real objects in the lived-in world. So having finally managed the extraction, how does the object move in the simplest case – that is, without any forces acting on it?

We suspect that you already know the answer, but here's an illuminating way of approaching the situation – all in very simple universes.

Start off by sitting next to a ball in a universe with just you and it (it's an imaginary universe, so you can switch gravity off as well, thus making it even simpler). It's stationary – that is, the separation between you and the object is unchanging. You can watch it for a long time, but symmetry arguments suggest that it'll sit there for just as long as you watch it: it has no reason to move off in any one direction. Stationary things remain stationary. That's a very simple rule.

Now Bob whizzes past, at constant velocity, so that the separation between you increases at 3 metre inverse second. He's also watching a stationary ball, whizzing along with him at 3 metre inverse second. What will Bob see his ball doing? It'll be stationary. There's no reason for it to do anything else.

But what will you see? Both Bob and the ball moving at constant velocity at 3 metre inverse second. So once the ball is moving, it'll keep moving at the same speed. What will Bob record, if he looks at you and your ball? Your ball's separation from him will be increasing at the same rate as his separation from you (3 metre inverse second). Otherwise you'd see it move. So if it is stationary for you, it is moving at constant speed for Bob.

Alice zips past at another constant speed. You can repeat the argument.

So Newton's first law is a consequence of two things only:

  • A simple rule that things that are stationary, remain stationary. (If they've no reason to start moving, they won't.)
  • The requirement that what you record will also be recorded by anyone who performs the same experiment at a constant speed is the principle of relativity.
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