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S2.1 Inertial reference frames: Newton`s 1st Law

Preamble

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Preamble

  • The pre-Newtonian question:
    What sustains an object’s velocity?
  • The post-Newtonian question:
    What controls an object’s acceleration?
  • The qualitative answers:
    • the external influence it feels

      – we call this force

    • its inertia (resistance) against acceleration

      —we call this mass

    • the reference frame from which it is viewed
  • Empirically, we find that a block sliding on a table has constant velocity if
    • the surface is made smooth enough
    • we view it from the earth’s reference frame
  • This kind of observation is generalised to give Newton’s 1st Law.
 

Key Point 2.1

Newton’s 1st Law. A body free of external ‘influences’ will have constant velocity (zero acceleration) with respect to any inertial reference frame.

About ‘influences’

About ‘influences’

You would instinctively write ‘forces’ instead of ‘influences’. But we haven’t yet got round to defining what we mean by a force. So we shouldn’t use that term yet. We assume that we can all agree on what is meant by ‘influence’ (though it’s not a very scientific word). Once we have got round to defining a ‘force’ you can come back here, score out ‘influences’ and write ‘forces’ instead. Some of you will wish we just wrote it that way from the start...
 
TInertial diver?

Learning Resources

Textbook: HRW Chapter 5.1-3