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Newton's Three Laws of Motion

  1. Oct 12, 2007 #1
    Are Newton's three laws of motion correct as Netwon stated them?

    1. The first law is the principle of inertia. It states that an object in motion will continue to move unless acted on by an external force. And that an object at rest will remain at rest unless acted on by an external force.
    2. The second law states that the change in momentum is proportional to the external force.
    3. The third law is that for every action there is an equal and opposite reaction.
     
  2. jcsd
  3. Oct 12, 2007 #2
    On the third law the direction is also opposite F(1,2)=-F(2,1)
     
  4. Oct 12, 2007 #3
    Yes, but are all these laws still considered correct today?
     
  5. Oct 12, 2007 #4

    brewnog

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    Absolutely, in their context. They're still the basis for every piece of machinery, every building; pretty much everything designed by man.
     
  6. Oct 12, 2007 #5

    russ_watters

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    Except insofar as they break down at very high speeds where relativity takes over, yes.
     
  7. Oct 12, 2007 #6

    Yes, but the three laws are general ideas about motion not equations and as general ideas are they not correct?
     
  8. Oct 12, 2007 #7
    russ_watters
    Or, as I understand it, in very dense gravitational fields (general relativity) or on very small scales (quantum mechanics).
     
  9. Oct 12, 2007 #8
    But even in relativity its correct to say that the change in momentum is proportional to the external force.
     
  10. Oct 13, 2007 #9

    Hootenanny

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    Yes, but this force need not be in the same direction as the change in momentum.
     
  11. Oct 13, 2007 #10

    russ_watters

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    The equations equations are connected to their verbal descriptions. Newton's first law, as you stated it, is basically a verbal description of f=ma.

    And yes, again, they are correct in their domain.
     
  12. Oct 13, 2007 #11
    The first law (a = 0 if f = 0) is a special case of the second law (f = ma).
     
  13. Oct 13, 2007 #12
    The idea that: the change in momentum is proportional to the external force can
    also be expressed as F = dp/dt which was the original Newtonian notation
     
    Last edited: Oct 13, 2007
  14. Oct 14, 2007 #13
    or in quantum physics
     
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