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Newton's First Law of Motion

  1. Mar 25, 2015 #1
    Newton's first law says that when an object is at rest or moving in constant velocity, it will stay that way unless external force is applied. So, net force is zero.

    What I don't get is on an object moving in constant velocity. How is the object begin moving? Is netforce applied initially before it reaches its constant velocity?
  2. jcsd
  3. Mar 25, 2015 #2


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    How it got that way is not part of the first law of motion, but yes, to get something moving on, for example, a flat surface, you have to give it a push. If the surface is frictionless it just keeps moving. If you don't give it a push it just keeps sitting there.
  4. Mar 26, 2015 #3


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    The object itself doesn’t have to begin to do anything. You can analyze any object in a reference frame where it always has been moving.
  5. Mar 26, 2015 #4
    Consider a frictionless surface.
  6. Mar 26, 2015 #5
    Yes, some force must have been applied to the object in it's past in order to accelerate it to the velocity which it presently has.
    (In relation to the same given frame of reference)
  7. Mar 26, 2015 #6
    Before Newton, people didn't know or realize that things could move on their own. He taught people that if something gets a push in one direction, it will need a push is the opposite direction to stop.
  8. Mar 26, 2015 #7


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    Why would one assume that in the infinite past that the object was at rest in an arbitrarily selected frame of reference?
  9. Mar 27, 2015 #8
    Because it's unlikely that the moving object does have an infinite past.
    It could of course be that the object was formed out of matter which already has momentum - like the solar system,
    However the momentum in that case hasn't come from nothing, it's momentum that was previously included in a nebula, and now is included in the Sun.
    Last edited: Mar 27, 2015
  10. Mar 27, 2015 #9


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    What do you mean by "it could be"?. It is absolutely certain that there are inertial frames (infinitely many) where it always had momentum.
  11. Mar 27, 2015 #10
    Poor wording on my part, yes I agree there certainly are frames where an object always had momentum,
    In relation to the original question," How is the object begin moving , Is net force applied initially before it reaches its constant velocity?",
    I was trying to explain that if the object was motionless at one stage and now it isn't, then a force must have been applied to it.
    On the hand an object which comes into existence in an already moving condition, and still is moving now, doesn't 'begin to move' and no force need have been applied to it since it started existing.
  12. Mar 27, 2015 #11


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    The correct answer to that question is, that the object doesn't have to begin moving. It is always moving in some inertial frames and Newtons 1st Law applies in all of them.
  13. Apr 3, 2015 #12
    And rightly so, most objects on Earth stop moveing because of friction they don't need a push in the opposite direction to make them stop.Newton was probably refering to celestial bodies which would be a hard concept to grasp let alone explain at that time.
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