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F=ma, no acceleration with constant velocity?

  1. Mar 11, 2012 #1
    I got a 30kg rock in space, no gravity, no friction. It is traveling at a CONSTANT speed of 100m/s. According Newton's second law, There should be NO force because there is NO acceleration. But I know if it hits another object it will either move the object or put a hole in it so I know there is force there but according to F=ma there is no force. How do I calculate the force?

    Please, I am not taking physics, so I am learning this on my own. So please try to put this in words a layman can understand.
  2. jcsd
  3. Mar 11, 2012 #2


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    Staff: Mentor

    Sure, there is force during the impact, but when it isn't accelerating, no force.
  4. Mar 11, 2012 #3


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    Gold Member

    You may be having a common misunderstanding about forces. Forces describe interactions between objects, it is not a property of the objects themselves. If an object is not interacting with another object or field, then there are no forces at work.
  5. Mar 11, 2012 #4
    Sounds like your intuitive idea of 'force' is closer to 'momentum.'
  6. Mar 11, 2012 #5


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    Gold Member

    Your rock, moving freely in space was force free until the instant that it collided with the second object. When the collision occurred, there were forces imposed on your rock, and it was no longer a force free motion. It still obeys F = ma, but at that instant, F becomes nonzero, and therefore a becomes nonzero as well. As long as your rock is in contact with the second body (or any subsequent body), it will not be a force free motion. Whenever contact is broken, force free motion will again obtain, but it will be a new force free motion.
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