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Clarification about Law of Inertia

  1. Mar 19, 2009 #1
    Hi everyone, just some quick background information, I'm only in grade 9 so I'm not the brightest in physics as of yet however I know some information here and there.
    My question is about the Law of Inertia (Newton's first law).
    The law states that an object basically remains in motion unless acted upon by an external unbalanced force. So does this mean that say if I threw a rock in space which is a near vacuum, the rock should theoretically move forever with a constant velocity, is this correct?
    If so, if I hit the moving rock, would the force add on top of the original force moving the rock or would the original force be "gone" and the rock would move with the new force?
    Sorry if I worded this in a confusing way or if I posted this in the wrong forum. But thanks in advance to anyone willing to answer/ clarify!
  2. jcsd
  3. Mar 20, 2009 #2

    Doc Al

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    Staff: Mentor

    Right, assuming you're far enough away from any gravitating bodies.
    The point of Newton's first law is that no force is required to keep something moving at constant velocity. So there is no "original force" continuing to act on that thrown rock. (As soon as it leaves your hand, your hand stops exerting a force on it.)

    So when you hit the moving rock, your hit is the only force acting on it. And, from Newton's second law, that force produces an acceleration that changes the rock's motion.
  4. Mar 20, 2009 #3
    Oh alright thanks for the reply, I guess this leads to my second question:
    A while back I was reading about the Orion Project which is basically a space-craft to be powered by "nuclear-bombs". This gave me an idea (I might be missing something) but would it be possible to accelerate the space-craft to the speed of light because the space-craft is only gaining speed not loosing it (assuming its far from any gravitational field) based on the first & second laws, is that correct?
    Another thing I don't really understand is why it would take infinite acceleration to reach a finite number such as C(based on what Einstein said).
    I would appreciate if anyone directed me to a book that deals with these issues since I have another ton of questions.
    Thanks in advance
  5. Mar 20, 2009 #4


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    Yes, if you continue applying force to the back end of the rocket while there is no force the other way, the ship keeps accelerating.

    However, it does not take "infinite acceleration" to reach c. The law is F= ma and, by relativity, m increases without bound as the speed approaches c. If F is constant (as in your scenario) then a= F/m goes to 0.
  6. Mar 20, 2009 #5
    If you google on ''Special relativity'' you will find plenty of information that deals with relativistic velocities, masses, time dilation, length contraction, and so forth.
  7. Mar 20, 2009 #6
    Curiosity is step in right direction =)


    You have developed intuition about the thing you called "original force", but that thing is not force, it is momentum : velocity multiplied with mass of the rock. Force is applied to change the momentum, e.g. when you pushed it with your arm. After releasing it, no force is applied no more. Should you kick already moving rock, you add some more momentum to the rock so final momentum is result of vector addition of new and initial momentum ( "original force" as you misnamed it ). Momentum is not gone : there is a law that states that momentum can not be lost or created (only transfered ) : momentum conservation law. We have never observed process that violates this law.

    Hope I helped! I saw you were also interested in space travel, so you should check out


    It is a very interesting FAQ on a very interesting page : John Baez's site is as good as they get. I know you will find interest in :

    http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken]

    Last edited by a moderator: May 4, 2017
  8. Mar 20, 2009 #7
    Thanks everyone for your replies cleared up everything about this issue, and the links helped a lot xlines.
    However, about special relativity I read about it (google'd) and the websites usually assume you know enough about physics therefore I end up reading but not understanding. Is there a book that starts out with the basics about physics?
  9. Mar 21, 2009 #8
    What is it you don't understand? Galilean transformations? Then I think it would be a good idea to do some basic 1D kinematics first.
  10. Mar 21, 2009 #9
  11. Mar 26, 2009 #10
    alright that link contains answers to some questions I had. Final question, I've been thinking for a while, I learned in science class that pulleys multiply force. So couldn't you use a pulley to multiply the force of a motor to get 2(input) (I think 2 was the mechanical advantage). So that way you would receive more output than input, is this possible?
  12. Mar 27, 2009 #11

    Doc Al

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    Using a pulley (or other simple machine providing a mechanical advantage) can multiply the force, but not the work (energy) produced. When you use a pulley that doubles the force, you must pull the rope twice as far to get the same output. So you never get more work out than you put in.
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