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Homework Help: Question about net force/acceleration/constant velocity

  1. Sep 27, 2012 #1
    I read in my textbook that an object can have constant velocity when net force and acceleration are equal to 0. For an example like a puck on frictionless ice that continues to move after it has had a force applied to it that is all good and fine, I understand that inertia keeps the puck moving.

    I am wondering how in an example similar to the above, the object can have constant velocity and net force can be equal to 0, with horizontally applied forces that are NOT equal to 0. For example, a puck being pushed by a hockey stick on some snowy ice, or me pushing a book across a table. Doesn't the applied force from the hockey stick or the force from my hand have to be greater than the force of friction from the ice or the table for the puck or book to move? if this is true, then net force is not equal to 0, yet the puck/book still has constant velocity and now all of Newton's principles make no sense to me. I'm hoping someone can clarify this/these concept(s).


    EDIT: I'll add one more question here, how would you calculate (not specifically, just conceptually) the acceleration of an object that has constant speed but changing direction only?
  2. jcsd
  3. Sep 27, 2012 #2


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    Welcome to PF,

    It seems like your question is of the form, "if I deliberately envision a scenario where Newton's laws are violated, then I find that Newton's laws aren't obeyed. What's going on?" I'm not being facetious here. I just think that contradiction is there because you put it there.

    If the puck has a non-zero net force acting on it, then it will have a non-zero acceleration. That's all there is to it.
  4. Sep 27, 2012 #3


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    Static friction has to be overcome to get the puck to initially move, and then kinetic (sliding) friction is involved. The puck will accelerate until the force is reduced to match kinetic friction.

    The math can be complex, but one example where this is commonly done is a car making maneuvers (turns) while at constant speed. The path can be just about any shape (spiral, parabola, hypebola, ellipse, circle, sine wave, ...) that doesn't have sharp inflection points (corners with a radius of 0).
    Last edited: Sep 27, 2012
  5. Sep 27, 2012 #4
    Good answer, thanks. And no, I wasn't trying to envision a scenario to violate Newton's laws. In the situation I presented they aren't violated, I just couldn't understand how they were operating in that context, as the examples that are given in my textbook/lectures are similar to the first example I gave.
    Last edited: Sep 27, 2012
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