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If there is no acceleration, does that mean no force?

  1. Sep 25, 2015 #1
    So last year, I was told by a friend that no Acceleration means no Force, Mathematically, this makes sense
    (500 Kg * 0 m/s^2 = 0 N), but intuitively, I can't wrap my head around it. Maybe this is because of language and the way we use the word force in normal conversation, but in physics it is also defined as a push or pull. So If I am pushing a box at a constant velocity (0 acceleration),does that mean I am not applying a force? Another thing that makes me believe that a force is applied is that I was told by my professor in lecture that (force is not actually
    Mass * Acceleration, but rather "It's What Happens".) Please help me wrap my head around this.
  2. jcsd
  3. Sep 25, 2015 #2


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    Gold Member

    It is being counter-balanced by surface friction**.
    If you removed the surface friction, but kept the force, the box would begin to accelerate. To keep it at a constant velocity, you would have to stop applying a force.

    **or, I suppose, air resistance, if you're, like, Superman or something.
  4. Sep 26, 2015 #3


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    Staff: Mentor

    Not true. No acceleration means no net force, not that there isn't any force. I'm certainly applying a force on my chair right now. 155 pounds of force to be exact.
  5. Sep 26, 2015 #4
    Does 10000 kgs of air count? Or it is cancelled out by the air underneath the chair.
  6. Sep 26, 2015 #5


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    Staff: Mentor

    Yes, it is cancelled by the air underneath. Well, more like opposed instead of cancelled. There is certainly a force on all sides of the chair from either the air, myself, or the ground (or a combination of them). There's no net force because they all oppose each other equally.
  7. Sep 28, 2015 #6


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    Not necessarily. It means that the sum or net force on the box is zero. For example if the force you apply exactly matches that of friction the the net force is zero. The box can be moving but not accelerating.

    You would have to ask your prof what he means.

    Back to the box example. It would be reasonable to write..

    Fnet = Fyou - Ffriction = mass * acceleration

    Then if the force you apply is just equal to friction the acceleration is zero.
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