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Homework Help: Can an object be in equilibrium if it is in motion?

  1. Jun 22, 2016 #1
    The problem statement, all variables and given/known data
    Can an object be in equilibrium if it is in motion? Explain
    * This is a prelab question to a lab that deals with equilibrium and torque

    The attempt at a solution
    My thought process:

    - Equilibrium means that:
    1) Net external forces = 0
    2) Net torque = 0
    - Net torque is related to angular acceleration, so if net torque = 0, then angular acceleration = 0.
    0 acceleration could mean constant velocity which would mean that the object is in motion.
    *Not sure if this second point is correct*
    -If an object is at constant velocity around an axis of rotation, are the external forces at equilibrium?
  2. jcsd
  3. Jun 22, 2016 #2
    Welcome to PF !

    Either the object is at rest or is moving with constant velocity . In the former case we say it is in static equilibrium . In the later it is in dynamic equilibrium .
    If the object is moving with constant speed around an axis of rotation then there must be a net centripetal force acting on it . The object is not at equilibrium .
  4. Jun 23, 2016 #3


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    Actually, there is no such thing, in and of itself and as a stand-alone statement like that. This may sound like nitpicking (and possibly for the purposes of this problem it is) but it is a very important point. All motion is relative. Something may be at rest in the frame of reference of the lab bench on which it is sitting, but it is not "at rest" or "in motion" in any absolute sense (unless it's accelerating, but that's a different story).
  5. Jun 23, 2016 #4


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    From a little Googling, the general view seems to be that one should distinguish translational from rotational equilibrium. An object spinning about its mass centre at a constant rate is considered to be in rotational equilibrium. If its mass centre is not accelerating then it would also be in translational equilibrium. This seems to fit with the OP's view.

    In fact, I added the 'mass centre' qualifier, others don't. But maybe the thinking is that if it is spinning about some other axis at constant speed then it is in rotational equilibrium (no net torque) but not translational equilibrium.
    Last edited: Jun 23, 2016
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