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Fictitious Forces ⇔ Constraint Forces? (re: D'Alembert's Principle)

  1. Sep 7, 2012 #1
    Are fictitious forces and constraint forces the same thing?
  2. jcsd
  3. Sep 7, 2012 #2
    No, fictitious forces are forces which arise from analyzing within a non-inertial reference frame. Constraint forces are those which arise from a geometrical configuration.

    For example, a constraint force would be perhaps a normal force exerted by the surface an object rests on such as an inclined plane problem. As long as the object is on the inclined plane, the object's motion is constrained to be along the inclined plane.

    For a fictitious force consider that the Earth is in fact a rotating reference frame and therefore non-inertial. This gives rise to the Coriolis force which is needed to correct calculations due to this fact. It is small in effect but needed especially in large-distanced calculations.
  4. Sep 8, 2012 #3
    Yes, but how can geometry cause forces? Isn't force a change in momentum? Doesn't force imply movement? If a force isn't doing work, how is it a force?
    Yes, but what causes the constraint force, if not inertial effects?
  5. Sep 8, 2012 #4

    Andrew Mason

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    Constraint forces self-adjust so that they are exactly equal and opposite to the force they are opposing. So they do not do work and they do not cause a change in momentum. They balance other forces that would otherwise do work/cause change in momentum.

  6. Sep 8, 2012 #5


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    One should remember that NET forces produce a change in momentum. Any one individual force may or may not lead to a change in momentum.

    A force is not required to do work. Work is the dot product of force and distance, so if the distance is 0, or if the force is applied perpendicular to the direction of motion, then the force does no work.
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