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Internal Forces

  1. Feb 29, 2016 #1
    1. The problem statement, all variables and given/known data
    My textbook says that internal forces have no effect on an object's motion and that external forces are required to change the motion of an object.

    But, we looked at an expirement where a raw egg was spun and then stopped. After being stopped, it went back to spinning because the egg yolks inside the shell resisted the change in motion due to inertia and kept moving, causing the egg shell to begin rotating again.

    Isn't this an example of an internal force affecting the motion of an object? This doesn't allign with what my textbook said.
    2. Relevant equations
    Newton's First Law

    3. The attempt at a solution
    I have no clue.
  2. jcsd
  3. Feb 29, 2016 #2


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    Internal forces have no effect on the motion of the center of mass of the object. That is to say that they have no effect on the object's momentum. That does not stop you from throwing your arms forward and having your body move backward as a result.

    Internal torques have no effect on the angular momentum of the object. That does not stop you from windmilling your arms forward and generating a backward rotation in the rest of your body.
  4. Feb 29, 2016 #3


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    It depends what you mean by "the object".
    If you consider the fluid and the shell as separate objects then the forces in question are not internal to either.
    If you consider the whole egg as the object then what do you consider its motion to be at any instant? Looking only at the outside does not give the whole picture. After releasing the egg, as the shell starts to rotate again both the total angular momentum of the egg and its total energy remain (almost) constant. (Some of the KE is turning into thermal energy, though.)
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