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Homework Help: Basic question

  1. Oct 22, 2003 #1
    Hi everyone this is hopefully a quick and easy question:

    If a person spins an object that rotates, is that considered torque?

    When that object that is rotating, are the forces that act upon it: gravity, air resistance, friction, and the beginning twist from that person, newton’s second law?
  2. jcsd
  3. Oct 22, 2003 #2
    If a force acts upon an object so that the object's rotation changes, then the torque is
    T = dL/dt.
    So, if the person speeds up the object's rotation then, yes, there is a nonzero torque.

    Concerning Newton's laws, I never know which is which, so please help me.
  4. Oct 22, 2003 #3
    Newton 2 law F = ma F = I* alpha
  5. Oct 22, 2003 #4
    I see. Should read
    T = I[alpha].

    Now you ask "are the forces ... Newton's second law".
    Sorry for nit-picking a bit, but of course forces cannot 'be' a law.
    Better to ask: "Does this law apply to a situation in which these forces act on a spinning object".

    My answer is: "Yes, but."

    Here's the 'but':
    T and [alpha] are both vectors. So I must be a tensor (something representable by a matrix).
    This means, physically, that a rotating body does not have >it's< moment of inertia I, but I is defined WRT an axis.

    In general, a spinning top will not have a constant axis of rotation, but will show nutation and precession. Just think of a gyroscope.

    OK, you could say "I do not care about nutation and stuff, I just assume the axis stays the same all the time".

    In this case, |T| = I|[alpha]| (a scalar equation) is correct.
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