1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Basic question
  1. Basic Questions (Replies: 4)

  2. Basic Questions (Replies: 12)

  3. Basic Question (Replies: 3)

  4. Basic question (Replies: 6)

Loading...