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Homework Help: A spinning top - moment of inertia/ Torque/ Angular momentum

  1. Aug 2, 2010 #1
    Hey guys,

    I have this really annoying last question on my assignment which is a pain. It combines 3 physics principles together.

    I am having problems specifically with 2) 3) and 4)

    2)I know that T = | r x F |, but what kind of general vector do I use to represent F?
    4) I have no clue how to do this one unfortunately.

    The question should be in the attached thumbnail

    Attached Files:

  2. jcsd
  3. Aug 2, 2010 #2
    The only force acting on the top is gravity.
  4. Aug 2, 2010 #3
    For part b, note that torque is a vector: [tex]\vec{T}=\vec{r}\times m\vec{g}[/tex] as the torque is calculated about the origin. You can express [tex]\vec{r}=d\hat{r}[/tex].

    For part c, the problem has already given you the answer. Just plug moment of inertial in.

    For part d, again, the problem has already shown you the way. Substitute L and T found earlier in the equation: [tex]\vec{T}=d\vec{L}/dt[/tex]. You have known that the top will rotate around the Y axis beside spinning very fast about its own axis of symmetry (because of this "very fast" spin, we can achieve L as in part c). That means the [tex]\hat{r}[/tex] vector will rotate around the Y axis just like the shaft. If so, then how would you relate [tex]d\hat{r}/dt[/tex] , the angular speed of the rotation about the Y axis [tex]\Omega[/tex] and the vector [tex]\hat{r} \times \hat{g}[/tex]. Note that [tex]\hat{g}[/tex] is only a unit vector whose direction is downward, nothing special. Find [tex]\Omega[/tex] and then the time needed.
  5. Aug 6, 2010 #4
    Nice one Jong trying to get the internet to solve the final question.
  6. Aug 6, 2010 #5
    hahahahhaa oh thats classic mate!
  7. Aug 9, 2010 #6
    hey clive this doesnt help at all so stop trying to get the answer from it.
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