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Homework Help: Harmonic movement

  1. Mar 1, 2009 #1
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Mar 1, 2009 #2

    tiny-tim

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    Hi transgalactic! :smile:

    No, the equation given is -∆mgR/2 sinθ = (I1 + I2)θ''

    and the left hand side is the torque (not minus the torque) … that's the way θ is defined …

    the rock inside pulls down, decreasing θ. :wink:
     
  4. Mar 2, 2009 #3
    but the this torque is going counter clock wise
    it should be taken as positive.
    i cant understand what it has to do with

    can you explain this part..
     
  5. Mar 2, 2009 #4

    tiny-tim

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    No, the torque of the inside rock about the centre is clockwise. :smile:
     
  6. Mar 2, 2009 #5
    you are correct
    but the problem is if the rock tilts to the left side
    the solution still gives a minus sign
    despite the fact that the torque is positive
    i was told that its because of the sinus graph
    ?
     
  7. Mar 2, 2009 #6

    tiny-tim

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    I'm not following you :confused:

    if the inside rock tilts to the left side, then sinθ is negative, so the equation means the angular acceleration must be positive … which it is! :smile:

    yes, the torque is the same sign as the angular acceleration (anti-clockwise and positive) …

    it will be in any apparatus …

    the equation doesn't say "negative torque = acceleration", it says "torque = acceleration", but the formula for the torque just happens to have a minus in it.
     
  8. Mar 2, 2009 #7
    i dont use negative sinx
    i dont have such thing
    i just take the hypotenuse multiply it by sinus theta *Mg
    and then look if its goes clock wise or counter clockwise.

    if the rock leans to the left and we loo from the point the rock touches the ground
    Mg*sin theta=Ma^2

    but in another solution it states
    -Mg*sin theta=Ma^2

    why?

    how to get the minus using the method i use??
     
  9. Mar 2, 2009 #8

    tiny-tim

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    erm … don't use your method!

    don't "look if its goes clock wise or counter clockwise"!

    physics is equations …

    if x is negative, then sinx is negative

    you can't write "just look at the diagram and you can see it's clockwise" in the middle of an exam proof! :smile:
     
  10. Mar 2, 2009 #9
    but the angle is a parameter
    and if x is negative
    then sinx will give us a negative value

    we cant say
    -sinx

    its just adding a minus to the sinus
     
  11. Mar 3, 2009 #10
    can you explain this stuff??
     
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