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Angular Momentum: Rotating Object

  1. Dec 11, 2014 #1
    1. The problem statement, all variables and given/known data

    I'm trying to understand an example from my textbook about angular momentum. This is the example given:
    upload_2014-12-11_16-18-1.png

    For the part in red: I don't understand where the cosine theta term came from. When you're calculating the magnitudes of torques, don't you just use FRsin(theta)? If someone could clear that up for me, it would be great! Thank you. upload_2014-12-11_16-18-1.png
    upload_2014-12-11_16-18-1.png
     
  2. jcsd
  3. Dec 11, 2014 #2

    haruspex

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    If the theta is the angle between the force vector and the distance vector, yes. But in the diagram, the angle between the vectors is the angle between the vertical and the seesaw. Theta is the angle between the horizontal and the seesaw.
     
  4. Dec 11, 2014 #3
    Ok. I think I get it; the cosine is used to get the moment arm in this case, correct?
     
  5. Dec 11, 2014 #4

    haruspex

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    That's one way to look at it. There are at least 3 ways, leading to the same answer:
    - distance cos (theta) = moment arm.
    - force cos (theta) = component of force perpendicular to distance
    - vector product of force and distance = force * distance * sin(90-theta)
     
  6. Dec 12, 2014 #5
    Ok, I understand. Thank you very much.
     
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