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Homework Help: Torque question need explaination

  1. Feb 24, 2013 #1
    1. The problem statement, all variables and given/known data
    1.Is it possible to calculate the torque acting on a rigid object without specifying an origin? Is the torque independent of the location of the origin?

    2.A mouse is initially at rest on a horizontal turntable mounted on a frictionless vertical axle. If the mouse begins to walk clockwise around the perimeter of the table, what happens to the turntable? Explain using Newton's Laws.

    2. Relevant equations
    No,equation. Need to explain in words.

    3. The attempt at a solution
    for #1 i put no, we cant find torque without a specifying an origin since we wont know the length. But i dont get the 2nd part about the independent thing.

    for #2 i put,
    If the mouse is initially at rest on a horizontal turntable and walks at a clockwise direction, then the horizontal turntable will move in a counterclockwise direction. Using Newton's third law, the mouse is walking and does not accelerate so all the forces equals zero. Since all the forces equals to zero that means the forces the turntable exerts back the equal but opposite force back to the mouse.
  2. jcsd
  3. Feb 25, 2013 #2
    It depends in what are you trying to calculate...

    *As you know, torque is a vector, so, as a vector it has magnitude and direction. If you try to find [tex]\overrightarrow{\tau}[/tex] the vector you have:

    [tex]\overrightarrow{\tau}=\overrightarrow{r}\times \overrightarrow{F}[/tex]

    r: position vector F: force vector

    In this case, you need a reference system

    *If you are trying to find magnitude you have to calculate [tex]|\overrightarrow{\tau}|[/tex](usually in basic physics known as [tex]|\tau|=|r|\cdot |F|\cdot |\sin \theta|[/tex]), in this case, you only need the [tex]F_{\perp}[/tex] at the point you're interested in and the distance to the axis of turning, there's no need to put an origin.

    Sometimes the problems with torque has lot of forces or asks for angular acceleration, so in this cases you need to work with them as vectors to get all the information.

    I apologize if you don't understand something, my english isn't so advanced, I hope you understand... greetings!
  4. Feb 25, 2013 #3
    Thank you for trying, those are the exact questions they asked me. So yeah...I need answer for the 2nd part of question #1, and confirmation of #2...

    Would greatly appreciated if someone can help me out before midnight, 1h 30min left.
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