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Torque, rotational inertia and angular acceleartion

  1. Oct 12, 2013 #1
    1. The problem statement, all variables and given/known data

    In order to link these into a logical sequence, how do they fit together

    2. Relevant equations

    I = rotational inertia a = angular acceleration (I know a different symbol is usually used)
    τ= force x moment arm distance

    3. The attempt at a solution

    so....to get my head around how these interact

    Torque is the amount of rotational force applied to a body.

    rotational inertia is how much the body will resist this force

    angular acceleration is how dpenedne ton these two.

    Situation one.

    If I applied 5Nm of torque to a body that had a rotational inertia greater than this then the body wouldn't move and its angular acceleration would be 0

    Situation two.

    If I applied 5Nm of torque to a body that had a rotational inertia less than this then the body would move BUTwhat if it moves at a constant velocity and doesn't accelerate ? It still has 0 acceleration but is moving ?

    I unsure how torque can length of moment arm x force AND moment of inertia x angular acceleration

    if there is is 0 angular acceleration then moment of inertia x 0 = 0 so ther eis 0 Torque

    but if I'm applying 5Nm of toruqe how can there be 0 torque ?

    I must have missed something fundamental somewhere
  2. jcsd
  3. Oct 12, 2013 #2


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    Homework Helper

    Yeah, it's called units. Look up the units of rotational inertia and angular acceleration.

    BTW, applying a torque to a rotating body always acts to change the angular velocity. If the body is rotating at a constant angular velocity, what must the net torque be on the system?

    Hint: Angular motion has formulas analogous to those of rectilinear motion.
  4. Oct 12, 2013 #3
    what happens though if the body isn't rotating and I want to start it rotating but can't apply enough force to do so ? I'm still applying torque though ....aren't I ?
  5. Oct 13, 2013 #4


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    Yes, you are. How quickly it starts to rotate depends on the amount of torque you apply and what the inertia of the body is.
  6. Oct 13, 2013 #5
    like SteamKing said, yes you are. But the net torque is zero, hence it doesn't rotate or simply some other torque resists the torque you are applying.
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