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Units problem

  1. Feb 1, 2015 #1
    1. The problem statement, all variables and given/known data

    So I am calculating torque and power for a disk of radius 0.5 m that is subjected to a force of 50 N at its periphery and is rotating at angular velocity of 100 rad/s. Find torque and power.

    2. Relevant equations

    Torque= radius x Force = 0.5m *50N = 25Nm

    Power= dW/dt= F*w(omega)= 25Nm * 100 rad/sec .... WHy the solutions book has an answer 2500 Nm/s. Why did they ignore the radians. I was thinking to convert the radians into meters or something like that. What am I not understanding here?
  2. jcsd
  3. Feb 1, 2015 #2


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    Staff: Mentor

    Radians are a sort of "unitless" unit. They arise out of ratios of like units, such as the ratio of the circumference of a circle to its radius (with suitable unitless constants involved). So when they get mixed up with "real" units they have a tendency to disappear. So, rad*m = m, rad*kg = kg, and so on.
  4. Feb 1, 2015 #3
    Thank you for the response. It makes sense now. Will do further research on this to make sure I understand.
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