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Breaking Heat in Joules

  1. Jan 29, 2010 #1
    1. The problem statement, all variables and given/known data

    Motor running, brakes applied. how much heat in joules will be dissipated?
    mass 250 kg
    diameter 30 cm
    3000 rpm
    torque 120 N-m




    2. Relevant equations

    W=1/2 *I *w2

    3. The attempt at a solution

    I keep going the route of:

    (1)/(2)*250*.3*315

    1*125*.3*315

    125*.3*315

    37.5*315

    11812.5 J

    Can anyone show me what I am doing wrong?
     
  2. jcsd
  3. Jan 29, 2010 #2

    Andrew Mason

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    You will have to explain the question. There is insufficient information here. Give us the complete wording of the question.

    AM
     
  4. Jan 29, 2010 #3
    If the power to the motor is shut off while the motor is running at 3000 rpm, and a brake is applyed, how much heat in Joules will be dissipated in the brakes in order to bring the motor to a stop.

    Figuring W=1/2*I*w2

    Sorry about the messy paper
     

    Attached Files:

  5. Jan 29, 2010 #4

    Andrew Mason

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    Ok. We needed the information in the attached paper about the design of the motor in order to determine the moment of inertia. What is the moment of inertia of the motor armature then?

    You seem to have the right idea here. The energy dissipated in the brake is equal to the kinetic energy of the rotating motor which is [itex]\frac{1}{2}I\omega^2[/itex]. You just have to determine I and convert 3000 rpm into angular speed and then plug in the values (using appropriate units, of course).

    AM
     
  6. Jan 29, 2010 #5
    Accord to my instruction "I" is I=Mr2
    M=250kg
    r2=225cm = 2.25 meter

    Is this correct?
     
  7. Jan 29, 2010 #6

    Andrew Mason

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    No. You have to treat the armature as a solid cylinder. Look up the moment of inertia of a solid cylinder.

    AM
     
  8. Feb 14, 2010 #7
    So I = 1/2 m r2 instead.
     
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