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Homework Help: Where the energy goes.?

  1. May 27, 2013 #1
    Flyheel the storage energy.

    The transfer from mechanical energy to electric energy was equal.
    The situation was, i got a 1/2 hp electric motor (EM) that can turning a 5 kg flywhell (∅ 28 cm) to 1500 RPM within 10 second. (connected by gear to each other)
    question was:
    1. at the 11 second, i turn off the EM, how much energy stored in flywheel until it stop turning.
    2. at the 11 second, i turn off the EM, then the flywheel start to reduce it speed to 1300 RPM. At that moment, i'm starting turn on the EM again. How much energy from EM needs to turning and to regain the top speed of the flywheel (from 1300 to 1500 RPM).?

    Thanks alot for the answer.
    Last edited: May 27, 2013
  2. jcsd
  3. May 27, 2013 #2


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  4. May 27, 2013 #3
    Thanks DennisN,

    according http://www.botlanta.org/converters/dale-calc/flywheel.html
    at 1300 RPM (5000 grams, dia = 283 mm) Disk KE (joule) = 463 j
    at 1500 RPM = 617 j
    is that mean, i need 617j-463j = 154 joule to regain the top speed.?

    If something connected by the flywheel (blade of windmill or pump), energy from EM and energy that stored on flywheel was turn to a mechanical energy, if flywheel connect to none of subject, the energy are use to increasing and maintainance the RPM when flywheel reducing the speed, and some other are.. gone with the wind.? :)

    back to question no 1, which expressed theory to answering that.?
    Last edited: May 27, 2013
  5. May 27, 2013 #4


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    Where does "wasted" energy generally go in mechanical systems?
  6. May 27, 2013 #5
    most are go/used to move "a thing", and finally get suck by "a thing". and disappear..
    like energy mechanical we used to pushing a wall, and does the wall stored our energy? or disappear?
    because energy can not be create or destroy, so it just flow... wasted, unused, disappear?
    like a energy from wind before it get captured by windmill. it nothing but a wind .. i'm i right.?

    i'm still need help for question no 1 :) please
  7. May 28, 2013 #6


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    You do not need energy to apply a force on a wall, if the wall does not move.

    Energy does not disappear, but it can get converted to another (non-mechanical) type of energy. This usually happens everywhere in mechanical systems. Hint: you can feel it.

    What do you mean with question 1, the energy stored at 1500 rpm? You calculated that value already.
  8. May 28, 2013 #7
    again i'm using calculating using : http://www.botlanta.org/converters/dale-calc/flywheel.html
    Metric (gram, mm)
    Mass = 15000
    Diameter = 280
    RPM = 1500

    Surface Speed (M/sec) = 21.99
    Interia = 0.147
    Disk KE (joules) = 1814.01
    Is this the "only" stored energy on flywheel from 1500 RPM until it stop turning.? or rapidly energy output persecond (and the amount was decrease following the slowing revolution of the flywheel it self) ie. it gives out put 1754 joules at 1475 RPM

    Centrifugal Force
    (Kg) = 5258.07
    (Newton) = 51828
  9. May 28, 2013 #8


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    no, thats not the answer mfb is looking for ;)

    read this post of his again ... carefully :)

  10. May 30, 2013 #9
    if it convert to (non mechanical ) type of energy. for mfb question the answer was heat. like when i using my weld machine, and yes too for most of friction betwen 2 or more mechanical thing, like friction on gear, V-belt, and even it not touching one to another, like friction betwen rotor and stator. The motor increasing their heat
    Last edited: May 30, 2013
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