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Help with physycs problems.

  1. Oct 30, 2006 #1
    need some help with some physics homework, would really appreciate explanations for these problems and not just answers :D, neways here they are!

    An automobile crankshaft transfers energy from the engine to the axle at the rate of 38.0 kW when rotating at a speed of 2900 rev/min. What torque does the crankshaft deliver?

    If a 38.0 N·m torque on a wheel causes angular acceleration 22.9 rad/s2, what is the wheel's rotational inertia?

    A disk, initially rotating at 150 rad/s, is slowed down with a constant angular acceleration of magnitude 3.4 rad/s2. (a) How much time does the disk take to stop? (b) Through what angle (rad) does the disk rotate during that time?

    Thank you ahead of time! :D
     
  2. jcsd
  3. Oct 30, 2006 #2

    Office_Shredder

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    I'm sure you've seen the expression P=FV, where P is power, F is force, and V is velocity. Can you think of an analogous situation for rotational motion?

    Going back to linear mechanics again, I'm sure you've seen f=ma What's the analogous situation for rotational motion?

    Can you think of a formula that, given initial velocity and deceleration, let's you calculate how long it takes for something moving in a straight line to stop? Try converting all of that into rotational terms. Then use a distance formula that, given initial velocity, time, and acceleration, let's you find distance, and try finding an analogous rotational equation
     
  4. Oct 30, 2006 #3
    I'm sorry to be a hassle, but that didnt really help me at all :\ , can someone show me some formulas to help me solve these questions ?
     
  5. Oct 30, 2006 #4

    Hootenanny

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    You must have met some formulae during your studies of rotational motion. Perhaps you could hazard a guess as to which would be appropriate here?
     
  6. Oct 30, 2006 #5
    i have some formulas, but im not really sure how/what to use. Would u like me to start listing formulas that i have?
    Once again, i'm sorry for any hassle. TY for help
     
  7. Oct 30, 2006 #6

    Hootenanny

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    It is no hassle at all, Officeshredder gave you some pretty good hints with his post. Okay lets break the first one down.
    Now, as Officeshredder said, the expression P=Fv, is valid for linear motion. Now, could you take a guess at what the rotational version would be? Or have a glance at your notes for something similar.
     
  8. Oct 30, 2006 #7
    KE = 1/2 Iw^2
    Would this one be suitable for our current problem?
     
  9. Oct 30, 2006 #8

    Hootenanny

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    I was thinking more of [itex]P=\tau\omega[/itex].
     
  10. Oct 30, 2006 #9
    Just realized i posted my problems backwards from the list of my HW, so i think that formula actually helps the last problem i listed.

    Our angular velocity is 2900 rev / min.
    Our power is 38 kW
    so (not quite sure how to make that symbol) = 38/2900 would be our solution?
     
  11. Oct 30, 2006 #10

    Hootenanny

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    We're almost there, but you must consider your units. Note that power is in kilowatts and our angular velocity is in rev/min, you must convert your values into SI units before doing the calculation; otherwise, you get yourself in all sorts of a mess. The SI units of power are watts and the SI units of angular velocity are radians per second.
     
  12. Oct 30, 2006 #11
    38000w/
    130(3.14)/sec
     
  13. Oct 30, 2006 #12

    Hootenanny

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    Your power conversion is correct, but you may wish to check your velocity conversion.
     
  14. Oct 30, 2006 #13
    2900 /60 = to seconds
    48.33 *2(pie) = 96.67(pie)

    not sure how i got 130, but how about this
     
  15. Oct 30, 2006 #14

    Hootenanny

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    Looks alot better :smile:
     
  16. Oct 30, 2006 #15
    would i use the same formula for the second problem?

    p=(38Nm) (22.9rad/s^2)
     
  17. Oct 30, 2006 #16

    Hootenanny

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    No, you need to find the moment of inertia (I). The formula you need is analogous to the linear equation F=ma.
     
  18. Oct 30, 2006 #17
  19. Oct 30, 2006 #18

    Hootenanny

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    Not quite. Think about F=ma, m is the property of matter which gives it inertia....
     
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