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Homework Help: Unit Conversions Grams to Watts

  1. Mar 4, 2014 #1
    1. The problem statement, all variables and given/known data
    The power available (output) is measured by the velocity at which you are moving and the thrust
    required to create that movement. NOTE: Conversions for proper units are provided below

    Power Available: Pout = Tc * v

    where: Pout = power available (watts)
    Tc = Calibrated thrust (grams)
    v = wind tunnel velocity (m/s)

    In order to determine the efficiency, the units must cancel. To convert the units so that the output
    power is in watts, use the following information:

    can't get the power right help?

    2. Relevant equations

    1 gram = 0.002205 lbs.
    1 m/s = 3.28 ft/s
    1 horsepower = 550 lb*ft/sec
    1 horsepower = 745.7 watts

    v = 3 m/s
    Tc = 0.0040 g

    3. The attempt at a solution

    (0.0040*0.002205) = 8.82 E-6

    (3*3.28) = 9.84 ft/s

    ((8.82 E-6 * 9.84) *550) / 745.7 = 6.401212284 E-5 watts
    Last edited: Mar 4, 2014
  2. jcsd
  3. Mar 4, 2014 #2
    I think you're right in everything except going from lb*ft/sec to watts. Check your unit conversions. I think you should divide by 550 and then multiply by 745.7.
  4. Mar 4, 2014 #3
    I got 0.000 watts using that method but that doesnt sound right because my efficiency calculations after are barely 1% and this data should keep to where they dont get above 80% according to the professor
  5. Mar 4, 2014 #4


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    Staff Emeritus
    Science Advisor
    Homework Helper

    These are extremely small magnitudes you are dealing with, so you should be using scientific notation in your calculations.

    You haven't provided any efficiency calculations, so it is not clear what your problem is.

    It's hard to envision what sort of mechanism you are dealing with (a gnat on afterburners, perhaps), but whatever it is, only microscopic amounts of power are being generated.
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