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Need help with a Trig question
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Thread: Need help with a Trig question

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  1. #1

    Need help with a Trig question

    Ok, Im kinda confused on a trig question I have for homework. Usually the answers are in the back to check the work, but not this one. Here is the question:

    The diameters of the front and rear tires of a tractor are 36 in and 64 in respectively. Find the RPM of the front and rear tires when the tractor is traveling 9 miles/hour.

    Now, here is my figuring:

    9 miles 1 5280 12 1 rev 1 hour
    ---------- * ---- * -------- * ----- * ------- * ---------
    1 hour 18in 1 mile 1 ft 2PI 60minutes

    So that reads 9 MIles over 1 Hour * 1 over 18 inches * 12 inches over 1 foot * 1 Rev over 2PI * 1 hour over 60 minutes. The 1 Rev over 2PI tranfers the answer to angular velocity needed for the RPM measurement, the 1 hour over 60 minutes gets rid of the hour under the 9 Miles, the 5280 ft gets rid of the miles and transfers to feet, and then the 12 inces over 1 ft gets rid of the feet. NOW, the answers I got were
    84 rpm for the small wheel 36 in diameter
    47.2rpm for the large wheels 64in diameter.

    Am I right or totally off base?

  2. #2

    Re:Need help with a Trig question

    hey mountainman,
    figure the number inches per minute which gave me 9504 in/min.

    From here I did a 2*pi*r * (x) RPM = 9504 in/min (because 2*pi*r = 1 rev /min)

    then i did a

    9504 in
    ---------------- = (x) RPM
    2pi*r in min

    solving for each x gave me:

    64 inch wheel = 23.63 RPM
    36 inch wheel = 42.01 RPM

    Then I scratched my head wondering why, why 'o' why my answers were double yours - then i saw it, i had the 64 and 36 as radius - so - your answers are correct.

    You totally right.

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