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Thread: [SOLVED] linear/angular speed?

  1. #1
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    [SOLVED] linear/angular speed?

    If a rocket car with 20 inch radius tires brakes the sound barrier (a speed in excess of 760 miles/hour), what is the angular speed of the wheels in revolutions/second?

    ok i am totally lost.....all i know is one revolution would probably be 40pi inches because that the circumfrence of the circle...and i thought i probably have to use the formula v=rw (v=linear speed, r=radius and w=angular speed)

    so i started with

    760mi/hr = 20w but im confused with the units??? i dont know if i even started it off right ???

    thank you(:
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  2. #2
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    Linear/angular speed

    Hello desperate_on_sunday_night
    Quote Originally Posted by desperate_on_sunday_night View Post
    If a rocket car with 20 inch radius tires brakes the sound barrier (a speed in excess of 760 miles/hour), what is the angular speed of the wheels in revolutions/second?

    ok i am totally lost.....all i know is one revolution would probably be 40pi inches because that the circumfrence of the circle...and i thought i probably have to use the formula v=rw (v=linear speed, r=radius and w=angular speed)

    so i started with

    760mi/hr = 20w but im confused with the units??? i dont know if i even started it off right ???

    thank you(:
    Use $\displaystyle v = r\omega$ if $\displaystyle \omega$ is measured in radians. Here, stick to your first idea, and work with the circumference of the tire, which is $\displaystyle 40\pi$ inches.

    So, in 1 revolution, the car moves forward $\displaystyle 40\pi$ inches $\displaystyle = 125.68$ inches.

    And we know that in one hour, the car would move forward $\displaystyle 760$ miles. So in one second it moves forward $\displaystyle 760 \div 3600$ miles, or $\displaystyle 760 \times 5280 \times 12 \div 3600$ inches, which $\displaystyle = 13376$ inches.

    So, how many revolutions do we need to make this many inches? Answer: $\displaystyle 13376 \div 125.68 = 106.5$. So there's your answer: just over $\displaystyle 106$ revolutions per second.

    Grandad
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