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Edge of a fan blade

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Edge of a fan blade

by yasem77 » Mon May 25, 2009 10:47 am
A point on the edge of a fan blade that is rotating in a plane is 10 centimeters from the center of the fan. What is the distance traveled, in centimeters, by this point in 15 seconds when the fan runs at the rate of 300 revolutions per minute ?

(a) 750pi
(b) 1500pi
(c) 1875pi
(d) 3000pi
(e) 7500pi

Does anybody understand the question? I have a big understanding problem. Can somebody help? Thanks.
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by axat » Mon May 25, 2009 12:00 pm
Look up at the fan and imagine a dot on one of the 3 blades. Now the distance of this from the centre of the fan is 10 cm, which is also the radius of the fan.

300 revolutions per minute means 300/4 revolutions in 15 seconds, i.e 75 revolutions.

Now the circumference of the fan( for it makes a circle when it is switched on) is 2(pi)r. i.e 20pi. That's the distance the point travels in 1 revolution. Therefore, it will travel 20 x 75 x (pi) in 15 seconds.

Answer is B. Is it?
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by tdadic84 » Mon May 25, 2009 12:07 pm
I agree to B...

300/60 = 5 per second
5 X 15 = 75 per 15 seconds...

circumfrance = 2piR = 10(2) pie = 20 pi . . . . 20 pi X 75 = 1500pi

hence b...
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