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Number - Revolutions / Partial Solution

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Randy Dandy
 
Reply Wed 30 Sep, 2015 11:44 am
A bicycle is traveling at 10 mph. Determine number of revolutions the wheels make per min.
Diameter of wheels - 28 in.

What is known:

A bike is traveling at 10 mph.
Diameter of wheels are 28 in.

Want to know:

Number of revolutions per min. the wheels make.

Partial solution with problem:

Circumference of wheel - pi x 28 in.

The wheels make r revolutions per min.

The wheels travel pi x 28 x r in. in 1 min.

The wheels travel pi x 28 x r x 60 in. in 1 hr.

How do I proceed from here ?

Non - homework question.


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engineer
 
  1  
Reply Wed 30 Sep, 2015 11:59 am
@Randy Dandy,
Clearly a homework question.

1) Convert 10 mph to inches per minute. You can do this instantly in Google by typing in "10 mph in inches per minute" but if you are smart, you will calculate it yourself by hand like you will have to do on the test.

2) Calculate the circumference of the wheel in inches. It is really inches per revolution.

3) Divide the speed in inches per minute by the circumference in inches per revolution to get revolutions per minute.

Randy Dandy
 
  1  
Reply Wed 30 Sep, 2015 12:13 pm
@engineer,
Not a homework question. I have been out of school almost 40 years and am trying to relearn math/algebra.
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Tes yeux noirs
 
  0  
Reply Wed 30 Sep, 2015 12:56 pm
Engineer, randydandy is an adult who does arithmetic and algebra problems for a hobby.

Convert mph to inches per minute then divide the result by the circumference of the wheel in inches.

Revs per minute = ((mph * 63360) / 60) / (pi x 28)


Randy Dandy
 
  0  
Reply Wed 30 Sep, 2015 02:57 pm
@Tes yeux noirs,
My attempt:

5280 x 10 = 52800 ft. = 52800 x 12 = 633600 in. / 60 = 10560 in. per min.
pi x 28 (diameter of wheels) = 87.6948 = 88 in.

10560 / 88 = 120 revolutions per minute.

Is that correct ?
Tes yeux noirs
 
  1  
Reply Wed 30 Sep, 2015 03:56 pm
@Randy Dandy,
Inches in 1 mile: 63360
Minutes in 1 hour: 60
pi = 3.14159265 to 8 d.p.

Inches per minute = (mph * 63360) / 60
Inches per minute = (10 * 63360) / 60
Inches per minute = 633600 / 60
Inches per minute = 633600 / 60
Inches per minute = 10560

circumference = pi x 28 inches
circumference = 3.14159265 x 28 inches
circumference = 87.9645942 inches

rpm = inches per minute / circumference
rpm = 10560 / 87.9645942
rpm = 120.0483000
Randy Dandy
 
  0  
Reply Wed 30 Sep, 2015 04:27 pm
@Tes yeux noirs,
Thanks.
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