Question

Please help!

A bicyclist races on a bicycle with 13-inch raduis wheels. When she is travleing at a speed of 49 ft/sec, how many revolutions per minute are her wheels making?

Answer 1

Use the circumference of the bicycle wheel.

Answer 2

Once you have the circumference, you'll know how many revolutions in a second because of the speed of 49 feet/sec. Once you get your answer in seconds, multiply by 60 for revs per minute.

Answer 3

so you find circumference first? with is c=3.14d?

Answer 4

Yes, but best to set up a string of factors as it gives the whole picture at once with all steps. Like so: 49 ft/sec x 60 sec/min x 1 rev/(pi)(d)(inches) x 12 inches/ft.

Answer 5

Actually, a great trick to use is to set up where the "measures" cancel out. We know we eventually want rev/min, so somewhere we will multiply by something that has sec/min to get min in the denominator. Also, another trick is multiplying by "1" as in 60 sec / min which is a constant and just converts units.

Answer 6

ya i got 432.1411073

is that right?

Answer 7

Depending on the value you used for pi, that would be about right. I used 3.14159 and got a value very close to yours.

Answer 8

Looks like you used 3.14 which is good enough.

Answer 9

oh ok! thanks!

Answer 10

More importantly, did you get the factors and the concept of cancel units and conversion? That's the key and will allow you to do these on your own.

Answer 11

ya sorta. i need to remmeber the conversions

Answer 12

A lot of these types of questions just are a matter of practice. You'll obviously have no trouble with things like 60 secs / min, but you may or may not remeber circumference formula.

Answer 13

ok hey what answer did you get? cuase it says that mines is wrong idk why. it says that i have to round to the nearest hundreth. so i put it 432.1 the answer i got is 432.1211

Answer 14

Well, hundredth is the second decimal position to the right (tenths is the first). So, you have to round to 2 decimal places. 432.14 should be right if you used (and if you are supposed to use) 3.14 for pi.

Answer 15

Usually, if there is a key to check by, the question tells you (or somewhere in the general directions) what values you are supposed to use for constants like pi.

Answer 16

still says im wrong.

Answer 17

432.14?

Answer 18

ya

Answer 19

What value are you supposed to use for pi?

Answer 20

the teacher uses the pi sign on the caltulator at times and other times 3.14

Answer 21

I would bet that the answer is differing based on the value for pi. Let's try a calculator value with lots of decimal places. Wait a sec and I'll try a different value for pi.

Answer 22

ok thanks tcarroll!

Answer 23

If you use your calculator pi, then you should get 431.92 rounded to 2 decimal places. How does that check out?

Answer 24

thats right ! ok so use the calculator pi?

Answer 25

Well, your teacher is supposed to tell you what value to use, but in the absence of him telling you that, it makes sense to use calculator pi. Otherwise, rounding to two decimal places with using 3.14 wouldn't make much sense.

Answer 26

It would be like saying a field goal kicker can kick 55.04 yards. No one gives stats like that. It's to yards rounded to whole yards. See what I mean?

Answer 27

ya i gotcha!! thank you!