## moongazer 3 years ago The speed of a car is 80 miles per hour. Find the radius of the car wheels in inches if the car makes 300 rev/min.

1. moongazer

I got r=44.82 inches is it correct?

2. amistre64

how many revolutions per hour does it make?

3. moongazer

5 rev per hour

4. amistre64

really? it does 300 revs in one minute; how can it do way less revs in one hour?

5. amistre64

300*60 ...

6. moongazer

sorry for that I interchanged it :) 18000 per hour

7. amistre64

18000 revs in one hour put to the ratio of 80 miles 18000/80 is our next step i believe

8. amistre64

of if we knew how many inches 80 miles was we could go more directly to an answer ....

9. moongazer

why did you made it into ratio?

10. amistre64

becasue that is what it is: 18000 revs = 80 miles i spose since we want to know how far 1 rev takes us its better to make the rev side equal to 1 tho divide both sides by 18000

11. amistre64

1 rev = the circumference of the wheel

12. moongazer

sorry I got confused :)

13. amistre64

if we convert 80 miles to inches; then we know how many inches 1 rev is; so we would know how many inches the circumference of the wheel is knowing circunference is equal to 2pi*radius; we solve for the radius r = C/2pi

14. moongazer

Sorry, but I am really confused with your method. :) I did angular and linear speed with this one.

15. amistre64

google says: 1mile=63360inches multiply both sides by 80 we get 80 miles = 5 068 800 inches 18000 revs = 5 068 800 inches divide both sides by 18000 to get 1 revolution 5 068 800/ 18000 = 281.6 281.6 inches is our circumference therefore, using what i posted up above; radius = 281.6/(2pi)

16. amistre64

i did the same, i converted angular (rpm) into linear (miles, inches)

17. amistre64

of course i get about a wheel that is about 7 feet in diameter lol

18. moongazer

brb. i'll try to understand your method. it looks good :) still, I think we got the same answer but different method of getting it.

19. amistre64

44.82, yes

20. moongazer

and I think your method is faster :)