# Homework Help: Rolling Friction and Bicycle Tires

1. Feb 5, 2006

### pureouchies4717

Two bicycle tires are set rolling with the same initial speed of 3.70 m/s along a long, straight road, and the distance each travels before its speed is reduced by half is measured. One tire is inflated to a pressure of 40 psi and goes a distance of 18.4 m; the other is at 105 psi and goes a distance of 94.0 m. Assume that the net horizontal force is due to rolling friction only and take the free-fall acceleration to be g = 9.80 m/s^2. What is the coefficient of rolling friction ur for the tire under low pressure?

2. Feb 5, 2006

### pureouchies4717

i tried to use this formula:

ur= v/[(d/v) x g]

and got

ur=.075, but it said i was wrong

^ what exactly does it mean to be off by a constant?

Last edited: Feb 5, 2006
3. Feb 5, 2006

anyone?

4. Feb 5, 2006

### Hootenanny

Staff Emeritus
Use the kinematic formula $v = u + at$ to obtain an acceleration. Then equate this with the force in $F_r = \mu R$

5. Feb 5, 2006

### pureouchies4717

hmm but the problem is that i dont have $u$; that is actually what im looking for

6. Feb 5, 2006

### Hootenanny

Staff Emeritus
Yes you do, $u$ refers to the intial velocity. $\mu$ is the coeffiecent of friction. Sorry for the different symbols, its just the notation I'm used to.

7. Feb 5, 2006

### pureouchies4717

o i see, but i still dont have time. and i dont really get what that "R" refers to. if its radius, i didnt get it in this problem

8. Feb 5, 2006

### Hootenanny

Staff Emeritus
R is the normal reaction force, which is the product of the mass and the gravitational field strength.

9. Feb 5, 2006

### pureouchies4717

i still dont understand since im not given mass

10. Feb 6, 2006

### Hootenanny

Staff Emeritus
Masses will cancel, this is the same process you used for the boy/swing/inclined plane question.
$$ma = \mu mg$$

11. Oct 17, 2007

### Gianna_07

so which equations did you use?

12. Sep 30, 2008

### cpark43

How do you know what time is? when using v = v + at..

13. Sep 30, 2008

### Hootenanny

Staff Emeritus
Looking back on this thread from two years ago, it would have been more appropriate to use the equation v2 = u2+2as. I don't know why I suggested the original one.