Rolling Friction and Bicycle Tires

[pureouchies4717]

my teacher Never went over this in class. ever. please help, thanks

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?

 

[pureouchies4717]

i tried to use this formula:

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

and got

ur=.075, but it said i was wrong

You are off by a constant. Be sure you are using the right equation of motion. Also, note that the distance given is not the stopping distance, but rather the distance that the object travels before its speed is reduced by half.

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

 

[pureouchies4717]

anyone?

 

[Hootenanny]

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

 

[pureouchies4717]

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

hmm but the problem is that i don't have $u$; that is actually what I am looking for

 

[Hootenanny]

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.

 

[pureouchies4717]

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.

o i see, but i still don't have time. and i don't really get what that "R" refers to. if its radius, i didnt get it in this problem

 

[Hootenanny]

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

 

[pureouchies4717]

i still don't understand since I am not given mass

 

[Hootenanny]

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

 

[Gianna_07]

so which equations did you use?

 

[cpark43]

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

 

[Hootenanny]

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

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.