Rolling Friction of Bicycle Tires

[neongoats]

Homework Statement

Two bicycle tires are set rolling with the same initial speed of 3.30m/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 17.3m ; the other is at 105 psi and goes a distance of 93.0m . Assume that the net horizontal force is due to rolling friction only and take the free-fall acceleration to be g = 9.80m/s2
What is the coefficient of rolling friction μr for the tire under low pressure?

Homework Equations

Fx=ma
Fx=μn
n=mg
v2=v02+2a(x−x0)

The Attempt at a Solution

First I solved for the acceleration:
1.65^2 = 3.3^2 + 2a(17.3)
a= -.24

Then I set Fx=ma and Fx=μn to be equal, and substituted n=mg in for n
ma=μmg
The masses cancel out so I get
a=μg
I tried solving for this
-.24=u(9.8)
and got -.024, but this answer was incorrect.

I feel good about this process but don't know I'm doing wrong? I found another question exactly like this on here but when I tried to do the calculations for acceleration I kept getting a different number, so I don't know if that's where my problem lies but if so I don't understand how my math is wrong.

 

[CWatters]

Have a think about the concept of a negative coefficient of friction.