# Angular acceleration of a tire...not sure how to find radius

1. Sep 30, 2015

1. The problem statement, all variables and given/known data:
Your car tire is rotating at 3.5 rev/s when suddenly you press down hard on the accelerator. After traveling 200 m, the tire’s rotation has increased to 6.0 rev/s. What was the tire’s angular acceleration? Give your answer in rad/s2.

2. Relevant equations:
2αΔΘ=ωf2i2

3. The attempt at a solution:
We have 200m as a distance. With the absence of a tire's diameter, I tried (200 m)/(2πr rads) but again I don't know the radius. The book's solution manual uses ΔΘ=Δx/r and then simply states that the tire's radius is 32cm. I have no idea where this number came from.

2. Sep 30, 2015

### tony873004

leave delta phi as x / r. Also, don't translate rev / second into rad /s. Instead, translate rev into 2 pi r, and you'll find the r's cancel.

3. Sep 30, 2015

I don't understand. Isn't the definition that rev=2π?

4. Oct 1, 2015

### haruspex

There is only one variable which involves the dimension of distance. That makes it useless. There is simply not enough information.