# Homework Help: A homework assignment including rotation of a rigid body

Tags:
1. Mar 14, 2015

### stipan_relix

1. The problem statement, all variables and given/known data
The problem is this: A car is evenly slowing down from 30 km/h to 0 in the time of 2 seconds. Radius of its wheels is 30 cm. What is its angular acceleration and what total angle will the wheel describe until it stops? How many turns does the wheel make and what is the length of the path the car travels while stopping?

2. Relevant equations
$$\alpha = \frac{\Delta \omega}{\Delta t}$$$$\omega = \frac{\phi}{t}$$$$\phi= \frac{\alpha}{2}t^2$$$$s = vt$$

3. The attempt at a solution
I honestly tried to use moment of inertia and tangential force equations and I just can't figure it out, every time I find a matching equation, something is missing and I can't solve it. Please help me, at least by some tips if you don't wanna do the whole problem. Thanks.

2. Mar 14, 2015

### Quantum Defect

You are over-thinking this. Think about how you can change the linear speed into a rotational velocity of the wheel. Hint: Today is appropriately 3/14/15, using the us convention for the date! From the initial angular velocity and the time to stop, you can calculate alpha, and on you go.

3. Mar 15, 2015

### stipan_relix

Thank you, I'll try it and report back!