# Angluar Kinematics

1. Sep 1, 2011

### bionut

1. The problem statement, all variables and given/known data

A cyclist is traveling at a speed of 30km/hr. If the diameter of the bike wheel is 0.70 meter, what is the angular velocity of the wheels in rad/sec?

2. Relevant equations

3. The attempt at a solution

Hi, I have racked my brain trying ot work this out, I know the answer is 23.7 rad/sec
(but I don't know hoe to get there ;-(!!
All I could come up with is 0.83m/s=speed and r=0.07m

2. Sep 1, 2011

### Hootenanny

Staff Emeritus
Welcome to Physics Forums.

The speed of the bicycle is the tangential speed of the wheels (assuming no slipping). So, the question now becomes, how is the tangential speed related to the angular speed of a disc?

3. Sep 1, 2011

### bionut

okay, not sure if im thinking is on the right track, but would the angular speed be in reference to revolutions/time period? Where 1 revolution - 2(pi)r?

4. Sep 1, 2011

### Hootenanny

Staff Emeritus
Almost, the definition of angular velocity (or speed) should be in your notes or textbook. Perhaps you could look it up.

5. Sep 1, 2011

### bionut

okay, all it says is angular speed = angular distance / change in time

If the total distance in 30 km = 3000m

r= 0.7m

I really just don't get it sorry.... Im sure its very basic and im missing something here.... ill kick myself when I find out thats for sure !!

6. Sep 1, 2011

### Hootenanny

Staff Emeritus
7. Sep 1, 2011

### bionut

Okay, thankyou for your help.... but im back to sqaure 1 lol.... gawd !!

okay if v=wr and s=2pi and the displacmeent is 2pi (6.8 rad) X 0.70 = 4.40, and v=30km/hr , is the tangential velcoity 0.83m/s

8. Sep 1, 2011

### Hootenanny

Staff Emeritus
Not quite. 30 km/hr is 8.33... m/s. So, the equation you need is

$$v = \omega r$$

Now, you have v (8.33 m/s) and you can work out the radius, r. And you need to work out $\omega$. Do you think you can do that?

9. Sep 1, 2011

### bionut

.... oh dear.... it is 8.3m/s .... no wonder I have spent 1/2 the day try to work it out ..... lol..... told you I would kickmyself.... instead I think i need paperbag lol...