When something rotates in a circle, it moves through a *position angle* \begin{align*} \theta \end{align*}*path length.* If the radius of the circle is larger, the path length traveled is longer. According to the arc length formula \begin{align*} s = r\theta \end{align*}

For an object moving in a circle, the objects tangential speed is directly proportional to the distance it is from the rotation axis. the tangential speed (as shown in the key equations) equals this distance multiplied by the angular speed (in radians/sec).

\begin{align*}\omega = 2\pi / T = 2\pi f \end{align*}

\begin{align*}
\omega = \frac{\Delta \theta}{\Delta t} = \frac{\Delta s}{rt}= \frac{v}{r} \end{align*}

\begin{align*} v = \omega r
\end{align*}

#### Example

A Merry Go Round is rotating once every 4 seconds. If you are on a horse that is 15 m from the rotation axis, how fast are you moving (i.e. what is your tangential speed)?

\begin{align*} v = r \omega \end{align*}

Now we need to convert the angular speed to units of radians per second.

\begin{align*} \omega = \frac{1}{4} \frac{\cancel{rotations} }{second} * \frac{2 \pi}{1} \frac{radians}{\cancel{rotation}}
= \frac{\pi}{2} \frac{radians}{second}
\end{align*}

\begin{align*} v = 15 m \times \frac{\pi}{2} = \frac {15}{2} \pi \approx 23.6 \frac{radians}{second} \end{align*}

### Simulation

### Review

- You are riding your bicycle and going 8 m/s. Your bicycle wheel is 0.25 m in radius.
- What is its angular speed in radians per second?
- What is the angular speed in rotations per minute?

- The angular speed of a record player is 33 rotations per minute. It has a diameter of 12 inches.
- What is the angular speed in radians per second?
- What is the tangential speed of the outer most part of the record?
- What is its tangential speed halfway out on the record?

**Review (Answers)**

- a. 32 rads/s b. 306 rot/min
- a.3.5 rads/s b. 0.53 m/s c. 0.26 m/s