1. Chapter 9
Circular Motion

2. Rotation and Revolution

3. The axis of rotation is internal.
The center of the circular motion (called the “axis”) is within the object (it is “internal”).
The axis of rotation is internal.

4. The axis of rotation is external.
Revolution
The center of the circular motion (called the “axis”) is outside, away from the object (it is “external”).
The axis of rotation is external.

5. Example:
Rotation
Revolution
Steering Wheel
You on a Ferris Wheel
Minute Hand on a Clock
“Revolving” Door
Car on a Race Track

6. Does the Moon Revolve or Rotate?

7. Tangential Velocity

8. Look at the fan blade. How fast is it going?
Notice the mark on one blade.

9. We need to calculate: Rotational Speed (ω) Tangential Speed (v)
They are related to each other by the radius.

10. A digital strobe light can help us determine the rotational speed.

11. Measuring Rotational Speed
If the flash rate (FPM) equals the rotational speed (RPM) the blade appear to stand still!
Read out tells flashes per minute (FPM)

12. Demonstration

13. Calculating Tangential Speed from Rotational Speed
Consider one trip around for the marked spot.
For all constant speed motion, velocity = distance/time
In this case, the distance is one circumference and the time is one period.
Therefore

14. Example: If the radius is 20 cm and the RPM=1000, find tangential speed.

15. Fix the radius units: r =20 cm = 0.20 m
Example: If the radius is 20 cm and the RPM=1000, find tangential speed.
Fix the radius units: r =20 cm = 0.20 m
Determine the period: T = 60sec/1000rev = 0.06 sec/rev

16. Example: If the radius is 20 cm and the RPM=1000, find tangential speed.
Fix the radius units: r =20 cm = 0.20 m
Determine the period: T = 60sec/1000rev = 0.06 sec/rev

17. Find: (Solutions given in class)
Practice Example:
A girl runs around a circular track that has a diameter of 50-m. She runs 5 laps in 7.5 minutes.
Find: (Solutions given in class)
Rotational speed (rev/s)
Period (s/rev)
c) Tangential speed (m/s)

18. Assignment 9a