Day 11

Wednesday, June 5, 2002

Announcements:

Preferences for next exam:

How many MC questions: 20 or 25?
RWP for Exam 2 due on day of exam or day after?

Chapter 7: Circular Motion and the Law of Gravity

angular speed: rate of change in angular displacement
angular acceleration: rate of change in angular speed

Note: angular speed and acceleration use radians: 2*PI radians = 360 degrees

The table on p. 181 shows the rotational analog of the corresponding linear motion variables.

Distinguish between:
- angular speed and tangential speed
- angular acceleration and tangential acceleration

Ponderable: For a car accelerating up the on-ramp of a highway, find:
a) linear acceleration of the car
b) tangential acceleration of a point on the wheel
c) angular acceleration of a wheel
d) the tangential speed of a point on the wheel when the car reaches 60 mph
e) the angular speed of a wheel when the car reaches 60 mph

Centripetal acceleration results from a force directed toward the center of the circular path.
- Forces that cause centripetal acceleration are gravity (for satellite motion) and the tension in a string
- Centrifugal forces do not exist. What feels like a force is actually inertia.

Gravitational force acts between any two objects that have mass: Fg = G*m1*m2/r^2

Kepler's laws of planetary motion (not emphasized in this course).

Concept questions

Is there a net force on a car as it rounds a flat curve at constant speed?
If you are riding in a car without your seat belt, and the car makes a sharp left turn, what force acts on you when you collide with the right-hand door?
A ladybug sits at the outer edge of a merry-go-round, and a gentleman bug sits halfway between her and the axis of rotation. The merry-go-round makes a complete revolution once each second. What is the angular speed of the gentleman bug compared with the ladybug?
A ladybug sits at the outer edge of a merry-go-round that is turning and slowing down. What is the direction of the radial component of the ladybug's acceleration?
A ladybug sits at the outer edge of a merry-go-round that is turning and slowing down. What is the direction of the tangential component of the ladybug's acceleration?
A rider in a "barrel of fun" finds herself stuck with her back to the wall (Problem 7.53). What is the correct diagram showing the forces that act on her?

Example 7.7: What is the angular speed of the yo-yo in Fig. 7.10 if the string is 40 cm long? What is the tension in the string? If the string is shortened, how do the speed, tension, and acceleration change?

Whirlygig demo: Why does the spinning mass speed up as the string gets shorter? How will the angular speed change as the radius is doubled?

Bucket of nails: How fast do I need to swing the bucket in a vertical circle to keep the nails from falling out?

The Texas Motor Speedway has 750-ft radius turns that are banked at 24 degrees. For what speed are these turns designed? What coefficient of friction is needed to keep a race car from skidding when v = 200 mph? What G-force does the driver experience at this speed?

Example 7.14: Find the altitude required for a satellite to remain in geosynchronous orbit.

Assignments:

HW8a (due Thursday)
HW7b (due Friday)
RWP7 - Proper Speed Limit (due Friday)