Thursday, June 6, 2002

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?

• Race Car Drivers Dizzy Over Physics
• Breakneck Pace

Chapter 8 - Rotational Equilibrium and Rotational Dynamics

Torque = Force x Lever Arm

Moment of inertia depends on the mass of an object and where that mass is distributed relative to the axis of rotation.

Rotational analog of F = ma:  Torque = (moment of inertia)(angular acceleration)

Conditions for equilibrium:
1)  Net external force is zero
2)  Net external torque is zero

Rotational kinetic energy:  KEr = 0.5*Iw²

Angular momentum:  L = Iw

Angular momentum can also be found using:  L = r x p

Angular momentum is constant if there is no net torque on a rotating object.
    Torque = dL/dt

Ponderables:

1. Which gives more torque when turning a screw:  a long or short screwdriver?
2. Why is it easier to turn a small screw into wood, but a large screw is difficult?

• Which object will fall over?
• How can you apply the maximum torque to a nut?
• If a meter stick balances at the 25 cm mark when a 1 kg mass is hung from the 0 cm end, what is the mass of the meter stick?

• center of mass of NC
• balancing bird and wine bottle
• inertia rods
• teeter totter

Assignments:

• HW7b (due Friday)
• RWP7 - Proper Speed Limit (due Friday)