Announcements:
Assignments:
Chapter 11 - Torque
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: Kr = 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
Demo: Rolling objects races - which one will win?
Summary of rolling object races:
large cart = weighted cart > solid wooden ball = bowling ball >
racquetball > solid cylinder = short solid cylinder = large solid cylinder
> thick-walled cylinder > thin-walled cylinder.
small car > monster truck
Full Coke can > can of soup > empty Coke can.
Problem: A constant 10-N force is exerted on a door handle at an angle of 30 degrees from the normal. The door has a mass of 50 kg, a height of 2 m, and a width of 1.5 m. How long will it take for the door to close if it is initially at a 90 degree angle and not moving? [2.8 s] How fast will the door be rotating when it closes? [1.1 rad/s]
Ponderable: What is the mass of a meter stick that balances at the 25 cm mark when a mass of 150 g is hung from one end?
Demos:
ConcepTest
questions