1. Chapter 7.2 Notes
Angular Momentum

2. Angular momentum tells us how difficult it is to stop a rotating object.

3. Equation for angular momentum = Inertia x angular velocity
L = Iw

4. Objects of equal mass but different shapes have different inertia.

5. Equations for Inertia range from:
Circular Orbit = I = 2/3mr2
Solid Cylinder = I = 1/2mr2
(I’ll tell you which equation to use)

6. A basketball has a mass of. 005 kg. The ball has a diameter of
A basketball has a mass of .005 kg. The ball has a diameter of .8m and is a sphere so we use the equation I = 2/3 mr2. The angular speed of the ball is 30 rad/s. What is the angular momentum?
Radius = Diameter / 2 = .8 m / 2 = .4 m
I=2/3mr2=2/3(.05g)(.4m)2 =.0005 kg/m2
L = Iw = (.0005)(30) = .016 kg·m2 / s

7. Newton’s 2nd law of motion with angular momentum – Equations = Torque = Angular momentum / time
T = L / t

8. An astronaut grabs a satellite to stop it from spinning
An astronaut grabs a satellite to stop it from spinning. The satellite mass is 900 kg, and it is spinning at 1 rad/s. The radius is .7 m. The astronaut must hold the satellite of .5 seconds, what torque is required to stop the spin? To find the Inertia use I = ½ mr2
T=L/t; First find L (angular momentum)
T = / .5 = 441

9. Angular impulse (equation) = Torque x time
Since angular impulse = change in angular momentum (equation) = Tt = L

10. A 100 kg potter’s wheel is 2 meters in radius
A 100 kg potter’s wheel is 2 meters in radius. What is the torque if the time is 10 seconds and the equation for I=1/2 mr2 and the angular speed of the wheel is 22 rad/s?
Tt = L
First find L (angular momentum)
T(10) = 4400
T = 4400 / 10 = 440

11. Law of conservation of angular momentum says that when no net external torque acts on a closed system, the total angular momentum of the system does not change.

12. Rotating Stool Demo

13. Law of conservation of angular momentum equation = L1 = L2
I1w1 = I2w2

15. If a skater starts with a moment of inertia of 5 with a speed of 2 than he brings in this arm to change his moment of inertia to 2. What happens to his speed?
I1w1 = I2w2
5(2) = 2(w2)
10 = 2w2
w2 = 5