121 Spring 2018 Week 9

Tuesday
we look at how a point mass (radius = 0) can have angular momentum, and the conical pendulum.

Before Class

Last quarter, I wrote this to students after a big exam:
“Big Exam #6 indicates that we have important fundamental misunderstandings of angular momentum, besides not recognizing that
angular momentum of a point mass about an external point = linear momentum x impact parameter:
– There is a common student misconception that momentum and angular momentum are the same thing or convertible to each other, just like linear kinetic energy and rotational kinetic energy are both kinetic energy. This is not true. Please check units right of these four things and notice that momentum isn’t angular momentum.
– Just as force and torque are different quantities with different units. Notice that force can exert a torque about a center of rotation only if it is connected to the center of rotation with a perpendicular radius, or partly perpendicular radius. Similarly, a body with momentum can have angular momentum about a center of rotation if there is a nonzero impact parameter.”

During Class
Practice conical pendulum
Practice slack lining?
Fill out class survey.

Thursday
This week, we review a little and also study the final topic: How torque is the rate of change of angular momentum, and how they are both vectors, so when torque is perpendicular to angular momentum, torque doesn’t increase or decrease angular momentum. Instead, torque causes the angular momentum to change direction. This is precession.
Before Class

• Watch Slack Lining. Is this how you solved the problem?
• Please see the video on angular momentum: Wheel Kick and Angular Momentum
• Please watch Veritassium Which Block Goes Higher?
• See this short video on Angular Momentum and Figure Skating
• See my explanation of how gyroscopes can stabilize vehicles, satellites, etc. I worked harder on this video than any other I’ve ever made. Let me know if it was worth it.
• See video on Inclined Plane using Only Energy (no kinematics at all!) (3.5 minutes)
• See Veritassium video on how angular momentum provides increased stability against rotation.
• Please see: Explanation of Veritassium Bullet Problem
• Please do this short survey #9
• PS#8 is posted on the main class website. Due next Tuesday.
• PS#7 solutions are posted on main class website. Please see them. I’m very sure that you will gain little if you just read through the solutions. You should work the problems with a study group and then check the answers, and only check the solutions after you figure out the problem or are unable to figure out the problem (after which you work the problem again.)
• I posted a “Math Competence” exam on the main class website. Please expect something like this on the final exam.
• I posted your feedback from Tuesday’s class on the main webpage.
• If you want to see some very interesting conservation of angular momentum in a torque-free environment, please consider seeing the aerial dancing this Friday or Saturday evening. Full disclosure: my daughter is in this.
• Post video for final project before midnight, this Saturday night.

During Class

• Angular momentum and Gyroscopic stability.
• What is counter steering on a motorcycle?
• Precession – what is it?