# 141 Winter 2018 Week 9

Monday
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

• Please see the video on angular momentum: Wheel Kick and Angular Momentum
• Veritassium Which Block Goes Higher?
• See this short video on Angular Momentum and Figure Skating
• Grading and Project #2: Hi everyone. Because there is no project #2, Your grades will be determined on a 95% total scale, essentially raising the total grade by a factor of 1.05.
• I’ve posted solutions to MT#2 on the main class website. Please read through the solutions, even if you aced the exam. In particular, I show you a nice way to solve the first problem very quickly that I think is worth seeing, and no one got the carousel problem perfectly, so please read through them.
• Check out the feedback from last Wednesday, posted on main class website.
• PS#9, due next Monday, posted on main class website.
• Finish PS#8 hand in today in class.
• See solutions for PS#7 posted on main class website.

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.

I ask that you again watch the video on youtube without the questions describing angular momentum of a point mass. Then, please do the last problem on the problem set, which is your BE!#6. Maybe in class Monday, you can tell me what I might have changed to make this video more effective. Please count on seeing this kind of problem in your future.

During Class

Tuesday Angular Momentum as Stability – Angular Momentum is a Vector!

Before Class

During Class

• Angular momentum and Gyroscopic stability.
• Precession – what is it?

Wednesday: Precession

Before Class

• Here is a precession video I made recently to explain precession. It’s likely my most well-received video to date.
• Please look over these activities on Gyroscope Worksheet. Can you try some of them?
• Group project is due this weekend, Midnight Saturday night. Please post the website for your youtube video – make sure it’s public so I can see it.
• Please take the fifth 10-question mechanics survey for discussion in class. Are we getting better at these?

In Class

After Class

Thursday:

During Class

• What is counter steering on a motorcycle?