# 121 Winter 2019 Week 9

Monday:
Before Class

During Class
Estimate normal force or tension, acceleration
Practice slack lining?

Tuesday

Before Class

During Class

• Big Exam #6

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.

Wednesday we review a little to get back in shape from the week gone by. Be ready to finish your projects.

Before Class

• I posted solutions to MT#2. Please check them out. Make sure you understand what you didn’t understand.
• PS#7 solutions posted. Please check them out.
• PS#8 is posted on main class website. I collected about 5 problem sets Monday. This is not unusual. However, I’m sure you understand that there is a strong correlation between homework success and final exam success. We have two weeks to get this stuff mastered. Please figure out how you’re going to do it.
• see video Veritassium Which Block Goes Higher?
• Remember the demo we did with the “frictionless” cart that shot a ball straight up? The ball landed in the cart even if the cart had a constant velocity! Furthermore, we saw that the ball landed in the cart even when the track was inclined at a high angle. Now that we’ve done the inclined plane, please consider how to show that the ball should land back in the cart even if the cart is on an inclined track. Did you draw a picture? Pick a lens?
• See video on Inclined Plane using Only Energy (no kinematics at all!) (3.5 minutes)
• See this short video on Angular Momentum and Figure Skating
• See Veritassium video on how angular momentum provides increased stability against rotation.
• See video on how gyroscopes can stabilize boats. How do you think this works?
• Please watch this very short video of something you should never never do. Please ask yourself what happened here and why the one boy was hurt and not the other.

During Class

• Why must we spin frisbees or footballs to make them fly stable?
• Precession – what is it?

Thursday: We look at angular momentum as stability.

Before Class

• Please see: Explanation of Veritassium Bullet Problem
• I’ve posted your feedback from yesterday’s class on the main class website, and made some comments. In particular, if you’re interested in more of this kind of learning, please consider taking one of my other classes: Energy, Society, and the Environment (PSc 320, winter quarter), Appropriate Technology Classes (PSc 391 in Fall, PSc 392 Spring – 391 is not a prerequisite for 392).
• Please take this short Survey #9
• Here is a precession video I made recently to explain precession. It’s likely my most well-received video to date.