# Spring 16 Mechanics Week 9

Day 1:
How can a point mass (with no radius and no moment of inertia) have angular momentum? It all depends on the impact parameter!!

Before Class

In Class

After Class

Day 2: More on how Torque = rate of change of angular momentum

What’s going on with PlayPosit and/or YouTube? I spoke today with PlayPosit (wow, they answer the phone!) and all is well. You all have credit if you watched the videos and you don’t have to leave a comment. They made some changes last week that must have disconnected communication with YouTube, so YouTube doesn’t know you are watching anymore. They were not aware of this and are looking into it now.

Before Class

• Please see the video on angular momentum: Wheel Kick and Figure Skating
• Solutions to MT#2 posted on main website, PS#8 solutions posted on main website. Please make sure as soon as possible that you would get all these problems correct if you were to encounter them again.

In Class

• Big Exam #7

After Class

Day 3:

Precession:
Torque is rate of change of angular momentum so dL = (torque)*dt. However, angular momentum is a vector, so you also need torque to change the direction of angular momentum.
Before Class

• Here is a precession video I made just this weekend for you guys. I think it’s pretty good, and I’m assigning it too, so i expect you’ll watch it and let me know how it works.
• Please see Precession by Veritassium , a super professionally made video. You’ll let me know if it’s more helpful than my video.
• Optional: Here is an old precession video I made a few years ago. It’s long and slow with lousey graphics, but it has close to 3000 views, so some people found it helpful: Pete’s OLD Precession Video
• Explanation of Veritassium Bullet Problem
• Hey you guys! I needs some help moving the demos from building 13 (Mechanical Engineering) to building 53 at 2:30 PM on Wednesday. If you can help me, please send me an Email at pschwart@calpoly.edu
• Solutions to MT#2 posted on main website, PS#8 solutions posted on main website. Please make sure as soon as possible that you would get all these problems correct if you were to encounter them again.

In Class

• Angular momentum and Gyroscopic stability. We have some cool demos for this.
• Why must we spin frisbees or footballs to make them fly stable? What is counter steering on a motorcycle?
• How is this related to precession?

After Class

Day 4:

Before Class

• Problem set #9 posted, check it out.
• Rotational Big Picture
• Rotational Systems
• Watch
• Consider this question: Are we (our class) a community?… and If our class became a community, how could you tell?

In Class

• A day at the races. Some round objects are going to roll down a flat surface without slipping – a hollow ring, a solid sphere, a hollow sphere – all of different sizes and masses. What determines which one wins? You might think about this while watching the rotational systems video.

After Class