141 Fall 2019 Week 7


Before Class

  • You may have been missing the “after-party party”, so I want you to know about it. It seems that 5:00 MTWR is an ideal time to talk physics and reflect about class, as judged by the effervescent activity just outside our door after class. I was thinking I’d change office hours to be at 5:00, but I think we’ll just keep things as they are and you should know that you are invited to the after-class discussions.
  • Please watch this review video of rotation: Big picture rotation! You will notice two references to precession (that we haven’t covered yet) from 5:11 – 6:00 and and from 9:08 – 9:24 that you may find interesting, but you are not responsible for until the final exam. Likely, we’ll revisit this video at the end of the quarter, but it’s also a very good review right now. You’ll have to tell me during class if this review was a good idea, and if I should remake it without these references, or if this preparation is fine.
  • See one way for Solving flywheel with hanging mass (this is the worked solution) (2 minutes)
  • See a system of masses using work and friction (2 minutes)
  • See a system of masses using dynamics (3.5 minutes)
  • Watch solution to problem Barbell Spinning in Space
  • Make sure you’ve Emailed me your Empathy Intervention Statement.

Some more study resources:

During Class

  • Discussion, demos, preparation

Tuesday, Assessment #7 We have done linear mechanics in a straight line and rotational mechanics. Now we will introduce components, angles, and trigonometry and spend the last 3 1/2 weeks reviewing the same curriculum with the inclusion of angles. This is a great opportunity to improve conceptual knowledge.
There was considerable confusion expressed about the “barbell spinning in space” video. Why did we integrate? We used an angular momentum lens because we know that Torque changes Angular Momentum! there are no outside torques in space, so angular momentum most be conserved. Because the moment of inertia, I => (1/9) its original value, omega goes up by a factor of 9. We calculate that the kinetic energy must also increase by a factor of nine… meaning that the work we did in pulling the masses closer together is equal to 8 times the original kinetic energy! Then we need to calculate work to see if it is in fact 8 times the original rotational kinetic energy? We use W = F*dx, right? – However, the tension in the string changes as we pull the masses closer because the centripetal acceleration increases because the masses spin faster and faster at a smaller radius as we pull them closer. So we can’t just multiply the Force (tension) by the distance because the force is always changing. Thus, we add all the little bits of work done for each tiny section of string pulled inward. This is an integral. Feel free to watch this video again to understand what I did.

  • Watch Intro Video to 2 Dimensions (3.8 minutes)
  • See the video on estimating vector components (1.5 minutes)
  • Read 7.0 in your textbook…. but really read it, OK? It’s super short. Please consider the questions. You’ll want to solve them all before next week’s assessment.

Wednesday: 2D mechanics (no trig).

Before class

  • Notes for final exam – don’t let your formula sheet get any longer. As we add a few (very few) new formulas, you will want to drop others that are obvious to you now.
  • See the video on 2D Components of Force (8.5 minutes)
  • See 2-D kinematics (3.5 minutes)
  • Watch this famous music video… how did they make that video? I mean, do you love it? You’ve seen it already, right?
  • Read about project #2 on the main class website.
  • Read chapter 7.1

During Class

  • Practice Components
  • Discuss project #2



  • See the video on 2D work (5 minutes)
  • Please take this short Survey #7
  • See video on 2D momentum transfer (6.5 minutes)
  • Solutions to PS#7 are posted. Please check your work.
  • PS #8 is posted. Please read through it
  • Please read 7.1 in your textbook and consider all the questions that you certainly want to master before next week’s assessment.
  • Plan your project #2.

In Class

  • Talk about projects – Project proposal due by next Thursday.

After Class