141 Fall 2018 Week 7


Before Class

  • Solutions to PS#7 posted on the main class website.
  • 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.
  • Hey, so we never got time to go overSurvey #6. Here are the student answers. Please check again 7 – 10 (the loop-de-loop) as these have “a diversity” of answers.
  • 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 posted your empathy intervention on our website before class time. Log your experience on the Empathy Self Intervention website. Add your post to the top. The user name is “interventor” with a small “i”, and the password is “Intervention” with a capital “I”.

Some more study resources:

During Class

  • Discussion, demos, preparation


Tuesday, Midterm #2
The midterm will cover everything we’ve done up until now. The material since MT#1 will be focused on. However, of course everything from before will be necessary to understand the more recent material.

There was considerable confusion expressed about the “barbell spinning in space” video. Why did we integrate? We used an angular momentum lens because there are no outside torques in space, so angular momentum most be conserved. Because 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.

Wednesday: 2D mechanics (no trig).
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.

Before class

  • Do the MT#2 in fine form. The exam is posted on the main website. update!!! I’ve finished grading questions 1 and 2: “total disaster”. OMG. I posted the midterm with comments… I’ll update the comments as we update the election results. You all will figure out what you did wrong and how to do it right. I’ll figure out how to make this work for all of us. See you tomorrow…. don’t celebrate too late.
  • 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.
  • Watch Intro Video to 2 Dimensions (3.8 minutes)
  • See the video on estimating vector components (1.5 minutes)
  • 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 7.0 in your textbook.
  • Read PS#8, posted on main class website.
  • Read about project #2 on the main class website.
  • *** hey, office hours are changed to 11:00 AM today

I’m offering a special service to help prepare for the final exam that didn’t get full credit on a MT#2 question. I think it is important that each person understand exactly what it is that they need to writing in order to receive full credit for each question. Talking to someone who got the question right may be a good start, but I think it’s crucially important that each of us go through actions of the correct answer in order to confirm clear expectations of what constitutes a correct answer. If you want I will “grade” your corrections for each problem that you want me to in order to make sure you got it right. What I request is that you decide which questions you want me to grade. Then you do the problem as if you were handing it in on an exam. Of course you will not benefit if you copy someone else’s work. After you understand what you did wrong and how to do each problem correctly, please sit down and do the problem with only a formula sheet. Then staple the problems together and submit them to me. You don’t need to include the original MT#2. Next Monday please give me these separately from handing in the problem set. I’ll “grade” them and hand them back Tuesday, so we will get a better idea of how to successfully complete a physics problem.

In Class**

  • Review Midterm #2 – Exam Returned
  • 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)
  • Please read 7.1 in your textbook
  • Finish MT#2 corrections with PS #8
  • Read over your classmates empathy interventions on the website. We will discuss in class.
  • POOP! I realize that didn’t put 7.0 Components, in chapter 7. Please read it, and you’ll need it for the problem set. I’ve updated Chapter 7 on the textbook website.

In Class

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

After Class