121 Winter 2019 Week 5

Monday Midterm #1

Midterm #1, Before the exam:

  • I’ve posted solutions to PS#4 on the main class website.
  • Please take a look at my statement about cheating on the main class website. It’s important to me that I communicate what I’ve seen myself do in the past.
  • First take  MT#1, from Spring 2017 under test conditions. Then watch the following four videos, which are my efforts at the four problems from this 141 class. While you can expect your exam to be slightly less demanding, it would be great preparation for you to be prepared to answer questions such as these. Many students are not sure how much description to provide. Some don’t provide enough narrative. Others fill the entire page with an essay. I’ve done my best to provide an example that is what I’m looking for: #1 Spring Energy Conversion, #2 Power of Running up the Stairs, #3 Throwing the rock upwards with a parachute, #4 Parachute Opening

Tuesday: Switch Working Groups, and Start Rotational Mechanics

PS#5 is due next Monday and the first part is complete corrections for MT#1.

This week, we will almost identically repeat what we’ve done the first 4 weeks, but this time for rotational motion.

All the concepts are the same, so taking three weeks, should work well. Your learning objective is to recognize the analogies between the linear concepts and the rotational concepts, and how to use the analogous equations to solve rotational problems.

Before Class

  • I posted MT#1 with some comments on how folks did, and some guidance on the main class website.  Please start your corrections/talking with friends about these questions. I’m still grading the last questions Tuesday at 10 AM, but I anticipate I’ll have your tests back for you at class time.
  • I posted PS#5 on the main class website. Please read through them and attempt them corresponding to chapters read. This is the best way to stay on top of what’s going on.
  • 4 Lenses of Rotational Motion Oh my, this video is way way too long. Please watch it in two sittings and take notes.
  • Please read our short introduction to rotation, 4.0
  • Please read direction of rotation, 4.1
  • See Circular Motion, Introduction
  • Please read 4.2. rotation connection to linear motion
  • PS#5 begins with redoing MT#1. Please start on that.

During Class
Bicycle Wheel Rotation

Hand in Project #1

I should have MT#1 back to you for discussion.


Know how to calculate Torque
In a statics problem where alpha = 0, can you set the sum of the torques = zero (like you did with forces for linear motion)

Before Class

  • For tomorrow, please finish **Perfect** MT#1.
  • Please read 4.3, Torque, Work, Power
  • Please See Video: moment of inertia, which includes the fundamentals of rotational dynamics and rotational kinetics.
  • Please read 4.4, Moment of Inertia
  • Rotational physics video: many students indicated that it was fast and confusing. After class Wednesday, maybe it is clearer? I encourage you to watch it again sometime soon to see if it makes better sense.
  • Do PS #5. Are the rotational problems easier than the linear problems were in week#1 (because you’ve practiced with the “linear analogues”)?
  • Watch the videos: Torque and the Lever
  • Please take this short Survey #4  

In Class

Working out with the “inertia wand”

After Class


Next Monday: Angular Dynamics and Statics.
Master the use dynamics (vector sum of the torques = I * alpha) just as you did for force and acceleration in linear motion
Augment your use of Work-Energy Theorem with rotational kinetic energy.

Before Class:

  • Finish **perfect** MT#1, and I will grade it for you. You can just submit the questions you want graded.
  • Read 4.5 about finding moment of inertia
  • Along with the above, you should watch the end ~ 5minutes of the moment of inertia video again. I think you can skip ahead the second time you watch it. If not, right click on the video and copy the youtube video link. Many students found this video confusing and complicated. For the future, I’m going to split it up into three videos: single mass, several masses, solid body (integration). Integrating infinitesimal mass is a new thing. However, this skill will be very important for PHYS 133, and later for many engineering classes. So, while this skill isn’t crucially important to conceptual understanding (we don’t even have a lens for it… just “math” lens), it’s a skill for the future.
  • Read 4.6, introduction to statics.
  • Then watch the video about the standard diving board problem
  • Please watch this video on torque = time derivative of angular momentum
  • Please read 4.7 Intro to Angular Momentum
  • I want you to be aware of one thing: just like F=ma, for rotational problems we use Torque = I*alpha. Please identify in your problem set which questions involve this Rotational Dynamics. Then, please describe a protocol for rotational dynamics (Forces) that is a rotational analogue of our protocol for linear dynamics.
  • I ask you to be prepared to “think with your hands”. Please see this NPR article that shows students who take notes by hand retain more information than students who type notes into a computer. My inference (which may not be correct) is that if you did neither, then you’d retain even less.
  • we look at centripetal acceleration.
  • Did you know that studies indicate texting costs students an average of half a letter grade in their classes. Should we do something about this? NPR story about cell phones and classes

In Class
BIG EXAM! #4 on rotation

After Class