141 Fall 2017 Week 5

Monday Midterm #1Before the exam:

  • Watch these videos four videos, which are my efforts at the four problems from Spring 2017 MT#1. Many students were 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 (oops! I left of a “delta x” in calculating the energy conversion from work in this video… sorry), #2 Power of Running up the Stairs, #3 Throwing the rock upwards with a parachute, #4 Parachute Opening .
  • See some student solutions from Spring 2017 MT#1: MT#1 Student Solutions
  • I posted some solutions for PS#4 on the main class website. This is mostly answers and discussion as to how to get the answers. There are no drawings or compete calculations.


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.

We will start mixing chapters from OpenStax when I find them appropriate. This will be longer and more technical than my short text notes. I may also write some more chapters, but it is less necessary now that we have finished linear mechanics. It wasn’t appropriate for us to use a standard text before because of the way we introduced all the concepts at once. However, now some of the chapters are appropriate for us because we’ve finished linear motion.

Before Class

  • I posted MT#1 on the main class website. I also posted another copy with some comments and direction. Please start your corrections/talking with friends about these questions.
  • Problem Set #5 posted. Please have a look at it and do the problems as you cover the material.
  • See 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#4 begins with redoing MT#1. Please start on that.

During Class
Bicycle Wheel Rotation


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

  • 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

In Class

Working out with the “inertia wand”

After Class


Thursday: 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:

  • Watch the videos: Torque and the Lever
  • Read 4.5 about finding moment of inertia
  • Read 4.6, introduction to statics.
  • Then watch the video about the standard diving board problem
  • 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
  • I’ve heard folks are concerned about how to do MT#2 corrections without your graded exam. I think you can still do a good job learning about what you might have missed during the exam. Please work with your group mates. At the same time, I will have your exams back to you on Monday and collect the corrections on Tuesday!
  • I’ll see you at 9:00 and 11:00 Friday in Office Hours?

In Class
BIG EXAM! #4 on rotation

After Class