141 Fall 2019 Week 5

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:

  • Watch the videos: Torque and the Lever
  • Please take this short Survey #4
  • Rotational physics video: many students indicated that it was fast and confusing. After class Thursday, maybe it is clearer? I encourage you to watch it again sometime soon to see if it makes better sense.
  • Read 4.5 about finding moment of inertia
  • Along with the above, you should watch the end ~ 5 minutes 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
  • 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
  • By now, you should be very comfortable with the four lenses… what to look for, and how to “motivate them”… for instance, “I will look through an energy lens (identification) because the spring potential energy is converted to kinetic energy (motivating).” If you are not, please refer to chapters 1.8 or 2.8 until you are…. THIS is what you should have for your test notes if you’re not comfortable. Formula are of no use until you’ve identified and motivated a lens.

In Class

After Class

we will examine an object that circulates into a circular path… like how the moon doesn’t travel in a straight line because the attractive force of gravity between the earth pulls the moon into a circular path.


Conserving Angular Momentum, when Sum of the Torque = zero

Before Class

    • 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.
    • SO! when you watch videos, do you answer the questions to get them done and move on, or do you actually read the comments for a wrong or right answer? I find this information very important and put it there because it confronts common misunderstandings and/or mistakes students make. Please give it a try and let me know what you think.
    • I’ve updated my office hours, changing Friday to 1:10 PM. Please see the main class website for office hours for me and Professor Klay… again, please feel free to visit  her office hours.

If we look at the earth spinning in space, we might notice that there’s nothing making it turn… that it is free of external torques. We might consider that we’d then look at the earth through the lens of angular momentum. What could we learn? Please see video on Coriolis Effect made by students from Fall, 2014. Look for a number of things:

    • Do we understand the Coriolis Effect?
    • Do we see the value in looking through the angular momentum lens?
    • Are we getting ideas for how we’d like to make our final video project?

In Class:
Dropping kids on merry-go-round, barbell spinning in space

Wednesday: Centripetal Acceleration, gravity and inverse square law

we look at universal gravity and the inverse square law.


circular dynamics with more than one force. It’s just like the elevator problem, but the acceleration is centripetal acceleration.
Before Class

During Class

  • Be prepared to address the question, “when I stand on the scale on the equator and on the North Pole, where do I weigh more?”