# 141 Fall 2017 Week 6

Monday
Conserving Angular Momentum, when Sum of the Torque = zeroBefore Class

• Please watch this video on torque = time derivative of angular momentum
• Please 4.7 Intro to Angular Momentum
• There’s only one physics video, so I want you to be aware of one more 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.
• Hand in improved project #1 tomorrow. Please staple the new project on top of the old project, so I have both of them to read.

In Class:
Revisiting the “inertia wand”
Dropping kids on merry-go-round, barbell spinning in space

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.
Tuesday we look at centripetal acceleration.

Before Class

• Hand in improved project #1 today in class.
• 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.
• Watch derivation of Centripetal Acceleration and how to use it. Make sure you can do this for Big Exam.
• 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?

Wednesday: we look at universal gravity and the inverse square law.

• I just posted PS#6 on the main class website. Please have a look.
• See video on Universal Gravity and Inverse Square Law
• Veritassium music gravity video
• I’ve posted some very good student answers for MT#1. Please check it out on the main class website. There are several solutions for each problem so you can look through and see which speaks to you best.

Thursday
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?”