141 Winter 2018 Week 5

Monday: Torque
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: which includes the fundamentals of rotational dynamics and rotational kinetics.
• Please read 4.4, Moment of Inertia
• Watch the videos:
• Read 4.5 about finding moment of inertia
• Finish PS#4
• Work on corrections for your midterm #1

In Class

Working out with the “inertia wand”
Which Way does the tricycle move when I push on the pedal with an outside force? Will the outcome be the same as it was for the last class?
Not handing in PS #4

After Class

Tuesday: Statics. and Conserving Angular Momentum, when Sum of the Torque = zero

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:

• HEY!! No one turned in their problem set #4 yesterday. This is important stuff that we’re starting on. Please do the problem set and hand it in today… Tuesday!
• 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
• Please watch this video on torque = time derivative of angular momentum
• Please read 4.7 Intro to Angular Momentum
• Please do this 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.
• Tomorrow!!! Hand in corrected MT #1 with narrative about what you’d expect if you had another half hour of time.
• PS#5 posted on the main class website.

In Class
Hand in PS #4 – for real this time!
Pete prints and distributes 25 copies of chapter 5.

After Class
Extra office hour in 180-262 from 7:00 – 8:00
Another extra office hour at 10 AM on Wednesday.

Wednesday: what forces must be acting on something in order to make it move in a circle?

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.

Before Class
Extra office hour at 10 AM at my office!

• Please see PS #5 on main class website. Please read (at least) read through and consider all the questions for chapters that we are covering today.
• 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 understand and can use this for Big Exam.
• Read 5.0 Centripetal Acceleration
• 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?
• 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.
• TODAY! Hand in corrected MT #1 with narrative about what you’d expect if you had another half hour of time.

In Class
BIG EXAM! #4
I put some masses on a rotating table and spin it. After a while, the masses shoot off the table. What force pushes the masses off the rotating table?

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

• Just now, 9 AM I chopped the last 4 questions off of PS#5 and will add them to PS#6. Thus, I start a new policy: I will not put questions on a problem set until the day after they have been covered in class and videos. I explain why I made this change, but feel free not to read it: The original intention of including Monday’s material was that once students had seen the lecture (video), they should be accountable for the material (problem set). However, what I noticed is that many students watch the videos immediately before class. Thus, if questions on the problem set cover Monday’s preparation, students would not have the lecture (videos) preparation before doing the problem set. Consequently, it seems a good idea to have Monday’s class material and videos covered on the subsequent problem set. I put this statement here to leave class time open for physics discussion. However, I welcome any feedback you have regarding this policy change….. Please do PS#5, due Monday.
• Finish corrected Project #1 to hand in during class.
• Please read 5.1 Inverse square relations
• See video on Universal Gravity and Inverse Square Law
• Read 5.2 Universal Gravity
• Veritassium music gravity video

In Class
Inverse Square Law!
Hand in Project #1.