Spring 17 Mechanics 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:

In Class
Collect PS#4

After Class

Conserving Angular Momentum, when Sum of the Torque = zero

  • Angular momentum = I*alpha, kind of like linear momentum = m*v
  • Torque SOMETIMES = I * alpha, as stated above in rotational dynamics. Torque ALWAYS = rate of change of angular momentum
  • For instance if Torque = zero, then angular momentum is conserved (just like we conserve momentum if there’s no outside force) it doesn’t mean that alpha = zero, like acceleration = 0 if the sum of the forces = 0, Think of the spinning skater pulling in her arms, yielding a positive angular acceleration.

Before Class

  • Check out our Facebook page for communication and organizing meetings.
  • Please watch this video on torque = time derivative of angular momentum
  • Please read 4.6 Introduction 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.
  • Please see PS#5

In Class:
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.

Wednesday, we look at centripetal acceleration. PS#5 Posted! Please take a look!

Before 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.
  • 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?

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