# 121 Spring 2018 Week 6

Tuesday: 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

• In class, we discussed the possibility of moving MT#2. We are not going to do that. MT#2 will happen next Thursday. Week 7
• I’ve added solutions to MT#1 on the main class website
• I’ve added directions for BE! #4 to the main class website.
• Finish PS#5, on main class website.
• 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 the comments important and put this information 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 #5.
• Project #2: You make a mechanics video of your choice. Please start planning it.
• 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? This video was a project #2 from two students. Another example is the “measuring the speed of a bullet” from week 3.
• See video on Universal Gravity and Inverse Square Law
• Veritassium music gravity video
• Please do this short survey #5. Please make sure you identify a lens while you draw a picture.

In Class

Hand in PS #5
Review Statics
Inverse Square Law!
I put some masses on a rotating table and spin it. After a while, the masses shoot off the table. What pushes the masses off the rotating table?

Thursday: MT#2 is next Thursday

• Solutions posted for PS#4 and PS#5. Please take a look.
• Please read 5.3 Loop the Loop: Circular motion in the vertical plane that you can find in our textbook. Then, please address the questions.
• Please make sure you try Exercises 3,4 and 5.
• Please watch this very short video of something you should never never do. Please ask yourself what happened here and why the one boy was hurt and not the others.
• Please Watch Skateboarding Loop of Death
• Be prepared to address the question (5.3, Q7), “when I stand on the scale on the equator and on the North Pole, where do I weigh more?” Will the anit-FBD gene dominate your behavior?
• Please read 5.4 Gravitational Potential Wells, and Escape Speed that you can find in our textbook
• Just if you’re interested: Here is a Veritassium optional Video explaining the neutron star collision detected by gravity waves, gamma ray bursts, and optical telescopes.
• Read 6.0 Systems of masses.
• Watch an introduction to simple systems This is an important video to understand systems.
• Please check out the standard process for solving the system with simultaneous dynamics equations, one equation for each mass: Atwood Machine by individual masses I won’t spend any more time on this method, but you are welcome to use it if you like.
• Please do this short survey #6. If the answer isn’t totally obvious to you, please make a drawing while you think.

During Class

• Be prepared to address the question, “when I stand on the scale on the equator and on the North Pole, where does the scale read more?”
• How do you keep the water in the bucket
• How do you calculate acceleration of a “falling” system?