Springs

Today we used your labs to figure out Hooke's Law...which we actually learned and took notes on back in October. Then you did a bunch of practice problems:

p.466 #1, 4, 5

Also bring your lab notebook Monday/Tuesday.

Here are some things that can be modelled like springs:
Dolphin Tails
Chemical Bonds
Robert Hooke

Spring Intro Lab

Your take-home test for Rotational Motion is due today - don't forget to staple the test paper to your answers, or you will lose 5 points :(

We started our new unit by basically discovering Hooke's Law with a lab, which we'll go over tomorrow. The lab was on the board:

Your only homework is to finish the lab if you didn't in class.

Fun With Angular Momentum

Today we looked at a bunch of real-life consequences of angular momentum. We learned a "magic" trick involving a ring and a necklace, played with a gyroscope, and finally we watched a couple of videos that use angular momentum to explain real-world stuff:

Why cats land on their feet - Smarter Every Day
Aerial Skiing - The New York Times

Homework: Ch.8 (p. 271) #58 & 72

I also had your practice test ready today, so you may have picked one up. You will have a multiple choice quiz thing on Friday, then you will receive a take-home test for the free response section. The free response section will be due on the following Thursday (so you'll have it for 6 days).

Here are the answers to the practice test:

Abridged answers

I also have some videos, but they are for an older version and some of the problems have changed.

#4
#5
#7 (PART C HAS CHANGED)
#8
#9 (energy method), #9 (dynamics method) (THIS HAS CHANGED)
#10
#11 (THIS HAS CHANGED)

Angular Momentum

First we did Plickers, and then we did this problem together:

The answer is b, but why? No energy is lost due to friction, since there would only be static friction (to make the ball roll) and static friction doesn't take energy out of the system the way kinetic friction does.

Next we took some notes about angular momentum. There aren't a lot of notes, and what there was ended up on the whiteboard. I also have notes from a past year when I was sick: Angular Momentum. Below you will find the answers for the example problems in that guide.

Suggested homework is p. 271 #55 (look up the mass, radius, and orbital radius of Earth with The Google) & 62

Finding the angular momentum of a bike wheel:


Finding the angular momentum with a clump of mud sticking to the rim of the wheel:

Finding the new angular speed of a star that has shrunk dramatically:

One last picture with all of the equations:

Rotational Kinetic Energy

Today we learned about rotational kinetic energy and did some problems with it:

Notes: Rotational Kinetic Energy

  Suggested homework p. 270 #47 & 50

Practice

Today we had another problem set to work on:

p.263 Conceptual #6 & 12 + p.273 #70, 74, 78

You had the whole period as work time.

Here are solutions to yesterday's suggested homework problems:

#39


#40 (this sets up a system of three equations and three unknowns, which you then need to solve)


#87 (this sets up a system of three equations and three unknowns, which you then need to solve)

Combining Linear and Rotational Dynamics

Today we walked through a problem that combines rotational and linear dynamics. The basic strategy goes like this:

1. Draw a free body diagram of the linear thing.
2. Write Newton's 2nd Law for the linear thing, fill in what you can.
3. Draw a free body diagram for the rotational thing with the forces drawn at the point they really are applied to the body.
4. Write Newton's 2nd Law (rotational version) for the rotational thing, fill in what you can.
5. Write (linear acceleration) = (radius)(angular acceleration) for the point where the linear and rational objects interact.
6. You should now have a system of three equations and three (or less) unknowns; solve!

Note: if there is more that one linear thing or rotational thing, you might end up with more than three equations and three unknowns. Fun! Also, if a problem has a rope going over a pulley that has mass, the tensions are different for the pieces of rope "entering" and "leaving" the pulley. Double fun!

Here is the problem we walked through:

Two years ago I was actually gone for this lecture, so here are some videos I made (thanks past me!):
Part 1: YouTube
Part 2: YouTube

Homework: p.269 #35, 40, 87. We'll have a work day tomorrow, so if you have trouble don't take too much time struggling with it. I can help you tomorrow.