121 Winter 2019 Week 4

Monday Elastic Collisions

  • Finish PS #3.
  • See: Elastic Collisions with Walter Lewin at MIT
  • Read 3.0, Changing Reference frames and then, see the video below.
  • Elastic Collisions with Pete at Cal Poly by Changing Reference Frames This is my most popular video – after ~ 6 years, it has over 25,000 views. See if you can tell me why it’s the most popular of my videos.
  • Read 3.1, elastic collision in 1 dimension.
  • If you like, please check out 8.4 from the original OpenStax textbook to see how the rest of the world solves elastic collisions in 1-Dimension using simultaneous equations: the conservation of momentum and kinetic energy.
  • Please take the following short survey #3.
  • See Student Project Video: Measuring Speed of Bullet. There’s a few math mistakes in the video, but it is worth looking at the two different ways to measure the speed of a bullet. 
  • You wake up in the middle of the night to find there’s a fire in the next room and the only way to save yourself is to throw a ball at your door to close it. You have two balls of the same mass at your disposal. One ball is perfectly elastic, and the other one is perfectly inelastic. Which ball will be most effective at closing the door? Why?
  • Optional reading on NPR about active learning in Physics Education Research. The author, Carl Wieman who won the Nobel Prize in Physics for Bose Einstein Condensation gave a talk at Cal Poly some 12 years ago calling on us to evaluate and innovate teaching with the same scientific process we use for other scientific research. Since then, he’s become prominent in Physics Education Research. He’s published a considerable amount about active learning strategies and over the years has become more outspoken about the futility of the lecture model and need to change.
  • Another NPR article about education:
    • “The first step is to teach Socratically, by asking questions and having students think out loud. This works much better than lecturing.”
    • “Teachers who find their kids’ ideas fascinating are just better teachers than teachers who find the subject matter fascinating,”

Important to consider for today’s class… your life may depend on it. You have two balls to throw against a door in order to close the door with the impact of the collision.

During Class

Surviving a fire in your dorm room


During Class

  • Elastic Collisions

Wednesday: Friction!

Before Class

  • See PS#4, posted on main class website.
  • Solutions for PS#2 and PS#3 posted on main class website.
  • See the updated more detailed project #1 description on the main class website.
  • View Big Picture of Mechanics to get an overview of the first 4 weeks of mechanics.
  • If you haven’t already, make sure you read 2.8, a review about the lens method, updated to include the newly-covered vectors to go along with the above video.
  • Read section 3.2 about friction in your textbook. If you want to know more about the molecular nature of friction and biomedical consequences, please see this OpenStax description.
  • Please see the video I made for you all about dragsters and friction. 
  • See this #1 Spring Energy Conversion from a past Midterm #1.
  • I posted a study guide for MT#1 on the main class website.
  • Is thinking about the midterm stress you out? Did you know that this can be good for you? Please see this TED Talk. If you recognize that stress is getting you ready for the “big game” (or exam) it may well help you rather than hinder you and make you healthier!

During Class

  • Examples of Friction, optional Big Exam #4 (?)

Thursday! Review for Midterm.
Before Class

  • I posted a study guide for MT#1 on the main class website.
  • Matt Walker (121 mentor in learning center) will be hosting a discussion/question session Sunday at noon in Baker near my office (180-608).
  • This is a slightly different version of the accelerating car, but try this video out if it helps you understand Powerful car accelerating I reference a question from the problem set but this is an old video and I didn’t assign this question this quarter. Thus is is all the more important that you learn from the video.
  • Study for Tuesday’s midterm!: Take MT#1 from last spring’s 121 class under test conditions: 50 min. Do the whole thing, It’s great to do it with your fiends, but pretend it’s an exam with no talking for the entire 50 minutes. Then you can compare answers afterwards.
  • After you finish the Practice MT#1 (above), please Solutions to the MT#1 that you just took. Then address the questions again to see if you can improve. You might do this as a group.

Questions about midterm:
– The midterm covers everything we’ve talked about in class, saw in videos, or had in assigned readings (including friction). In short, if you’ve been exposed to it though the class, then please expect that it could be covered on the exam

What about “smart” calculators? You are not allowed to use calculators that graph or can integrate or take derivatives or anything like that… My strong preference is that you use no calculator whatsoever and just estimate… you can write “no calculator” on your exam and I will like it! However, if you feel better about having a calculator, I understand. If you do use a calculator, please find a simple calculator with only trig, square roots and simple math. – Thanks

Next Monday!: MT#1. I’ve posted a number of assigned videos to be watched before Monday’s midterm. You might consider doing the related questions as soon as you can and then watching the videos over the weekend with friends to review the material together. In any case, please read through next Monday’s preparation as soon as you’re able.