Week 4 121 W2020


Before Class: YIKES! PlayPosit isn’t working for some reason… I just spoke with them and they are working on it. However, I provide the direct YouTube link if the PlayPosit link doesn’t work.

  • Potential Energy Graphs video A considerable number of students are confused by the potential energy graph video. This is a big concept to learn and requires some extra time and effort. Doing the homework example will be of great benefit. YIKES! PlayPosit isn’t working for some reason. If you can’t access the video, please see this through the Potential Energy Graph YouTube Direct Link.
  • Read 2.7 Energy Graphs 
  • See a video about how to measure power. I don’t have the direct YouTube link for this one, so if you can’t see it now, you can see it tomorrow.
  • Finish PS#3
  • Finish Project #1

During Class
Review Roller Coaster Problem through momentum lens.


Before Class

  • Kaitlin is holding office hours M, W from 1-2, immediately before class, by my office. I’ve added her office hours to the main class website.
  • Read 2.8, a review about the lens method (section 1.8) , updated to include the newly-covered vectors. Remember I said that 1.8 was the most important section in the class?… that statement is now updated – section 2.8 is a road map to solving mechanics problems… this is something to be practiced… maybe put a few reminders on your formula sheet? 
  • See Firehose Video
  • 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. AND this is a wonderful example of how good your project #2 can be.
  • If you like, please see the graph on maximum human power over time I think I showed you in class.
  • How about the power of one super athlete?
  • REALLY finish project #1.

During Class

Hand in project #1

Measuring the speed you can throw with a ballistics pendulum! This is how we measured the speed of bullets before we had ultra fast cameras.


Wednesday Elastic Collisions

  • Study for Assessment #4
  • I posted solutions to PS#3 on the main class website.
  • Please calculate the speed of the ball that was thrown by Drew. As I recall, the mass of the cooler is 2.0 kg, the mass of the ball is 41 g, the length of the string that the cooler was hanging from is 60 cm, and the cooker swung back ~ 7 cm. First, use some Pythagorean Theorem to find that the cooler gained 4.1 mm in the process. How fast did Drew throw the ball? In particular, was the speed of the ball closer to 0.25 m/s, 2 m/s, 15 m/s, 40 m/s, or 100 m/s?
  • Kaitlin is holding office hours M, W from 1-2, immediately before class, by my office. I’ve added her office hours to the main class website.
  • 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 32,000 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.
  • 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

  • Assessment #4
  • Elastic Collisions

Thursday: Friction!

  • I posted PS#4 immediately after class Wednesday. Please check it out. I might add a few questions by class time Thursday.
  • How about measuring the power of another super athlete: please see this video
  • Please see the video I made for you about dragsters and friction.  
  • 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.
  • Optional: another education article on NPR: How you think with your hand, not your brain (or at least not your computer).
  • Assessment update 11:00 PM. I imagine that many of you see your oversight after watching the video above… however, the larger issue is that people are still formula hunting. It seems I can feel your anxiety as I read your answers. It seems you’re in such a hurry to get to the answer, you’re not going to “waste” your time on the motivational statement… which is the most important part of the problem… If you write “energy because we’re dealing with power….” your statement alone shoujld tell you that you’re formula hunting. If you have the words “we’re dealing with…” in your motivational statement, it’s a bad motivational statement. First, did you draw a good picture indicating that the person is running at a constant speed… because you carefully read the question. Then maybe you say, “oh, power. I am looking at power, so there must be a change in energy or work being done…… I’ll contemplate the before and after pictures I drew… then OK, if it’s an energy lens, maybe there’s a change in energy or work changes energy. speed doesn’t change (so there’s no kinetic energy)… is there any work. Is the energy changing between the beginning and end? Oh, I’m at a higher elevation, I’ve gained gravitational potential energy… so the transition is from something to gravitational potential energy… Is energy conserved in the system or did some work happen? Oh, my work changed my gravitational potential energy…. So I can low write: “I’m picking an energy lens because I’m doing work that is changing into gravitational potential energy.” I imagine it’s not this clear for many of you… and we’re just learning. However, you need to make more time and space for the drawing and motivational statement. ALSO, students are often confused right now in the curriculum. However, almost everyone gets the hang of it by the end of the quarter. I’m sure you’ll get it. Just keep it up. Work together, talk with your people, come to office hours.

During Class

  • Surviving a fire in your dorm room: You have to shut the door by throwing a ball against it. Would an elastic ball or an inelastic ball be more effective?