Fall 2016 Mechanics Week 4

Friction, Elastic Collisions and Reference Frames. Midterm #1 is Thursday

For Tuesday: I’ve updated PS#3 on the main webpage, adding some more important questions. Please do these questions before class. Please get together with your classmates to work on them. Visit office hours together.
First Hour
Before Class

  • My attempt at PS #1 is posted on the main class website as well as most of the answers for PS #2.
  • See: Elastic Collisions with Walter Lewin at MIT
  • Then before seeing my video below, please read this very short statement about 2.9 changing references frames. Do the exercise.
  • Elastic Collisions with Pete at Cal Poly by Changing Reference Frames This is my most popular video – after ~ 4 years, it has over 13,000 views. See if you can tell me why it’s the most popular of my videos.
  • Read 8.4 from the original OpenStax textbook to see how the rest of the world looks at elastic collisions in 1-Dimension. Please reflect on the following two questions
    • How do you compare this chapter to the chapters that I write?
    • How do you compare this method to the picture drawing method that I present in the video?
  • 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 ball. 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 Weiman who won the Nobel Prize in Physics for Bose Einstein Condensate 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 today:
    • “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,”

During Class
Elastic Collisions
Surviving a fire in your door room: we talked about at the end of class Tuesday (sorry for keeping you later than 4:00)

Second Hour
Goals: Learn about Friction

  • Dynamics: Friction is a force = to the product of the coefficient of friction and the normal force between two bodies.
  • Dynamics and momentum: Friction acts in the opposite direction of the relative motion of the two bodies exchanging momentum. So say, you are spinning your car’s tires when the light turns green. The tires are moving backwards relative to the ground. This pushes the tires forward and the ground backwards. You can see this especially when some of the road gravel flies backwards.
  • Energy: Friction turns kinetic energy or mechanical work into heat energy. In the above example, the tires and road get hot. In drag racing, the cars melt some rubber down on the starting pad by spinning their tires.

Before Class

  • Read 5.1 friction from the original OpenStax textbook. Please reflect on how this works for you compared to the short chapters I write like: 3.0 friction.
  • Check out the video I made for you guys about dragsters and friction.
  • Prepare for midterm by doing last quarter’s midterm exam or midterms from other previous classes, accessible from the general sharedcurriculum website. Bring in your answers, questions, etc. Your midterm will be similar to this with the following difference. There may be some questions on your midterm that do not have a numerical answer. For instance, I might ask you to just set up a problem or consider a situation without calculating a numerical result.
  • Please see this video on using calculus to look at kinematics
  • graphing the motion, force, energy, power of an object: motion of an object, We didn’t do this problem on our problem set, but a similar problem could be on the midterm, so please watch it, and try to do it before I explain it… and come with questions if you have difficulty.
  • Review how I solve the Ballistics Pendulum Video from an old problem set.
  • Optional: another education article on NPR: How you think with your hand, not your brain (or at least not your computer).

During Class
We do an experiment with the Ballistics Pendulum

After Class
Hey, I wish I could look at even more midterms!… Hey take a look here: MT#1_W16
MT#1_Answers, Pete’s MIT#1 Solutions
If you want to make maximum benefit from this resource, you will NOT start by looking at the solutions. My recommendation is that you:
1) sit down and “take” last quarter’s MT#1 in 50 minutes under a test like situation, quietly alone.
2) “grade” your midterm with the answers and make corrections without the solutions. I recommend you do this with a group of students and feel free to check videos and/or your textbok to get more information. I bet it will be fun to do during office hours on Monday! allow up to two hours for this without looking at the solutions
3) Only after you have worked out the answers on your own, consider the solutions… the solutions are only the way that I did them, so make sure you recognize the value in the way you worked the problems too.

Thursday Big exam today. The plan is to discuss and review for ~20 minutes, have a one hour exam, and then debrief as a class and get video projects back.

  • I am not collecting PS#3 until after MT#1, so you can use it to study from. I have posted on the main webpage guidance on how to approach the problems and where to find solutions.
  • i added a brief description of 3.0 friction with three important examples. We didn’t do any friction problems in class, so I encourage you to read this and do the examples because friction is covered in the midterm.
  • Watch solutions for the last question on PS#3: Throwing Box Off Cliff
  • View Big Picture of Mechanics to get an overview of the first 4 weeks of mechanics.
  • Solutions for PS#1 sliding down a curved ramp question. I also go over some calculus that may be interesting here. This is from a past class, so the numbers are different.
  • 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.

During Class

  • My take on how to know which lens you’re to use:
    • MOMENTUM: When there is an interaction between two objects, such as a collision, or an “inverse collision” (when one object pushes or pulls on another object). Sometimes you consider both bodies, but sometimes you just consider one, like when the earth pulls on your body, and we really don’t consider what happens to the motion of the earth.
    • ENERGY: When there is a before and after that converts energy. Things like height and speed have a corresponding energy terms like PE and KE.
    • DYNAMICS: if there are forces and acceleration.
    • KINEMATICS: when everything in front of you has to do with displacement and the time rates of change. In particular, you already have these equations, and don’t have to figure them out by examining forces, momentum, or energy.
  • as we examined in class, some interactions require more than one lens – such as which curved frictionless track brought the ball to the end in the shortest amount of time. We need to consider energy (or dynamics) to compare their speeds, and then kinematics to compare times from speeds.

Questions about midterm:
– The midterm covers everything we’ve talked about in class, saw in videos, or had in assigned readings. In short, if you’ve been exposed to it though the class, then please expect that it could be covered on the exam.
– I added a brief description of 3.0 friction with three important examples. We didn’t do any friction problems in class, so I encourage you to read this and do the examples because friction is covered in the midterm.

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