# Spring 16 Mechanics Week 4

Day 1: Elastic Collisions

Before Class
Read section 8.4 in the text and the previous sections to provide background – 8.3 may be new to you and worth the look. You can solve the elastic collision with the simultaneous equations as they do in example 8.4, conserving both momentum and energy. However, you could also solve this by drawing pictures as I do in the video posted for today.

• Elastic Collisions with Walter Lewin at MIT he’s super famous and was at MIT when I was a student there.
• Elastic Collisions by Changing Reference Frames This is my most popular video – after ~ 3.5 years, it has over 10,000 views.
• Please see this video on using calculus to look at kinematics
• 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.” and
“Teachers who find their kids’ ideas fascinating are just better teachers than teachers who find the subject matter fascinating,”

During Class
Surviving a fire in your door room: see above.
Collect PS#3

After Class
Hey, I wish I could look at last quarter’s midterm #1!… 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.

Day 2

Before Class

• Prepare for midterm by doing last quarter’s midterm exam. 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.
• See solutions to PS#3 posted on main class website.
• Solutions for PS#1 sliding down a curved ramp question. I also go over some calculus that may be interesting here. This is from last year, 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.
• 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 PS#3.
• Yet another education article on NPR: How you think with your hand, not your brain (or at least not your computer).
• Watch video on throwing a block off a cliff.
• If I have a Problem Set or Big Exam! because you handed it in late or didn’t pick it up, please come find it pinned outside my door.

During Class

• I won’t be here for class or office hour on Tuesday. I am giving a talk at UCSB. Yes, I made this commitment when I thought we were having a strike. The learning assistants will collect the classwork.

Review for MT#1… you know that lens identification is crucial to solving the problem.

• My take on how to know which lens you’re to use:
• ENERGY: When there is a before and after that converts energy. Such that everything you’re interested in (like height and speed) have a corresponding energy term (like PE and KE).
• 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. Especially important if motion of both bodies are to be considered (as opposed to when the earth pulls on your body, when we really don’t consider what happens to the motion of the earth).
• DYNAMICS: if there are forces and energy.
• 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.

After Class: Review more problems.

Day 3: MT#1 – 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. Please find a simple calculator with only trig, square roots and simple math. – Thanks
After Exam: Relax, redo/improve your video projects and see videos about rotation!