# W16 Mechanics Week 4

Day 1: Elastic Collisions, PS #3 is posted. I added a 4th problem Thursday morning.

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 Educanon video posted for today.

During Class
More examples of elastic collisions
Collect PS#3
Collect Mandatory project #1. I’ve added the following to the syllabus:

#### …If you do each of the two projects adequately, you will receive full credit. If I find the project lacking, I’ll ask you to improve it for full credit. Your grade will be docked a half letter grade for each project not completed adequately.

After Class
Hey, I wish I could look at last quarter’s midterm #1!… look what I found:
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 text to get more information. I bet it will be fun to do during office hours on Wednesday! allow up to two hours of this without looking at the solutions
3) Only after you have worked out the answers on your own, consider the solutions… this is only the way that I did them, so make sure you recognize the value in the way you worked the problems too.

Day 2
I just posted answers for PS#3 are on main page.
Before Class

• Prepare for midterm
• 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?
• Solutions for Questions 2 and 3. 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 Question 4 (the accelerating car) This is from last year, so the numbers are different.
• 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

During Class
We will do some demos and solve some problems.

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.

During Class

• More examples. Practice identifying flows of energy and lens identification.

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. Additionally, if all you can find today is a “smart” calculator, then you can use it (only on this exam), but do not use any of the smart functions such as graphing and differentiation and integration. In future exams, please find a simple calculator with only trig, square roots and simple math. – Thanks
After Exam: Remember to see videos for tomorrow’s class! We start with 2 dimensions!

Day 4: Begin 2 Dimensions – Qualitative worksheet. Pete in Seattle at the ETHOS Conference, but you folks will come and review MT#1, and will actively learn both about past material in reviewing the exam and work on two dimensions by handing in a worksheet stapled together with your group. I was hoping that I could have your exam back to you for Friday, but I think in the interest of taking care of myself – getting up at 4 AM to bicycle to the airport, it would be best for me to sleep and plan. I’ll grade the exams on the airplane. I appreciate your patience and understanding in this. I’ll post answers and you can discuss the exam during class and Emily will do some great demos and collect the worksheets.

Before Class

• Check out PS#4 (main webpage) and consider it while you watch the videos
• see estimating vector components
• Watch Intro to 2-D Forces
• Watch Intro to 2-D Kinematics
• A copy of Midterm #1 with answers is posted on the main website. Please have a look.
• PS#4 is posted on the main webpage. Please read it immediately so you know what we’ll be working with.

In class:

• Discuss MT#1
• Explore Components, Do and hand in worksheet.