# F15 Mechanics Week 4

Solutions to Big Exam #2 and answers for PS#1 and PS#2 are on main page. More solutions to come.
Before Class

• Prepare for midterm
• 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?
• LON CAPA has been set for class time today, Monday! The answers are already available. Make sure you do them before class.
• 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.
• Please see a video on using calculus to look at kinematics

These videos were added Sunday Morning, so if you looked at this page between midnight Saturday night and 8:00 AM Sunday and then concluded there were no videos, please let me know and watch them as soon as you can.
During Class
We will do some demos and solve some problems.

After Class

Day 2:
Tuesday: Review for MT #1 – check out last quarter’s midterm by going to previous class. Link is on main page – The material is slightly different, as we will not address 2-D problems such as an inclined plane until MT#2.

For review of the Ballistics Pendulum, you could review: Ballistics Pendulum Video, but this is due for Wednesday, before the exam. It’s like the BE#3 and Problem #1 for PS#3, but it has different numbers. Answers to PS#3 posted on main page.

Before Class

• For an “A”:
1. Develop lens – what is the concept(s) you are using and why do you know this is the right concept?
2. Illustrate scenario with adequate drawing and explanation
3. Work the problem correctly
4. attention to units
• 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
After Exam: Remember to see videos for tomorrow’s class! We start with 2 dimensions!

Day 4: Begin 2 Dimensions

Before Class

• Check out PS#4 (main webpage) and consider it while you watch the videos
• 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.