# W16 Mechanics Week 2

### We will measure distance, speed, acceleration, momentum, force, energy, power.

Day 1:

Before Class
What if you want to see a video the second time and also want to see the questions? Just click on the small vertical bars at the bottom of the video that indicate where the questions are located in the video.

1. View Scaling
2. View Pushing Box Problem.
3. View this 3 minute video : Energy, Work, and Power. Be prepared to answer this question: a 1000 kg car starts from rest and accelerates to 30 m/s in 5 seconds. Please calculate the average power the car produces to do this. Please put answer in Watts and Horsepower.
• 7.7 Power
• Units 1.2. Remember how we should cancel units all the time. Please practice this every time you do a problem.
5. Prepare problem set #1 (available on main class page). Come to class ready to hand in something to get feedback. Come with questions, difficulties, etc.

Day 2:
We are going to graph some of our own motion on Thursday. I show you how I did it in #3 below. #2 is a super cool way to do it (in my opinion), but some students had difficulty getting the software to work.

Before Class. I just added PS#2 to the main class webpage. Please take a look before class. You should find some of the questions to be familiar.

1. Please view the video kinematic graphs, LIVE! and watch me draw graphs on a board to represent my motion on a bicycle.
2. View Smarter Every Day Grasshoppers, and try to repeat his experiment with the other grasshopper. You may have an easy time doing this, or you may have great difficulty because of software compatibility. A past student wrote this to help tracker instructions. Please work together. In the face of fighting with malicious software, you need good friends. All the same, if it doesn’t work for you, please do your best to estimate the acceleration using my Excel method in the graphing video I did below. Ian was able to get tracker to work on his Mac only after he transformed the video into a .mov… here it is Grasshopper .mov
3. I graphed my motion without Tracker. Please view: Graphing running
4. See a video about how to measure power.
5. See a: Graphing a ball toss.
• 2.8 Graphical Analysis of One-Dimensional Motion
• 7.8 Work, Energy, and Power in Humans
7. LON-CAPA, You have two problems to do. Please do them before class. One difficulty students had in measuring speed was simply that frequency is the inverse of the time period. That is if a light flashes at a frequency of 10 per second, then the time period between flashes is one tenth of a second.

During Class
Be prepared for a practice exam. We will have exams every Tuesday. Most of them won’t count toward your final grade and we do them to practice the testing situation.

Day 3:

Before Class

1. See a video about how to measure power.
2. Please view: Graphing a ball toss.
3. Please see how I solved the Grasshopper Problem Using Excel.
4. Plan your activity at Via Carta Bicycle Path.
5. Please see PS #2 on main webpage.
6. See Big Exam #1 answers (that earned a D on the exam) and the solutions (that would earn an “A”), on the main web page.

During Class

• Meet at Via Carta!! Bring a cell phone or video camera. Bring something heavy to throw as high as you can and/or a beach ball, etc.
• If you have a laptop, please bring it, so we can start analyzing your work.
• I hope to hand back Big Exam 1. While your tests are better than the first Big Exam in previous classes, many students have yet to make the transition to prioritize communication over numerical answers. Additionally, I’m sure that many students may feel overwhelmed by the amount of information required on the test. I remind you that we started with all the concepts at once, so I do not expect that you will have them already mastered. However, I do have an expectation that you will have them mastered by week 4 when we have our first midterm. So Big Exam #1 should serve as a very good marker as to what you need to master in the next two weeks.

After Class

Day 4: PS #2 posted on Main Pate
The vector sum of the Forces = mass * acceleration.

Before Class

1. Please learn about Free Body Diagrams and Equal and Opposite Forces.
3. The Elevator Dynamics Problem
4. Bozeman Science FBDs
5. Do LON-CAPA “Week 2″… it’s due Monday, but please do it before class tomorrow if you can.
7. Big Exam #1 Solutions… I uploaded both pages of the solutions. Please see the work I did as it defines what I’m looking for on the exams you will be taking.

During Class

What is equilibrium? How do we find the resultant force? We introduce the “dynamics protocol” (Quiz on this tomorrow):
“The Protocol for Dynamics”:
0) Identify that this problem is about forces => It’s a dynamics problem
1) “oh shit, I don’t know anything”… there’s no formula for a dynamics problem!
2) “but I do know that (vector sum) F = ma”
3) I can draw a force (free body) diagram labeling all the forces at the point of action.
4) I can ask myself “is it in equilibrium?”
=> yes – then the sum of the forces is zero and the vectors must close on themselves
=> no – then I ask another very important question, “which way is it accelerating?” and know that the forces must add to be in the same direction as the acceleration.
5) I can make a Vector Sum of the Forces diagram showing how the forces add to give a net force …. this net force must be in the same direction as the acceleration.

• Pulling on a string on a ball on a string.