This week we will bring vectors into our lens method summary and begin to explore collisions via reference frames. We will also introduce friction and see where and when it is useful and a nuisance.

Learning Outcomes

By the end of this week you will be able to:

Use vectors in your application of the four lenses
Explain the difference between inelastic, elastic, and partially elastic collisions
Analyze elastic collisions by considering a change of reference frame
Apply momentum and energy conservation to solve inelastic collision problems
Discuss the limits of human power
Describe the difference between static and dynamic friction
Evaluate how friction forces change depending on physical conditions
Solve dynamics problems involving friction

Day 13 | Monday 1/24 | Big Picture, Reference Frames

Have you read the responses to the Anonymous Survey yet? Did the things others wrote resonate with you? Were you surprised by anything? This week we update the roadmap to the lens method of problem solving by including vectors. The "Big Picture" mechanics video includes a summary that might be useful on your idea sheet... We will consider an alternate way to solve elastic collision problems by changing reference frames and compare it to the traditional method. Which one do you prefer?

Goals:

By the end of the today you will be able to...

Use vectors in your application of the four lenses
Describe how our perception of a situation depends on the reference frame from which we view it
Solve a two-body nuclear decay problem

Before Class:

Textbook: Read Ch 2.8 - Update on Applying the Four Lenses [1 pg, 0 ex]
Video: Watch Big Picture Mechanics [5m59s]
Textbook: Read Ch 3.0 - Changing Reference Frames [1.5 pg, 2 ex]
Video: Watch Elastic Collisions with Walter Lewin MIT [2m25s]
External: Please look at this way of solving elastic collisions using algebra and compare it to using reference frames (tomorrow).
Discussion: Please leave a statement or question on our Week 4 Reading Discussion
Survey: Take this Short Survey - Day 13 [5 questions]

During Class:

We will consider how our frame of reference changes our perspective on a situation...in physics
We will look at collisions from different reference frames
We will compare problem-solving methods
We will consider a nuclear decay problem

Day 14 | Tuesday 1/25 | Elastic Collision by Reference Frame

Using reference frames can simplify collision problems but it also takes some thinking. Today you will explore collisions and maybe discover your preferred method.

Goals:

By the end of today you will be able to...

Analyze elastic collisions by considering a change of reference frame
Apply energy and momentum conservation to solve inelastic collision problems
Express your preference for algebra vs. reference frames

Before Class:

Discussion board grades: I read the Week 3 discussion board today. Very few people received full credit. I expect a thoughtful statement and also a response to someone else's statement. Please do a good job from here on out.
I uploaded a new Homework with the extra "fire hose" problem we did in class. Please give it a try: Upload Problem Set #4, to Canvas before class.
Video: Watch Measuring the Speed of a Bullet [7m55s]
Textbook: Read Ch 3.1 - Using Changing Reference Frames [2.5 pg, 2 ex]
Discussion: Please leave a statement or question on our Week 4 Reading Discussion
Video: Watch Elastic Collision, Changing Reference Frames [9m27s]
Exercise: Can you calculate the speed of the bullet in this 45 s video? (optional)... Looking at the height change to be about 46 cm, I calculated a speed of ~ 324 m/s, about the speed of sound.
Exercise: Try calculating the speed of the bean bag or plastic hairy thing using this video from Prof Klay's class or this video from an earlier class. What lenses can you use?

During Class:

We will solve an elastic collision using changing reference frames
We will investigate the ballistics pendulum

Day 15 | Wednesday 1/26 | Fire Hose

Today's preparation is review of the material up until now. These videos should help you review the material and also provide support for understanding our first project. As a general rule, we cover things thoroughly before finding them on an assessment. In particular, don't expect to see elastic collisions on the assessment today.

Goals:

By the end of today you will be able to...

Describe the force and power exerted by water from a fire hose
Discuss the limits of human power

Before Class:

PETE HAS COVID??? I've been for sure exposed to COVID late last week. I tested negative last night a rapid home test, so for sure my viral load is low. Consistent with CDC and Cal Poly rules, I will come to class and office hours. However, I will wear a mask and shield and I will have office hours outside.

Video: Watch Kinematic Functions [3m32s]
Video: Watch Force of Water From Fire Hose [3m22s]
Video: Watch Power to a Toaster [3m11s]
Video: Watch Running Stairs Power [5m29s]
Reflect: Please review this graph of maximum human power over time. Where do Robert (German cyclist) and Pete (physics professor) appear on this graph. Does this make sense?
Discussion: Please leave a thoughtful comment, statement, or question/response about the video review on our Week 4 Reading Discussion.
Study: We will have our in-class Weekly Assessment #4. Please look at the solutions to Problem Set #4, to see how to improve. Follow the problem-solving rubric to know what to expect.
Discussion board grades: I read the Week 3 discussion board today. Very few people received full credit. I expect a thoughtful statement and also a response to someone else's statement. Please do a good job from here on out.
I posted solutions to Problem Set #4 [T 1/25]
I've started reading through the projects. I have not yet looked at one that is correctly finished. Please read through the description of what you need to have there. You can improve the grade by resubmitting to do what is needed. If I have not yet graded your project and you know that you need to add something, please just add the new document entitled "improved".

During Class:

We will discuss the fire hose problem
We will review the limits of human power
We will take Weekly Assessment #4 [20 min]
We will end by considering an old problem

Day 16 | Thursday 1/27 | Friction

Friction is an interesting phenomenon. In many applications, we want to minimize friction to maximize energy transfer or efficiency, but in other applications, we must maximize friction to gain any traction or save a life. One person's garbage is another person's treasure, as they say.,,

Goals:

By the end of today you will be able to...

Explain the difference between static and dynamic friction
Evaluate how friction forces change depending on physical conditions
Explain the difference between inelastic, elastic, and partially elastic collisions
Solve dynamics problems involving friction

Before Class:

Reflect: You wake up in the middle of the night to find there's a fire in the next room!!! and the only way to save yourself is to throw a ball at your door to close it. You have two balls of the same mass at your disposal. One ball is perfectly elastic, and the other one is perfectly inelastic. Which ball will be most effective at closing the door? Why? Which lens is the most appropriate?
Textbook: Read Ch 3.2 - Friction [3 pg, 4 ex]
Discussion: Please leave a statement or question on our our Week 4 Reading Discussion.
Video: Watch Friction: Introduction and Applications [5m18s]
Video: Watch Dragster Physics [1m31s]
Survey: Take this Short Survey - Day 16 [2 questions]
I'm grading Assessment #4 Wednesday evening... and many students are not setting up the problems... if it's dynamics, then what is the body you are looking at? Did you make a FBD? a VSotFD? Did you start with the vector sum of the forces = m*a? For the power, did you identify the energy conversions? The first question on Problem Set #5 [T 2/1] is to revisit this question and do a good job and learn and practice the dynamics protocol. So, for tomorrow, please redo assessment #4... this isn't a requirement of course, but a recommendation that you read 2.8 and get a good start with lenses.
As you see above, I've posted Problem Set #5 [T 2/1]

During Class:

We will consider the physics of dragsters
We will practice solving problems with friction
We will throw some balls and save some lives

Week 4 Wrap-up

We are finishing up the first four weeks of instruction and wrapping up our investigations of one dimensional linear motion. Next week we will begin learning about rotational motion. The same tools we have developed up til now will have analogous applications for spinning masses but we will need to incorporate some new concepts.

Take a moment to review our learning objectives from this week and try to evaluate how confident you feel in your mastery of them. Are there any areas where you feel you need more practice? Try reviewing some old problems from Pete's website for more practice.