20190923

Online reading assignment: work and energy

Physics 205A, fall semester 2019
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing a presentation on work and energy.

Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.
"Work is the result of the magnitude of force times the magnitude of displacement of an object. Because displacement must be present, work does not apply to immovable objects. "

"How translational kinetic energy increases or decreases depending on if work is done on or against the object."

"Work is the magnitude of the force times the magnitude of the displacement and does not have direction, therefore, it is a scalar quantity. If an object does not move, then the force acting on the object does no work. When the force and displacement do not point in the same direction, only the component of the force along the displacement is used to find work."

"Work is scalar meaning there is no direction necessarily. There are two facets of work, as it comes positively and negatively. Positive work indicate the work done on the object, and negative work indicates the work done by the object. If an object is in constant motion, then the net positive and negative work equal zero. Work on a moving object dependent on the direction means the net positive work is greater than the negative work. We can use the work-energy theorem which is essentially W=1/2mvf^2 and can be manipulated to find certain values however you life. Giving you the ability to find the values necessary to complete a problem."

"Oh boy. This one was a doozy! So first, the textbook talked about joules. I like joules. They’re simple enough, made sense. THEN the textbook talked about Kinetic Energy, how that relates to the kinematic equations, how we factor in Newton’s second law and some of the theory behind forces. FINALLY it drops the bomb of two very involved problems. My mind was blown, and I’ll need the next 48 hours to process how I feel."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.
"I'm a little confused on how we're supposed to apply kinematics for kinetic energy and work. Are we trying to find a velocity? is it given? or does it just depend on what the problem asks?"

"I can't picture how the angles between forces and displacement correlate to what's going on. I don't know how to use the angles to see when work is negative."

"The idea of work is still a little confusing. I'm unsure of where this comes from or how we would measure it."

"Pretty much just kinematic energy as a whole, and why it's important. At this point, it seems like a whole bunch of formulaic substitutions that were made somewhat arbitrarily. I know that that's entirely not true, and that there’s plenty of usage. However, I need to figure out how and why."

"I think I just need to listen to the lecture in class to get a better understanding of it. No specific questions."

"I would benefit from an example in class of pushing an object with an acute angle/obtuse angle and whether it results in positive work or negative work."

"I'm sure there are somethings in there I don't know, we'll see."

Explain how the SI (Système International) unit for work is related to the SI unit for force.
"Force × distance = work, which is newtons (N) × meters (m) = joules (J)."

"the SI unit for work is the Joule while the SI unit for force is newton. The joule for work is found by the force multiplied by the displacement of the force on an object."

"Force can be put upon an object, but it isn't work until it moves. Therefore the unit for work has to account for movement."

"Because you need force to calculate the work."

"They both have to do with the newton."

Match the description of the work exerted by these forces for each object. (Only correct responses shown.)
Pushing in the same direction of motion: work done "on" the object (positive work). [87%]
Pushing opposite the direction of motion: work done "against" the object (negative work). [93%]
Pushing 90° sideways to the direction of motion: no work done. [47%]
Pulling such that the angle between the force and motion is an acute angle (between 0° and 90°): work done "on" the object (positive work). [64%]
Pulling such that the angle between the force and motion is an obtuse angle (between 90° and 180°): work done "against" the object (negative work). [60%]

For the catapulted squirrel, the bungee cord force does work __________ the squirrel, which __________ the squirrel's translational kinetic energy.
 on; increases. **************************************** [40] against; decreases. **** [4] (Unsure/lost/guessing/help!) * [1]

For the braking car, the brakes do work __________ the car, which __________ the car's translational kinetic energy.
 on; increases. ****** [6] against; decreases. *************************************** [39] (Unsure/lost/guessing/help!) [0]

For Mrs. P-dog being catapulted, the bungee cords do work __________ Mrs. P-dog, while the weight force does work __________ Mrs. P-dog.
 on; on. ** [2] on; against. ********************************* [33] against; on. ******** [8] against; against. [0] (Unsure/lost/guessing/help!) ** [2]

For Mrs. P-dog's translational kinetic energy to be increased while being catapulted, the amount of work from the bungee cords must be __________ the amount of work from the weight force.
 less than. * [1] the same as. **** [4] greater than. ************************************** [38] (Not enough information is given.) [0] (Unsure/lost/guessing/help!) ** [2]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Can we go over how angles change work?"

"Please go over these in class! Thank you."

"I guessed on most of the questions because I need help :("

"Will we be getting a lot of things like, 'this is how the work works,' or any other things like that?" (It will be more like, "this is how energy conservation works." Work and kinetic energy will be just components of energy conservation.)

"I would like a brief discussion of how the signs work. More closely the (+)=(+) or (-)=(-), is this every time or is there specific situations this isn't always true?" (The signs on both sides of an equal sign must have the same ± sign; otherwise it's not an equal sign.)

"How can you change an object's translational energy if it is infinitely small on Earth?" (With other infinitely small objects. Like us.)

"Why is kinetic energy important?" (...asked the dinosaur just before Earth was hit by the Chicxulub impactor.)

1 comment:

Patrick M. Len said...

Some additional questions from last week's post-lab assignment:

"Can you review why the error bars were so small and yet the percent error [for experimentally validating the linear trend line] was a decently-sized value, about 30%?" (The linear trend line was more than adequate to model the behavior of your cart's acceleration for small to medium hanging masses (acceleration proportional to hanging mass m), but the acceleration is definitely non-linear (acceleration is actually dependent on both the hanging mass m and cart mass M: (m/(M+m)), and if you had a modestly large hanging mass, this definitely non-linear behavior should show up as a sizable percent error between the experimental and predicted acceleration values.)

"I was wondering if there are any tutors on campus for physics? Or any study sessions available, I feel like I need a brush up." (Yes, you can check the drop-in tutoring schedule at the SLO campus Student Success Center.)

"I'm just wondering if we are still going to learn how to write post-lab reports as you had mentioned about a couple of times." (Right now the emphasis in lab is on good data-taking and graphing practices. Starting next week, in addition to the usual experiments, lab time will also cover the format for writing abstracts, procedures, and evidence-supported concluding statements. Then for the last part of the semester, lab time will only be used for taking data, and you will write individual lab reports to be turned in at the start of next lab. We'll gradually build up to that.)

"Will there be a lab exam?" (Not really, but at the end of the semester you will be expected to be able to write properly formatted lab reports.")