## 20171002

### Online reading assignment: work and energy

Physics 205A, fall semester 2017
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.
"Translational kinetic energy is the energy of motion, it is dependent on the objects mass and the square of its velocity. Work done on the object will result in an increased change of change in transitional kinetic energy, while work done against the object will decrease it."

"Translational kinetic energy is the energy of motion. To change the translational kinetic energy of an object, a force must do a certain amount of work either on, or against the motion of the object. Work on an object increases the speed by the work against the object decreases speed."

"Work is the force that is applied to an object multiplied by the distance/displacement it traveled."

"I understand the concept of work and that to do work on or against something that a force must be applied."

"Kinetic energy is energy in motion compared to potential energy which is not moving."

"When the force is in the same direction as the displacement work is positive. The opposite is also true when the force is in the opposite direction as displacement then work is negative."

"When a force performs work on an object, there will always be a change in kinetic energy."

"A stationary object has no translational kinetic energy, and the faster an object moves, the more translational kinetic energy it has. Also, if a force does work on an object, then the work will have a positive sign, and the translational kinetic energy of the object will increase."

"The SI units I understood quite well."

"I understand the meaning of work and how it is used in the real world, just not exactly how to calculate angles and directions."

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 was kind of confused about how the angles and the cosθ that was thrown in there."

"The squirrel catapult part with the 90° angle thing is the only part of the reading that I found confusing."

"I'm a little confused on the work = F·scosθ equation."

"I didn't find any main concepts confusing, but a little review in class would be helpful. Just clarifying application to real-life circumstances like what was done in the online presentation would be helpful."

"The concept of joule was a little confusing. As well as the kinetic energy section. I understand the general idea, but a little more practice and explanation would help."

"Applying the work-energy theorem."

"The examples and proofs in the book confused me, as they tend to. I am somewhat confused by the angle relating to force and how that is applied to the kinetic energy equations."

"I don't understand translational kinetic energy, the work energy-theorem and how I would begin to apply these to problems."

"All of it. So far so good. This section is pretty easy to understand."

"Nothing too hard nor confusing was found."

"Most of the stuff I read was pretty clear."

Explain how the SI (Système International) unit for work is related to the SI unit for force.
"Work is related to force by way of newtons (the SI unit of force), and when you multiply them by meters, the joule (unit of energy) is found."

"A joule is the work done by a force of one newton acting to move an object a distance of one meter in the direction in which the force is applied."

"I'm confused by this question."

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). [81%]
Pushing opposite the direction of motion: work done "against" the object (negative work). [92%]
Pushing 90° sideways to the direction of motion: no work done. [46%]
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). [54%]
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). [54%] For the catapulted squirrel, the bungee cord force does work __________ the squirrel, which __________ the squirrel's translational kinetic energy.
 on; increases. *****************************************  against; decreases. *****  (Unsure/lost/guessing/help!) ** For the braking car, the brakes do work __________ the car, which __________ the car's translational kinetic energy.
 on; increases. ***  against; decreases. ******************************************  (Unsure/lost/guessing/help!) *** 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. ********  on; against. ********************************  against; on. ***  against; against. **  (Unsure/lost/guessing/help!) *** 

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. **  the same as. ****  greater than. ***************************************  (Not enough information is given.)  (Unsure/lost/guessing/help!) *** 

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Is 9.80 m/s2 the same as in 'g's?' Otherwise, everything is pretty straightforward." Yes, if you measure acceleration (or weight) in g's, then that's how many times this acceleration (or weight) is greater than that due to gravity."

"I need more explanation on connecting force and motion."

"I was confused by the work-energy theorem. I would appreciate some review of that as well as work in general."

"This all makes sense, but in a scarily easy way where I'm pretty sure I am completely wrong."

"The example questions did not click to me. I do not understand the difference of work and force."

"I feel I'm tackling the major concepts of the class at home, while not fully understanding most of them, and then coming into class and just brushing the surface of these concepts with some problems but little explanation and/or direction on how to go about solving these problems."

"Work on an object makes sense, but again it becomes a problem of integrating what I currently know into what is new information. Hopefully in-class work will aid in my understanding."

"I would like to work example questions in class."

"I think I understand most of the material but could benefit from more examples of how work effects transitional kinetic energy in different situations. Additionally some review on the changing of the angle of the force and displacement vector and how it effects the work being done on an object."

"The bungee cord question was the most confusing for me out of all the questions. Very quickly, if there is time, I would like you to clarify the answer to this question in class tomorrow. Thanks."

"I really am starting to understand physics better now than I have before."

"Nothing really confusing; I just wish you could teach this instead of trying to teach myself."(Well, I'm okay with lecturing in class as long as I know specifically what you need to hear; or if you're not too confused, then I can build upon what you already know. But I have to have to you try to go through this material for yourself first (which can be frustrating!), at least to familiarize yourself with the new definitions and concepts before we talk about them in class.)

"The increased description during the presentation preview increased my clarity on the topics. If possible can you keep that level of explanation or even increase it?" (I think when there are plenty of applications at the end of some presentations, I can try to break down one application as an extended example for you while leaving the rest for you to do on the reading assignments.)

"Why is it that there would be no work done on an object at 90° sideways to the motion?" (Forces exerted perpendicular to the direction of motion would not affect its speed, but would only deflect the direction of its motion. So, basically uniform circular motion, where the speed remains constant while the direction continuously changes, due to the net force in the centripetal direction, sideways to the velocity vector.)

"Is it possible to have a force that is doing both, working on and against an object?" (No, either a force points along the direction of motion, or opposite the direction of motion, or sideways. If the force is diagonal to the direction of motion, then you can break it down into components parallel to and perpendicular to the direction of motion.)

"Relating back to vectors, if the angle between force and displacement were to be past 90°, would the angle be taken from the +x-axis in the counter clockwise direction?" (Yes, because force and displacement, when drawn "tail-to-tail," would make an obtuse (greater than 90°) angle.)

"More of a concept question but why would it always be cos rather than say sin or tan to calculate work?" (If displacement is set along the +x-axis, then you use cosine because you are basically finding the (horizontal) component of force "along" the direction of motion.)

"If an object is moving to the right, then would gravity have no effect on its kinetic energy because gravity is 90° straight down?" (Yes.)

"Can work being done on a object if it's stationary? Or would it be against it?" (Work has force and displacement in it, so while a force can be exerted on an object that is stationary, the displacement (straight-line vector from start-to-finish) would be zero, so work be zero as well.)

"Are the midterms set up the same as the quizzes are?" (No; see comment below.)

"In the midterm do you ask questions like above and like those found in the blog? and also definitions? Or will it be essentially solving problems?" (The midterm will have four questions where you have to apply concepts in your explanations; there will only be one problem where you need to strictly use equations to calculate stuff.)