20160926

Online reading assignment: work and energy

Physics 205A, fall semester 2016
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 energy of movement, and that it is affected by work. If work is done against the object then it will slow down, and if it is done on the object then it will speed up."

"Work done on an object will be force directed along the direction of the motion, and the work will have a positive sign. The work done against an object will be force exerted opposite the direction of its motion, and the work will have a negative sign."

"If the displacement of an object is zero, the work is zero no matter how much force is applied. A joule is a newton·meter. The difference between initial and final kinetic energy is the amount of work being done."

"Work is a scalar quantity, in units of joules, that requires a force and a displacement. Work-energy-theorem relates work to the change in kinetic energy. Kinetic energy is also a scalar quantity."

"Even if a force is acting on an object with mass, if there is no displacement of the object, them the amount of work being done on the object is still zero. I also understand the relationship between the force and displacement (work being done on an object) will change the kinetic energy of the object."

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.
"The actual equations for work and translational kinetic energy look a little intimidating and confusing, but I think that once we use them in examples that I will understand them better."

"I would benefit from more examples of the work-energy theorem equation."

"I don't really understand how if the displacement s is zero, then work is zero, even if a force is applied."

"I'm not 100% sure how the angle between the force and displacement vectors figure into calculating work."

"I am somewhat confused about the work-energy theorem which states that W = KEfinalKEinitial. I would think that work would be equal to the sum of these quantities, and not the difference."

"It is a bit confusing on what 'does' work on what."

"The equation for calculating translational kinetic energy. Also how this tied in with the initial and final kinetic energies of an equation."

"I don't really understand why if the forces are perpendicular they are not doing work?"

"I'm a little confused on why we focus so much attention on the initial to final change in kinetic energy."

Explain how the SI (Système International) unit for work is related to the SI unit for force.
"The unit for work, the joule, is calculated using the force unit of newtons times the displacement in meters of an object's travel."

"They are similar in that they both have the component of Newtons, but what work has that force doesn't is the displacement in meters and is measured in joules (N·m)."

"They are both measured in joules, because they are similar forces that cause kinetic change?"

"Work is equal to the mass times the distance?"

"The SI unit for work is related to the SI unit for force because it shows us how all the units come together as one?"

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). [92%]
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. [65%]
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). [57%]
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). [57%]

For the catapulted squirrel, the bungee cord force does work __________ the squirrel, which __________ the squirrel's translational kinetic energy.
 on; increases. ******************************************** [44] against; decreases. ***** [5] (Unsure/lost/guessing/help!) [0]

For the braking car, the brakes do work __________ the car, which __________ the car's translational kinetic energy.
 on; increases. ** [2] against; decreases. ********************************************** [46] (Unsure/lost/guessing/help!) * [1]

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. *** [3] on; against. ******************************** [32] against; on. ******* [7] against; against. ** [2] (Unsure/lost/guessing/help!) ***** [5]

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. ** [2] the same as. [0] greater than. ******************************************* [43] (Not enough information is given.) * [1] (Unsure/lost/guessing/help!) *** [3]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Will we review these questions in class?" (Yes.)

"How does the angle between the force and displacement vectors figure into calculating work?" (More on this in class.)

"Will you give these equations on the midterm/quiz?" (Yes, if those equations have already appeared at the bottom of a practice quiz, or worksheet packet. Everything else, you will need to memorize.)

"Poor squirrel." (No, no, good times. The squirrel was having good times.)

"What format should we expect on the midterm?" (Four conceptual short-answer questions and one worked-out numerical problem. The study guide for specific topics covered on the midterm is now posted on the course website.)

"How has Mrs. P-dog been these days?" (She's fine, being awesome as always. Eh, squirrel, maybe not so much.)