## 20141110

### Online reading assignment: simple harmonic motion

Physics 205A, fall semester 2014
Cuesta College, San Luis Obispo, CA

Students have a 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 simple harmonic motion.

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.
"Harmonic motion is the position versus time that will be sine or cosine function. For a simple pendulum (a certain mass on a string), the period depends on the square root of the string length and gravitational acceleration. So, the shorter the string, the shorter the period."

"Harmonic motion is repeated along the same path with a back-and-forth motion. The total mechanical energy must equal Ktr + Uelas. These values do not each have to be exactly half of the total energy, but together they must equal the constant total mechanical energy."

"In order to complete one cycle of motion, the particle must be at the same point and heading in the same direction as it was at the start of the cycle."

"Increasing string length will increase the the time of the pendulum. As will mass when attached to a spring. The Ktr is zero at the turnaround points, as all of the energy is in the elastic potential form."

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.
"For some reason the truck example kind of confuses me, I think I understand but wouldn't mind going over that example."

"How different masses would affect the pendulum period, because mass is not in the simple pendulum period equation."

The total energy of a mass-spring system would have zero translational kinetic energy and all elastic potential energy at the:
 origin (x = 0). ******* [7] turnaround points (x = A). ***************************************** [41] (Both of the above choices.) * [1] (Neither of the above choices.) * [1] (Unsure/lost/guessing/help!) ***** [5]

The total energy of a mass-spring system would have all translational kinetic energy and zero elastic potential energy at the:
 origin (x = 0). ************************************* [37] turnaround points (x = A). ********* [9] (Both of the above choices.) ** [2] (Neither of the above choices.) ** [2] (Unsure/lost/guessing/help!) ***** [5]

For these identical mass billiard balls hanging from strings of different lengths, the billiard balls hanging from shorter length strings have periods that are _________ the periods of the billiard balls hanging from longer length strings.
 shorter than. ******************************************* [43] equal to. ******** [8] longer than. * [1] (Unsure/lost/guessing/help!) *** [3]

For the different mass riders on swings of the same lengths, the more massive rider has a period __________ the period of the less massive rider.
 shorter than. *** [3] equal to. ************************ [24] longer than. ************************* [25] (Unsure/lost/guessing/help!) *** [3]

For the overloaded truck with a vertical oscillation period of approximately half a second, after dumping its load (thus decreasing the mass connected to the springs in its suspension) would __________ the period of oscillation.
 decrease. ****************************** [30] have no affect on. *** [3] increase. *************** [15] (Unsure/lost/guessing/help!) ******* [7]

For this mass connected to two springs, its springs weakening over time (decreasing its spring constant) would __________ the period of oscillation.
 decrease. ****************** [18] have no affect on. ******* [5] increase. ********************** [22] (Unsure/lost/guessing/help!) ******** [8]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"On the reading assignments it is really helpful if we go over the concept then I find it easier to compete the problems." (In theory, that would always be true. Realistically, though, I need data. From your answers to these (hard!) questions the night before, I can prepare exactly which selected concepts confused you and need to be explained in the following class, instead of a pre-packaged, one-size-fits-all lecture that would assume ahead of time what you need or don't need.)

"It is always extremely joyful when you go over everything in class. What I thought I understood is never what it really means." (Well, okay, but we don't have time to go over everything during class, so there are limits on the amount of joyfulness we can have.)

"I'm taking college-level calculus and understand the functions of sines, cosines and their friends." (I think you mean "frenemies.")

"Will you be hypnotizing us in class?" (Well, we might have time to watch this mesmerizing pendulum wave video.)

"Will I pass this course?" (I think we had this conversation already. But now that we have more data, let's have this conversation again.)