## 20161107

### Online reading assignment: simple harmonic motion

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 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.
"Simple harmonic motion means you ignore any outside work such as friction or drag. An example of this condition would continue forever."

"Energy conservation--the idea that its only kinetic energy and potential elastic energy that come into play make sense to me."

"How increasing the mass and pendulum length would increase the time to complete one cycle of motion, and increasing the gravitational constant or the strength of the spring would decrease the time it would take to complete the cycle."

"The restoring force of an ideal spring is Fx = –k·x. The k refers to the spring constant while x is the displacement of the spring from its unstrained length. The minus sign indicates that this restoring force will point in a direction that is opposite to the displacement of the spring."

"The total energy conservation with always be equal to the translational kinetic energy plus the potential energy. If one goes down the other must go up and vice versa to conserve the energy in the system. This is always because in simple harmonic motion, we ignore external non-conservative work."

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 really confused about energy conservation."

"I don't know when I'm supposed to use the simple pendulum equation or the mass-spring equation. I do understand the equations themselves though."

"Mass-spring period is slightly confusing. Because it is dependent on the mass attached to the spring. With the pendulum doesn't it also depend on the mass attached to the string?"

"Why Newton's second law is applicable to simple harmonic motion."

"The pendulum period vs. the mass-spring period. Also, the section of the book that talked about the simple harmonic motion and the reference circle was very confusing."

"I don't understand why the x value of the elastic potential energy equation is squared, or why any equation squares its variable."

"I found little confusing, it's really straightforward."

"This section did not confuse me, I feel like I understand it pretty well."

The total energy of a mass-spring system would have zero translational kinetic energy and all elastic potential energy at the:
 origin (x = 0). * [1] turnaround points (x = ±A). ******************************* [31] (Both of the above choices.) ** [2] (Neither of the above choices.) * [1] (Unsure/lost/guessing/help!) **** [4]

The total energy of a mass-spring system would have all translational kinetic energy and zero elastic potential energy at the:
 origin (x = 0). ********************** [22] turnaround points (x = ±A). [3] (Both of the above choices.) ** [2] (Neither of the above choices.) **** [4] (Unsure/lost/guessing/help!) ******** [8]

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. ********************************* [33] equal to. ** [2] longer than. [0] (Unsure/lost/guessing/help!) **** [4]

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. ****** [6] equal to. *********** [11] longer than. ******************* [19] (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. ******************** [20] have no affect on. *** [3] increase. ******* [7] (Unsure/lost/guessing/help!) ********* [9]

For this mass connected to two springs, its springs weakening over time (decreasing its spring constant) would __________ the period of oscillation.
 decrease. ********** [10] have no affect on. **** [4] increase. ****************** [18] (Unsure/lost/guessing/help!) ******* [7]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Does increasing the period of oscillation mean it takes longer for one cycle of motion?" (Yes. By definition the period is the time it takes for one cycle of motion to complete.)

"Please go over the pendulum stuff...how about just go over everything since it all ties together."

"I'm confused on the relationship between KEtr and PEelas. Why does their total equal the same amount? And how do I locate the location where E = KEtr?" (If no mechanical energy is lost to friction or drag, then you will always get to keep the same amount of total mechanical energy E you started with, but the total E may be in the form of KEtr and/or PEelas. As a mass on a spring oscillates back and forth, when it is momentarily stationary (making KEtr = 0) in its motion is when all of its energy is the form of PEalas; and when the spring is momentarily relaxed (making PEalas = 0) is when all of its energy is the form of KEtr only.)