## 20161102

### Online reading assignment: elasticity

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 elasticity.

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.
"Compression is a squeezing force, tension is a stretching force, and tensile stress is force applied to the center of a strip of material perpendicularly."

"To compress or stretch a spring, a force must be applied to it, by newtons third law the springs also applies an opposite force of the same magnitude. The reaction force is applied to the object attached to the spring."

"Hooke's law of restoring force of an ideal spring is Fx = –k·x."

"Compression and tension. Compression is squishing something and tension is when you stretch something."

"The difference between tension and compression. I also understand how those forces cause strain."

"How elastic deformation in a spring is non-permanent and reversible while plastic deformation of a spring undergoes permanent deformation and is non-reversible. Hooke's law can be used to find the tension force, the length or extension of the spring, and/or the value of the spring constant."

"This section was on elasticity. Tension is when you stretch something and compression is when you squish something. Tensile stress is applying a force to a certain cross-section sample to stretch it, while tensile strain is a measure of how the material responds. Hooke's law is used to calculate tensile/compressive stress. Stress is exerted in Pa, and the strain is unitless in the equation."

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 don't really understand where ∆L comes from. I don't know 'what change in L' means (especially in the context of the 2×4 problem)."

"I am confused by some of the variables for the equations from the reading. So, I would like to go over those and to also apply it to situations to make the subjects more relatable."

"I don't understand how to balance the hooke's law equation for two different lengths of material. If one side of the equation remains constant doesn't that mean the whole equation remains unchanged?"

"I didn't really find anything confusing."

"I found the Hooke's law to be a little confusing in the blog. The Y variable is throwing me off."

"I don't understand some of the textbook problems that they worked out. I feel like they were mixing up ∆L and L but I know it must be my mistake."

"I found the whole strain thing a little confusing, kinda had me overall 'confuzzled.'"

"I don't understand why the forces between atoms act like springs. How were the values for Young's modulus of solid materials derived?"

What is the SI (Système International) unit for stress?
"Pa."

"Pascals."

"N/m2, aka Pascals (Pa)."

Explain why strain is a unitless quantity.
"Is it because the units cancel out?"

"Because length is both in the numerator and denominator so they cancel out."

"Change in length divided by original length. Since both the 'change in length' and 'original length' have the same units, strain is a unit-less quantity."

"Because strain is how the material responds?"

"It depends on the material and how it responds to stress?"

"Because of the infinite ways to represent it in different scenarios. It depends on the cross-sectional area, the material, etc.?"

What is the SI (Système International) unit for Young's modulus?
"Pa."

"Pascals."

"N/m2."

"It is named after the French polymath Blaise Pascal."

The __________ lengths of vertical suspension bridge cable are stretched by a greater amount ∆L from their original lengths.
 shorter. *********** [11] longer. *************** [15] (There is a tie.) ******* [7] (Unsure/lost/guessing/help!) ***** [5]

The __________ columns of 2×4s support the least amount of force.
 narrower (two 2×4s). *************** [15] wider (three 2×4s). ****** [6] (There is a tie.) ********** [10] (Unsure/lost/guessing/help!) ******* [7]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Did not get a chance to read this section yet. Cramming for an exam in another class I forgot until now."

"I don't fully understand what Young's modulus represents."

"How does cross-sectional area apply to Hooke's law?"

"I am not sure about these last two questions. Can we go over them in class?"

"If a suspension bridge's max load was reached, evenly displaced, which wire would snap first?" (Assuming that they all carry the same stress, and are designed to stretch the same ∆L amount, then the wire that experiences the most strain would probably snap first--which would be the shortest one.)