Online reading assignment: elasticity

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 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.

"Strain is the ratio of the change in length to the length when a tensile force is applied. Stress is the ratio of the tensile force to the cross-sectional area."

"Tensile force stretches something, which results in strain. Compressive force squishes something, which results in strain."

"Four things to consider with elasticity are tension, compression, stress, and strain. Tension is when you stretch something whereas compression is when you squish something. Both tensile stress and compressive stress cause strain. Hooke's law is the quantitative relation between tensile/compressive stress (force exerted per unit area, in N/m², or Pa), and strain (given as a fractional length change, unitless), with a material-dependent Young's modulus (in units of Pa) that characterizes the response of the material to these stresses."

"Didn't read it yet."

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 understand stress. I don't understand all the variables being used."

"I don't understand how Y can be independent of the length and cross section of an area? Is the object like a really solid metal that has no give?"

"I'm still trying to figure out why strain is a unitless measure. Possibly because any change it makes is so minuscule that it cannot be measured?"

"Pretty intuitive, really."

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

"N/m² or Pa."

"idk."

"N times m-squared."

"F/A."

Explain why strain is a unitless quantity.

"Strain is measured in proportion to what is being strained, so instead of being strained by say 2 meters, it is strained by 10%, which is a unitless value that is only related to the object being strained."

"It is defined as change in length, divided by initial length, but since these quantities both have dimensions of length, the result is a dimensionless quantity."

"Because it is a measurement of deformation."

"I'm not sure."

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

"N/m² or Pa."

"k times N times meters-squared."

The __________ lengths of vertical suspension bridge cable are stretched by a greater amount ∆L from their original lengths.

shorter.	********* [9]
longer.	******************************* [31]
(There is a tie.)	********** [10]
(Unsure/lost/guessing/help!)	****** [6]

The __________ columns of 2×4s support the least amount of force.

narrower (two 2×4s).	******************** [20]
wider (three 2×4s).	************ [12]
(There is a tie.)	************* [13]
(Unsure/lost/guessing/help!)	*********** [11]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"I didn't find anything in this section too confusing, but I always appreciate examples in class."

"Will we be given Young's modulus values? Does it just describe how strong materials are?" (Yes, unless it is being solved for. Yes, large Young's modulus materials don't show much strain when stress is applied, given that thickness and lengths are equal.)

"What exactly is the cross-sectional area for cables?" (The area of their circular shape at either end.)

"The string we used in the last lab had a high Young's modulus and low tensile strength. I hope the bond Measure 'L' that just passed for Cuesta College goes into providing strings that don't break so easily." (I would be more concerned with using bond measure funds for the construction of new buildings that don't break easily. #Lyeah)