Online reading assignment: standing waves

Physics 205A, fall semester 2019
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 standing waves.


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.

"Thick strings have lower fundamental frequencies than thinner strings when they are plucked."

"I understand that the fundamental frequency of a string depends on the wave speed v (which depends on its tension and thickness), and length L. The frequencies that this string will resonate at are then merely integer multiples of the fundamental frequency."

"What I understand is some of the standing waves stuff. I know the equation for getting the fundamental frequency of something. I also understand which things are independent and dependent parameters."

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 feel like this is more in depth of the previous waves section, this feels similar and not difficult, just more of it."

"Putting all the different parts together, and not mixing up the independent and dependent parameters."

"I'm having trouble understanding resonance and how nodes are created."

"The difference between node and antinode wasnt as clear to me. Will need some more clarification in class."

(Assuming that their tensions are approximately equal), the __________ strings have a slower wave speed.

thicker.	************************************** [38]
thinner.	*** [3]
(There is a tie.)	** [2]
(Unsure/lost/guessing/help!)	* [1]

After these same-length guitar strings are plucked (assuming that their tensions are approximately equal), the __________ strings vibrate at a lower fundamental frequency.

thicker.	************************************ [36]
thinner.	** [2]
(There is a tie.)	*** [3]
(Unsure/lost/guessing/help!)	*** [3]

After the bass string is plucked, sliding a finger down to decrease its length would __________ the speed of waves along the string.

decrease.	****************** [18]
not change.	******** [8]
increase.	***************** [17]
(Unsure/lost/guessing/help!)	* [1]

After the bass string is plucked, sliding a finger down to decrease its length would __________ the fundamental frequency of the string.

decrease.	*********** [11]
not change.	********* [9]
increase.	********************** [22]
(Unsure/lost/guessing/help!)	** [2]

For standing waves on a string, classify each of these parameters are being "independent" (able to be changed without affecting other independent parameters), or "dependent" (will be changed when independent values are changed).
(Only correct responses shown.)

Wave speed v: independent [59%]
String length L: independent. [43%]
Fundamental frequency f₁: dependent. [50%]

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

"May we focus on the fundamental frequencies? I did not understand the equation."

"Go over deez."

"Is there a particular reason we left waves and then came back to them later?" (We lost a Monday due to Veteran's Day, so we'll pick it up here just before you do the standing waves lab later today. Also this stuff is not on Quiz 6 or on Quiz 7, but you'll see it on the Final Exam.)

"How was your Thanksgiving?" (Eh, it was okay. The day after Thanksgiving went much better.)

"I'm full from the turkey."

Physics midterm question: comparing vertical forces supporting tilted beams

Physics 205A Midterm 2, fall semester 2019
Cuesta College, San Luis Obispo, CA

A force F₁ pulls up at the end of a uniform beam to hold it stationary at an angle of 80° above the horizontal, and a force F₂ pulls up at the end of an identical uniform beam to hold it stationary at an angle of 10° above the horizontal. (Calculate all torques with respect to the pivot, located at the base of the beams.) Discuss why these forces 
 F₁ and F₂ have the same magnitude. Explain your reasoning using diagram(s) with locations of forces and perpendicular lever arms, the properties of torques, and Newton's laws.

Solution and grading rubric:

• p:
  Complete free-body diagrams with forces and perpendicular lever arms, and discusses/demonstrates:
  1. the magnitude of the weight force w is the same for both higher and lower beams; and
  2. for each beam, the lever arm for the applied force F is twice the lever for the weight force w (2⋅ℓ_w = ℓ_F); and
  3. Newton's first law for rotations applies to both higher and lower beams, where the ccw force torque F⋅(ℓ_F) and cw weight torque w⋅(ℓ_w) must balance each other out, and so: F⋅(ℓ_F) = w⋅(ℓ_w), F = w⋅(ℓ_w/ℓ_F) = w⋅(ℓ_w/(2⋅ℓ_w)) = w/2; such that
  4. the applied forces on the higher and lower beam must be equal in magnitude, as they are both equal to one-half of the weight of the beam.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Does not explicitly note that the ℓ_F lever arm is always twice the ℓ_w lever arm for both situations. Instead, argues that since the ℓ_F and ℓ_w values for the higher beam are both bigger than the respective ℓ_F and ℓ_w values for the lower beam, then the higher beam F = w⋅(ℓ_w/ℓ_F) = w⋅(bigger/bigger) must be equal to the lower beam F = w⋅(ℓ_w/ℓ_F) = w⋅(smaller/smaller), but only implicitly demonstrates how the "bigger/bigger" ratio is exactly equal to the "smaller/smaller" ratio by use of a scaled drawing instead of using geometry/trigonometry, etc.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. As (r), but does not clearly/correctly show ℓ_F and ℓ_w lever arms for both situations. At least has two sets of Newton's first law for rotations, one for the higher beam and one for the the lower beam, setting the ccw torques equal to the cw torques.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying Newton's first law to torques, forces, and perpendicular lever arms.
• x:
  Implementation of ideas, but credit given for effort rather than merit. Approach other than that of applying Newton's first law to torques, forces, and perpendicular lever arms.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 70854, 70855
Exam code: midterm02sQm5
p: 3 students
r: 18 students
t: 14 students
v: 14 students
x: 3 students
y: 0 students
z: 0 students

A sample "p" response (from student 5281):

Physics quiz question: linear mass density of viola string

Physics 205A Quiz 6, fall semester 2019
Cuesta College, San Luis Obispo, CA

Transverse waves travel at a speed of 357 m/s along a viola's A-string, stretched to a tension of 52.9 N[*][**]. The linear mass density of this string is:
(A) 4.15×10^–4 kg/m.
(B) 2.20×10^–2 kg/m.
(C) 0.148 kg/m.
(D) 0.385 kg/m.

[*] theviolaworkshop.com/page16.html.
[**] gamutmusic.com/viola-tensions.

Correct answer (highlight to unhide): (A)

The speed v of transverse waves along the viola string depends on the tension F and the linear mass density (mass per unit length) (m/L):

v = √(F/(m/L)).

Solving for the linear mass density results in:

v² = F/(m/L),

(m/L) = F/(v²),

(m/L) = (52.9 N)/(357 m/s)² = 0.000415067988 kg/m,

or to three significant figures, 4.15×10^–4 kg/m.

(Response (B) is (F/v)²; response (C) is F/v; response (D) is √(F/v).)

Sections 70854, 70855
Exam code: quiz06co6O
(A) : 35 students
(B) : 7 students
(C) : 8 students
(D) : 2 students

Success level: 67%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.78

Online reading assignment: waves

Physics 205A, fall semester 2019
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 waves.


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.

"Waves always create a disturbance and move energy from one place to another. A transverse wave has disturbance perpendicular to travel direction, and that a longitudinal wave has disturbance parallel to travel direction. The general parts of a wave, such as amplitude and period, as well as the equations that the book provided for those parts, also make sense to me."

"The motion of transverse waves is perpendicular to the direction the wave travels in. The motion of a longitudinal wave occurs parallel to its direction."

"Something I understood was waves because I learned it in chemistry. A wave can have a frequency, amplitude, wavelength, time, distance, and periods."

"As for the presentations, I am understanding what equations need to be used. I am better understanding the pendulum problems versus the oscillating springs. However I am having issues with my internet tonight so I can't see any of the GIFs so I am having a hard time with the waves presentation."

"Wavelength is the horizontal distance of a complete cycle of a wave, whereas a period is the time for one cycle to be completed. Amplitude is a measurement of distance between the highest or lowest point of a wave and the undisturbed position. Frequency is equal to one cycle per second, where the seconds represent the period. Since properties of the material/medium in which the wave travels determines the speed of a wave, we can find the speed by taking the square root of the force of tension divided by the mass per unit length."

"There are four different aspects of a wave; wave length, amplitude, frequency, and speed. Wavelength is the length that a wave stretches over itself. Amplitude is the height of a wave. Frequency is the amount of waves present or produced. Speed is the rate at which the waves move."

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 having trouble understanding the independent and dependent parameters in these equations."

"I do not understand the differences between the longitudal, periodic, transverse waves based on the pictures. The equations are also not making sense, mainly because there are so many variables to consider."

"Something that I found confusing was differentiating a period from a wavelength. The book did not do a great job at explaining the stuff."

"I think the most confusing is all these different symbols for each thing we learn. They are getting all jumbled up and it's annoying since I can't focus on what is actually what."

"Maybe it was because I had no available friends at the time of viewing these slides, but the example of the wavelengths being affected by either amplitude or frequency reached an amplitude above my head. Seemingly, the speed and wavelength are unaffected by the amplitude; however, frequency has some affect on those that I am confused about."

"I'm a little confused on how frequency and speed are independent but the wavelength is dependent on frequency and speed."

"From the presentation preview I was confused on the independent and dependent parameters at the beginning but then understood it once reading the what each symbol meant."

A string of a given length has a certain linear mass density (mass/length) value. If this string is cut in half, then its linear mass density will:

decrease.	************ [12]
remain constant.	************** [14]
increase.	************ [13]
(Unsure/lost/guessing/help!)	* [1]

The top and bottom waves have the same frequency, but different amplitudes. Identify which waves have the greater wave parameter.
(Only correct responses shown.)

Faster wave speed v: there is (approximately) a tie. [73%]
Longer wavelength λ: there is (approximately) a tie. [55%]

The top and bottom waves have the same amplitude, but different frequencies. Identify which waves have the greater wave parameter.
(Only correct responses shown.)

Faster wave speed v: there is (approximately) a tie. 38[%]
Longer wavelength λ: top wave (low frequency f). [75%]

This wave travels from the left section to the right section of this apparatus. Identify along which section the waves have the greater wave parameter.
(Only correct responses shown.)

Faster wave speed v: Along the right apparatus. [18%]
Longer wavelength λ: along the right apparatus. [88%]
Higher frequency f: there is (approximately) a tie. [18%]

For transverse waves on a string, classify each of these parameters are being "independent" (able to be changed without affecting other independent parameters), or "dependent" (will be changed when independent values are changed).
(Only correct responses shown.)

Amplitude A: independent. [65%]
Wave speed v: independent. [55%]
Frequency f: independent. [45%]
Wavelength λ: dependent. [65%]

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

"How does Newton's second law cause a wall with a string attached to it to pull on the string in the opposite direction of its standing wave in order to make it reflect back the way it came?" (That sounds like Newton's third law to me.)

"I don't really understand the independent and dependent parameters of transverse waves, some review on that would be great."

"I think I just need a brief review of the independent and dependent parameters to verify I am understanding correctly."

"What is the difference between wavelength and period? Also, what are the equations that we will be using?"

"I really like this part of physics."

"Just need to hear you explain this."

"I am still confused about how to apply resonance." (We'll have a lab specifically on that later.)

Online reading assignment: elasticity

Physics 205A, fall semester 2019
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.

"Tension is when you stretch something as opposed to compression when you squish something. Stress is the applying force to the object and strain is a measure of how the object/material responds."

"The two cases of elasticity, either with a tension force or a compression force. Tensile/compressive stress is the act of applying the force, whereas tensile/compressive strain is how the material behaves under tension/compression."

"When compressing a spring, or when it restores, the displacement of the spring is proportional to the force applied. Also, the tensile force is perpendicular to the area, and the shearing force is parallel with a surface."

"That k is the spring constant and x is the displacement of the spring from its unstrained length. The minus sign indicates that the restoring force always points in an opposite direction to the displacement of the spring from its length."

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.

"Nothing is too confusing to grasp from the text. Just a few equations that are used in certain circumstances."

"Hooke's law. I don't understand how materials with a material-dependent Young's modulus (in units of pascals) that characterizes the response of the material to these stresses."

"Using Hooke's law, and why exactly strain is unitless."

"From the presentation preview I was first confused about the tensile stress example but upon another glance I was able to understand it. The stress causes the object to stretch to a limit that does not cause it to break."

"Putting the complex theories into practical world applications and making the connection to conceptualizing while using the equations correctly."

"The elastic deformation equation is a little confusing because I'm not sure about all the components and variables."

"I don't understand hardly anything, I don't know what the variables stand for."

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

"N/m²."

"Pa."

Explain why strain is a unitless quantity.

"Strain is unitless because it is a proportion of two quantities with the same dimensions."

"It's a unitless quantity because it deals with the fractional change of length or volume."

"I am not sure."

"I don't know."

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

"N/m²."

"Pa."

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

shorter.	***** [5]
longer.	**************************** [28]
(There is a tie.)	*** [3]
(Unsure/lost/guessing/help!)	***** [5]

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

narrower (two 2×4s).	****************** [18]
wider (three 2×4s).	******** [8]
(There is a tie.)	********** [10]
(Unsure/lost/guessing/help!)	***** [5]

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

"Need some clearing up, book kind of went off the rails."

"Please go over these in class, I am confuuuuuused ):"

"I understand that Young's modulus is the ability of a material to withstand changes in length when under tension. Will this value always be provided for an individual material? Does it matter what the material is resting on... for example a piece of steel on concrete vs a wood table?" (Yes, the Young's modulus values will always be given for a problem (unless you need to solve for it); and no, it doesn't matter what the material (being tested) is resting on, provided that the supporting object is strong enough to handle whatever is being done to the material being tested.)

"Will we be doing a simple harmonic motion lab?" (Yes, for Lab 12.)

I don't have anything to say."

"Yay physics :)"

"I'm just enjoying the day."

"This irregular weather is killing me."

Online reading assignment: torque and rotations

Physics 205A, fall semester 2019
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 torque and rotations.


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.

"Magnitude of torque is equal to the magnitude of force times the lever arm, which means there is more torque when you push near the outside end of lever in the direction of the hinge. The line of action is parallel and overlapping the force applied."

"How to solve for torque; it can vary due to the position of the force and the length of an object that it accompanies. if a force is applied at the very end of a length while being perpendicular to the object you'll achieve maximum torque."

"The idea of Newtons's first and second law when applied to torque and rotations. Like force, when net torque is equal to zero, Newton's first law is in effect; moreover, if torque adds up to something other than zero, Newton's second law applies."

"The lug nuts do get supertight so it was helpful to see an example that I could personally relate to which makes torque and twist easier to understand. The force and direction (ccw/cw) also made sense because of the rotation or 'twist' and the force making it do so."

"The line of action and the lever arm are important concepts when talking about torque. The line of action being an extended line drawn co-linear with the force and the lever arm referring to the perpendicular distance between the line of action and the axis of rotation."

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.

"Exactly how to apply Newton’s laws to the rotational motion."

"The line of action and the perpendicular lever arm. Although I generally understood the examples of Newtons's laws in relation to torque, I need help in class--through examples--to see how these lines of actions are drawn and how they allow the determination of a lever arm or vice versa."

"I think I understand torque for the most part. I just have trouble visualizing how the line of action and lever arm work for different/more complicated real-world examples."

"Just the formulas obviously, but nothing practice won't help."

"I am confused about how torque can zero, or would that be considered a non-existent idea?"

"I am having a hard time understanding the equilibrium of a rigid body. I understand that if a rigid body is in equilibrium the sum of the externally applied forces is zero and the sum of the applied toques is zero, but what if it isn't in equilibrium, and how do we know if it is, and will we even have to worry about that?"

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

"A newton meter (N·m)."

"Newton·meters, not to be confused with joules."

Briefly describe how the line of action should be drawn for a given force.

"The line of action is drawn collinear with force."

"Extend a line along force vector."

"The line of action should be drawn co-linear with the force. It is an extended line and is usually dotted."

When a lever arm (or moment arm) is drawn, briefly explain where it starts, and how it should intersect the line of action for a force.

"The lever arm is defined as the perpendicular distance from the axis rotation to the line of action of the force."

"Starts at the axis of rotation origin, and is perpendicular to the line of action."

"Extend a line along this force vector--this is the line of action, which is just a guide to find the perpendicular lever arm, which starts at the pivot point, and at the other end must intersect the line of action perpendicularly."

For the stuck wrench (assuming that it is rigid and not flexing), Newton's rotational __________ law applies, and the clockwise torques and counterclockwise torques acting on the wrench are __________.

first; balanced.	********************************************* [45]
second; unbalanced.	**** [4]
(Unsure/lost/guessing/help!)	[0]

For the crane, Newton's rotational __________ law applies, and the clockwise torques and counterclockwise torques acting on the crane are __________.

first; balanced.	** [2]
second; unbalanced.	*********************************************** [47]
(Unsure/lost/guessing/help!)	[0]

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

"Can all the forces besides weight force be considered in calculating torque?" (Yes, definitely.)

"How would the guy use the spanner to apply a greater amount of torque? Use a longer spanner?" (Yes, or apply a perpendicular force to end of the original size spanner.)

"Am I just completely missing the section of Newton's rotational laws? I am assuming we are just applying the laws we already know?"

"Are Newton's rotational laws the same as his laws of motion?" (The textbook does not actually state Newton's laws as applied to rotations, but Newton's first law for rotations is when net torque is zero if rotational motion is constant (which includes no rotations), and Newton's second law for rotations is when the net force is non-zero, such that rotational motion will be changing. Newton's third law for rotations is very obscure; so we're not going there.)

Your use of 'esoteric' had a double meaning in that few people would use such a means to calculate torque (line of action and perpendicular lever arm) would be equaled by the number of people who would understand the meaning of 'esoteric.' LOL"

"I've never seen a crane fall." (It is bad luck to see a crane fall.)

Physics quiz archive: vectors, projectile motion, forces

Physics 205A Quiz 3, fall semester 2019
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855
Exam code: quiz03Ch3V



Sections 70854, 70855 results

0- 6 :	* [low = 6]
7-12 :	***
13-18 :	**************
19-24 :	******************* [mean = 21.7 +/- 5.8]
25-30 :	***************** [high = 30]

Online reading assignment: applications of Newton's laws (friction)

Physics 205A, fall semester 2019
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 on applications of Newton's laws (emphasizing static and kinetic friction).


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.

"There can only be two types of motion: constant and changing. There also only two types of net forces: zero and non-zero."

"Newton's third law of motion isn't necessarily about motion or net force, but the properties of force itself."

"We can use the mnemonic device 'POF-OST-ITO' in order to test if Newton's third law applies or not. POF stands for 'pair of opposite forces,' OST is 'of same type,' ITO 'involving two objects.'"

"Newton's third law relates only two forces of the same type acting on two different objects. However Newton's first law deals with two forces acting upon one object. Additionally, if the POF-OST-ITO test fails, then it is not a N3 scenario, but it is not yet safe to assume it is a N1 scenario either, until you do further investigation."

"Newton's second law shows how the acceleration depends on both the net force and the mass. The magnitude of acceleration is proportional to the net force acting on the object, and inversely proportional to the mass. The net force includes only the forces that the environment exerts on the object of interest. The kinetic friction force opposes the relative sliding motion."

"Everything is effected on by gravity. If something is touching a surface, it will have a normal force acting on it."

"The normal force can only exist when two surfaces are making contact with one another. If two objects are not making contact, then there is no normal force."

"Friction is another component when dealing with the motions of an object. It is a force that is parallel to the surface an object is moving on. Static friction is what makes an object remain stationary even when a force is applied, and only moves when the applied force is slightly greater than the maximum force of static friction."

"Static friction force comes into play when pulling on an object at rest, and if the force applied is small enough static friction will cancel out the applied force resulting in no movement."

"That an object initially at rest interacts with a surface through static friction. As the object moves, after overcoming the maximum static friction, the resistance turns into kinetic friction. Friction is a parallel and normal force is perpendicular to the surfaces in contact."

"Static friction is the friction that a surface has on a object that is resting on the surface and is directly proportional to the normal force on said object. Next, there is kinetic friction which is friction that occurs between two sliding surfaces. Also kinetic friction is typically less than the static friction."

"Static and kinetic frictional forces seem pretty straightforward. Static refers to when there is friction acting on an object but the object is not moving. Kinetic is when there is friction acting on an object but the object is moving."

"Was a little lost, so there isn't much I understand."

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 feel like I understand Newton's laws from the textbook. However, when it comes to applying the concepts to a free body diagram, I start getting confused with the direction of the forces and how they cancel out especially when things are stacked on top of each other."

"I still have trouble distinguishing between Newton's first law and second law."

"I was slightly confused as to why the pulleys and ropes would have equal tensions for the different worksheet examples in class. But after further research I understood that neither the mass nor acceleration of gravity changed, therefore the tension forces stayed the same."

"I get confused on how to determine which law is being used between Newton's first law or third law. I also don't understand the difference between static and kinetic friction forces and the magnitudes associated with them."

"The difference between static friction and kinetic friction aren't clear to me."

"The coefficient of friction and what it stands for might need some clarification for me."

"I would like some more work in class with problems on kinetic friction and static friction. I understand that static friction is friction for stationary objects, like a box on the ground, and that kinetic friction is friction for moving objects, like a box sliding across the ground. However, I'm still unsure of how this would work in real-world situations."

"I don't know how to put friction in an equation or how to use it to solve for something."

"Why are there no SI units for static/kinetic friction coefficient?"

"For the most part it is pretty easy to visualize friction, since we have all experienced it before."

"Didn't really find anything confusing."

"I understood most of it conceptually, I think it could get more confusing when these concepts are applied in a problem."

"Most of it."

What is the meaning of the "normal" in the "normal force?"

"The perpendicular force applied between two objects contacting each other."

"It just means the force is perpendicular to the surface."

"The 'normal' force refers to the perpendicular direction with respect to the surface."

"Normal means 'perpendicular.'"

"There will almost always be normal force on any given objects in contact, regardless of its state or location? That's what makes it 'normal,' right?

The SI (Système International) units of the static friction coefficient µ_s and the kinetic friction coefficient µ_k are:

" Unitless."

The coefficients because the units cancel."

"I do not believe these coefficients have units."

"Newtons?"

"kg·m²·s²?"

"I'm not entirely sure."

Identify the magnitude of the static friction force f_s for each of the following situations of a box that is initially stationary on a horizontal floor. (Only correct responses shown.)

No external horizontal forces applied to it, so it remains stuck to the floor:
f_s = 0. [77%]

An external horizontal force applied to it, but still remains stuck to the floor:
f_s = some value between 0 and µ_s·N. [83%]

An external horizontal force applied to it, at the threshold of nearly becoming unstuck:
f_s = µ_s·N. [72%]

Identify the magnitude of the kinetic friction force f_k for each of the following situations of a box that is already sliding across a horizontal floor. (Only correct responses shown.)

No external horizontal forces applied on it, so it slows down:
f_k = µ_k·N. [21%]

An external horizontal force applied in the forward direction, but not enough to keep the box going so it still gradually slows down:
f_k = µ_k·N. [23%]

An external horizontal force applied in the forward direction, just enough to keep the box going at a constant speed:
f_k = µ_k·N. [43%]

An external horizontal force applied in the forward direction, enough to gradually increase the speed of the box:
f_k = µ_k·N. [58%]

An external horizontal force applied in the backwards direction, such that the box slows down:
f_k = µ_k·N. [32%]

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

"Can we go over the friction coefficients? Please explain static friction force and kinetic friction force in detail in class! I could use some review on the equations for the magnitudes of static and kinetic frictional force. Is the friction coefficient different between objects that do not appear smooth. For example, is the constant for ice on ice different than say, bumpy ice on ice?" (Yes, the coefficient will be larger for rougher surfaces than for smoother sliding surfaces.)

"Are there ways to increase of decrease friction?" (Change the smoothness/roughness of the surfaces, or add a lubricant (which is "smooth" on a molecular level; long hydrocarbon chains can align with each other to roll like logs) between the two surfaces.)

"What is it that causes friction? Is it simply objects hooking onto each other at microscopic levels?"

"Does static friction force become kinetic friction force once the object starts moving?" (Yes, once the object is already unstuck.)

"What I found confusing was that static friction force and the applied force on a object are directly proportional, except for the fact that static friction has a maximum amount, as opposed to applied force which can keep increasing indefinitely. If the two are directly proportional, shouldn't the static friction force increase as long as the applied force increases, or shouldn't the applied force also have a maximum if the static friction has one as well?" (You can arbitrarily exert any amount of applied force on an object, but the static friction force will have a maximum amount because at some point your applied force will "unstick" the object so it will then begin to move. I suppose if you stuck together the two surfaces magic superglue, then you could exert an infinite amount of applied force, and the static friction force would then also be infinite, provided the magic superglue still holds the object stationary.)

"I hope I'm understanding this correctly! It seems to make sense so that's either a really good sign or a really bad sign."

"Definitely need some review, this weekend fried my brain."

"I don't like this :("

Online reading assignment: standing waves

Physics 205A, fall semester 2018
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 standing waves.


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.

"String tension and thickness determine the pitch of the note that the string creates when plucked."

"The frequency of the wave is set by the source. The speed of periodic waves along strings is set by the string tension and thickness. The fundamental frequency of a string depends on the wave speed v and length L."

"Different thicknesses of string vibrate at different fundamental frequencies when plucked. Thinner strings would have a higher fundamental frequency and thicker strings would have a lower fundamental frequency."

"Nodes are where there is no vibration at all and antinodes are the spaces where maximum vibration occurs. The distance between two nodes or antinodes of a standing wave is equal to half of its wavelength."

"Natural frequency is when resonance occurs. Guitar strings that are thicker resonate at a lower fundamental frequency than thinner strings. If a string were oscillated at twice its fundamental frequency, then it results in a pattern that creates a node in the center."

"All strings have their own fundamental frequency and you can have resonate frequencies by multiplying integers to the fundamental frequency."

"I didn't get to 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.

"The fundamental frequency equation and the resonant frequencies confused me some. How the fundamental frequency of a string depends on the wave speed v and length L."

"I'm still kind of confused on what a node actually is. Is it the portion that's completely still in the GIF animations? Or is it something completely different?"

"I was a little confused by fundamental frequency. I also found the questions kind of confusing."

"I feel like I understood this all pretty well."

(Assuming that their tensions are approximately equal), the __________ strings have a slower wave speed.

thicker.	******************************************* [43]
thinner.	[0]
(There is a tie.)	*** [3]
(Unsure/lost/guessing/help!)	[0]

After these same-length guitar strings are plucked (assuming that their tensions are approximately equal), the __________ strings vibrate at a lower fundamental frequency.

thicker.	********************************** [34]
thinner.	***** [5]
(There is a tie.)	******* [7]
(Unsure/lost/guessing/help!)	[0]

After the bass string is plucked, sliding a finger down to decrease its length would __________ the speed of waves along the string.

decrease.	***************** [17]
not change.	********* [9]
increase.	******************* [19]
(Unsure/lost/guessing/help!)	* [1]

After the bass string is plucked, sliding a finger down to decrease its length would __________ the fundamental frequency of the string.

decrease.	*********** [11]
not change.	******* [7]
increase.	*************************** [27]
(Unsure/lost/guessing/help!)	* [1]

For standing waves on a string, classify each of these parameters are being "independent" (able to be changed without affecting other independent parameters), or "dependent" (will be changed when independent values are changed).
(Only correct responses shown.)

Wave speed v: independent [39%]
String length L: independent. [59%]
Fundamental frequency f₁: dependent. [54%]

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

"Since fundamental frequency is equal to waves speed divided by 2·L, then wouldn't all of these parameters be dependent of each other because if the fundamental frequency f is changed then wave speed and or length would have to also be different values and vice versa?"

"I am currently learning how to play the guitar so this is cool to talk about!"

"I'm actually from Gig Harbor, Washington, which is on the other end of the Narrows Bridge from Tacoma. Galloping Gertie is a very familiar topic with me haha I have crossed the Narrows a thousand times. We learned about how they changed the structure of the bridge when they built the new one, and when you go to the Narrows Beach (Gig Harbor side) or Titlow Beach (Tacoma side) you can look up at the bottoms of the bridges and see all of the tresses. They had to make the bottom more open so there wouldn't be large solid surfaces to catch the wind the same way they did before. I remember when there was only one bridge that had traffic going both ways from the Harbor to Tacoma, but then there was so much traffic going back and forth that they built another one, and left the green one as east-to-west traffic (Tac to GH), and the new one as west-to-east traffic (GH to Tac). The tolls are still stupidly high, even though the bridge is paid off."

"I hope we go over all this in the lab on Monday better."

"How the length of the string effects the sound it makes."

"I enjoyed the partially/lightly flipped format of your class! Your class policies and procedures are very reasonable. I'm not sure what else to say to help improve your teaching methods; I believe you have done a wonderful job!"