Physics midterm question: distance traveled vs. displacement

Physics 205A Midterm 1, fall semester 2019
Cuesta College, San Luis Obispo, CA

The v_x(t) graph of a Physics 205A student walking along a horizontal road is shown at right. The student started at x = 0 at t = 0. Discuss whether the magnitude of the distance traveled by the student is equal to or greater than the magnitude of the displacement. Explain your reasoning using the properties of velocity, position, time, distance traveled, and/or displacement.

Solution and grading rubric:

• p:
  Correct. Demonstrates that the distance traveled is equal to the magnitude of displacement, as the student always travels in the same (positive) direction (only positive horizontal velocity values), without reversing direction (no negative horizontal velocity values).
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. Conceptual understanding of displacement and distance traveled, but somehow misinterprets/misapplies/ignores the given information.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
• x:
  Implementation of ideas, but credit given for effort rather than merit.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 70854, 70855
Exam code: midterm01duCk
p: 38 students
r: 0 students
t: 5 students
v: 4 students
x: 5 students
y: 0 students
z: 0 students

A sample "p" response (from student 5808):

Physics midterm problem: skateboard-launched rubber duck toy

Physics 205A Midterm 1, fall semester 2019
Cuesta College, San Luis Obispo, CA

"WRECKING BALL Vs. SEESAW from 45m! How High Will the Watermelon Go?"
How Ridiculous
youtu.be/1quHlRJLtgM

Brett Stanford, Derek Herron and Scott Gaunson for the "How Ridiculous" YouTube channel dropped a heavy ball on one end of a skateboard to launch a rubber duck toy from the other end. Video analysis shows that the toy was launched at an angle of 61° from the horizontal, and took 3.2 s from the moment it was launched from the ground to land back down on the ground. Determine the horizontal distance along the ground from where it was launched to where it landed. Neglect air resistance, and treat the toy as a point object that started on the ground. Show your work and explain your reasoning using properties of projectile motion.

[*] youtu.be/1quHlRJLtgM?t=335.

Solution and grading rubric:

• p:
  Correct. From the time of flight t = 3.2 s, solves for the initial vertical velocity component v_0y = +16 m/s. Next, using the launch angle of elevation θ = 61° finds the initial horizontal velocity component v_0x = +8.7 m/s, and subsequently uses that value and the time of flight t = 3.2 s to solve for the final horizontal position x = +28 m.
• r:
  Nearly correct, but includes minor math errors. At least successfully solves for the vertical v_0y and/or horizontal v_0x components of the initial velocity vector.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. At least some systematic attempt at using kinematic equations for projectile motion. May have made one or more erroneous assumptions about certain values, such as setting the final velocity components v_x = 0 and or v_y = 0, but still methodically solves for a (wrong) value of v_0y, and then (somehow) solves for a (wrong) value of v_0x using trigonometry to find a (wrong) value for the final horizontal position x.
• v:
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Some attempt at systematic use of kinematic equations for projectile motion.
• x:
  Implementation of ideas, but credit given for effort rather than merit. No clear attempt at kinematic equations for projectile motion.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 70854, 70855
Exam code: midterm01duCk
p: 15 students
r: 4 students
t: 9 students
v: 21 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 5832):
Another sample "p" response (from student 2342):

Physics quiz question: soccer ball vertical velocity component

Physics 205A Quiz 3, fall semester 2019
Cuesta College, San Luis Obispo, CA

A Physics 205A student kicks a soccer ball off a cliff with an initial velocity vector that has x- and y-components:

v_0x = +6.1 m/s,
v_0y = –3.2 m/s.

Neglect air resistance. Just before it hits the ground, the magnitude of the soccer ball's vertical velocity is:
(A) 0 m/s.
(B) some value between 0 m/s and 3.2 m/s.
(C) 3.2 m/s.
(D) some value greater than 3.2 m/s.

Correct answer (highlight to unhide): (D)

While the horizontal component of the soccer ball's velocity never changes, the vertical component of the soccer ball's velocity will always keep changing, due to the acceleration due to gravity. The soccer ball already starts with an initial downwards speed, and the vertical downwards speed will increase as it keeps moving along its trajectory.

Sections 70854, 70855
Exam code: quiz03Ch3V
(A) : 6 students
(B) : 5 students
(C) : 19 students
(D) : 24 students

Success level: 44%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.86

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: uniform circular motion

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 uniform circular 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.

"In circular motion, there is a net force that produces the acceleration of an object. The centripetal force always points towards the middle of a circle."

"That circular motion is covered by Newton's second law. Even restricted to uniform circular motion Newton's second law still applies."

"I understand that by Newton's second law, there must be some type of force to create centripetal acceleration. It makes sense to me that this force is pointing towards the center of the circle, as if the force pointed outward, the object would drift off its circular path."

"Since the word 'centripetal' means 'moving toward a center,' the centripetal acceleration vector always points toward the center of the circle and continually changes direction as the object moves. For an object in uniform circular motion, the velocity vector has different directions at different places on the circle. Uniform circular motion along the arc of an infinitely large circle entails no acceleration, because it is just like motion at a constant speed along a straight line. An object undergoing uniform circular motion can never be at equilibrium because there is acceleration since the direction is constantly changing. Sometimes the centripetal force consists of a single force such as tension, friction, the normal force or a component thereof or the gravitational force."

"The concept of centripetal force, the net force of an object undergoing circular motion with a specific direction which is in towards the center of the circle and specific magnitude m·v²/r. Centripetal means 'center-seeking.' This is the requirement for uniform circular motion to maintain a constant speed along a circular trajectory with acceleration directed in towards the center."

"Centripetal force! It's like centrifugal force but not. In all seriousness, a sample scenario is a car turning a corner. This change in direction of velocity requires a net force to keep pulling the car into the center of its curve."

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 the conceptual understanding of why the net force points towards the center of the circle."

"I'm a little confused on if this is the same or different from 'centrifugal force.'"

"I understand the majority of these situations, but the one about the girl swinging is tripping me up."

"Slightly confused on calculating ALL the net forces during circular motion."

"Friction and how it affects centripetal force (i.e. car driving in a curve along a track)."

"How circular motion works for a car drifting around a turn. I do not understand what direction the net force is going."

"Is centripetal acceleration the acceleration the object is going at each new direction in a circular motion? I'm a little perplexed by that concept."

"I guess I still haven't grasped the idea of vertical circular motion yet."

"I'm pretty sure that I understood everything in the chapter."

For the "drifting" car (skidding around a circular track at constant speed), Newton's __________ law applies to its motion, and the forces acting on it add up to a net force that:

first; is zero.	****** [6]
second; points to the left.	*************** [15]
second; points to the right.	************************ [24]
(Unsure/lost/guessing/help!)	* [1]

At the moment when the woman is at the bottom of her swinging trajectory (when the rope is vertical), Newton's __________ law applies to her motion, and the forces acting on her add up to a net force that:

first; is zero.	****** [6]
second; points upwards.	**************************** [28]
second; points downwards.	*********** [11]
(Unsure/lost/guessing/help!)	* [1]

At the moment when the motor scooter is on the left side of the screen (traveling out at you), Newton's __________ law applies to its motion, and the forces acting on it add up to a net force that:

first; is zero.	** [2]
second; points to the left.	************** [14]
second; points to the right.	************************** [26]
(Unsure/lost/guessing/help!)	**** [4]

At the moment when the car is at the very top of the loop-the-loop, Newton's __________ law applies to its motion, and the forces acting on it add up to a net force that:

first; is zero.	***** [5]
second; points upwards.	******* [7]
second; points downwards.	******************************** [32]
(Unsure/lost/guessing/help!)	** [2]

At the moment when a person is at the right edge of the screen (traveling out at you), Newton's __________ law applies to his/her motion, and the forces acting on him/her add up to a net force that:

first; is zero.	**** [4]
second; points to the left.	**************************** [28]
second; points to the right.	*********** [11]
(Unsure/lost/guessing/help!)	*** [3]

At the moment when the car is at the very top of its mid-air trajectory, Newton's __________ law applies to its motion, and the forces acting on it add up to a net force that:

first; is zero.	******** [8]
second; points upwards.	****** [6]
second; points downwards.	**************************** [28]
(Unsure/lost/guessing/help!)	**** [4]

At the moment when the skateboarder is at the very top of his mid-air trajectory, Newton's __________ law applies to his motion, and the forces acting on him add up to a net force that:

first; is zero.	****** [6]
second; points upwards.	****** [6]
second; points downwards.	******************************* [31]
(Unsure/lost/guessing/help!)	*** [3]

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

"I feel like to really grasp what these questions are asking we need a bit more clarification in class because it's easy to get mixed up." (That's okay, just try your best--I need to see how well you do on the first try, so I know how basic or advanced to tailor my lecture for this material. (Spoiler alert: very basic.)

I've got no idea how I did on those last questions. May we go over these questions in class? I think I got all of them wrong."

"How do I know if I got these questions right or not?" (We'll go over these in class, and as part of the next reading assignment, you have a link to click on so you can see these correct answers.)

"Can we go over how to tell which way the net force is pointing when objects are moving in a circle? How do you figure out which way the net force points?" (For objects undergoing uniform circular motion (moving in a circle with constant speed), the net force always points inwards, towards the center of the circle. Always. Points. Inwards.)

"So lost help!!"

"After going over the questions I am really confused on the idea of objects in motion. Do we determine the law at a specific moment in time or the entire sequence?" (At that one moment in time. Then for an entire sequence, it's just a whole bunch of consecutive moments in time, taken one by one.)

"I am still confused about the car and motorcycle skidding around a circular track--if I was to compare the motorcycle to the car, the motorcycle would almost be in a standstill, but compared to an outside frame of reference, they both move?" (Yes, since both the car and motorcycle are accelerating (constant speed, but changing direction), they are not proper inertial reference frames, and they will not be able to properly apply Newton's laws to their motions. As an outside (stationary) observer, you would be in an inertial reference frame, and would be able to properly apply Newton's laws to them as they undergo uniform circular motion.)

"I didn't really understand how when in circular motion we are feeling the forces that may feel like outward but really they are going in." (When you undergo uniform circular motion, you are not in a proper inertial reference frame, and cannot properly apply Newton's laws because you experience "pseudoforces" like something is pulling/pushing you outwards. An outside (stationary) observer would be in an inertial reference frame, and would be able to properly apply Newton's laws and describe the net force as inwards, despite what you (undergoing uniform circular motion) "perceives.")

"Is it still considered uniform circular motion if it is not a perfect circle? What about a racetrack that is an oval in shape?" (If we can approximate part of the oval as a circle (for any small enough section of an oval), then we can apply Newton's second law for uniform circular motion for that section.)

"Do we need to memorize all the weight, normal, tension, static friction, kinetic friction, Newton's laws and ΣF = m·a equations, or are they given on quizzes and exams like the kinematic equations of motion?" (You need to memorize w = m·g, f_{s, max} = μ_s·N, f_k = μ_k·N, ΣF = m·a, and "POF-OST-ITO." Instead of just giving you those equations on a quiz or exam, I think memorizing those equations will force you understand how and why to use them while applying Newton's laws on free-body diagrams. (Note that the normal force N and the tension force T don't have any equations; their magnitudes have to be deduced, as they vary depending on the particular situation.)

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 :("

Physics quiz question: Kia Telluride braking time

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

"2020 Kia Telluride"
Automotive Rhythms
flic.kr/p/2eei8ws

According to Car and Driver magazine[*] a 2020 Kia Telluride SX AWD sports utility vehicle was able to slow down from an initial speed of 31 m/s to a complete stop over a distance of 49 m. Assume that the road is horizontal, and that the acceleration of the Kia Telluride is constant and always points in the opposite direction from its velocity. While the Kia Telluride slowed down to a complete stop, the elapsed time was:
(A) 0.057 s.
(B) 0.32 s.
(C) 1.8 s.
(D) 3.2 s.

[*] David Beard, "The 2020 Kia Telluride Is Set to Win Big in the Three-Row SUV Segment," caranddriver.com/reviews/a26965174/2020-kia-telluride-drive/.

The following quantities are given (or assumed to be known):

(t₀ = 0 m),
(x₀ = 0 m),
x = +49 m,
v_0x = +31 m/s,
v_x = 0 m/s.

So in the equations for constant (average) acceleration motion in the horizontal direction, the following quantities are unknown, or are to be explicitly solved for:

v_x = v_0x + a_x·t,

x = (1/2)·(v_x + v_0x)·t,

x = v_0x·t + (1/2)·a_x·(t)²,

v_x² = v_0x² + 2·a_x·x.

With the unknown quantity t to be solved for appearing in the second equation, with all other quantities given (or assumed to be known), then:

x = (1/2)·(v_x + v_0x)·t,

t = 2·x/(v_x + v_0x) = 2·(+49 m)/(+31 m/s + 0 m/s) = 3.1612903226 s,

or to two significant figures, t = 3.2 s.

(Response (A) is sqrt(v_x)/(2·x); response (B) is v_x/(2·x); response (C) is sqrt(v_x/(9.80 m/s²)).)

Sections 70854, 70855
Exam code: quiz02Cs1o
(A) : 2 students
(B) : 4 students
(C) : 4 students
(D) : 44 students

Success level: 81%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.46

Physics quiz question: Casio watch prototype testing

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

Casio engineer Kikuo Ibe tested G-Shock watch prototypes by placing them in a rubber ball dropped from a window, falling 10 m down to the ground[*]. Neglect air resistance. Choose up to be the +y direction.

"Story"
Casio America Inc.
gshock.com/technology/story

After being released from rest, the rubber ball took __________ to reach the ground.
(A) 0.49 s.
(B) 1.0 s.
(C) 1.4 s.
(D) 2.0 s.

[*] gshock.com/technology/story.

Correct answer (highlight to unhide): (C)

The following quantities are given (or assumed to be known):

(t₀ = 0 s),
(y₀ = 0 m),
y = –10.0 m (below the starting point),
v_0y = 0 m/s (no initial velocity),
a_y = –9.80 m/s².

So in the equations for constant acceleration motion in the vertical direction, the following quantities are unknown, or are to be explicitly solved for:

v_y = v_0y + a_y·t,

y = (1/2)·(v_y + v_0y)·t,

y = v_0y·t + (1/2)·a_y·(t)²,

v_y² = v_0y² + 2·a_y·y.

With the unknown quantity t to be solved for appearing in the third equation, with all other quantities given (or assumed to be known), then:

y = v_0y·t + (1/2)·a_y·(t)²,

(–10.0 m) = (0 m/s)·t + (1/2)·(–9.80 m/s²)·t²,

1.4285714286 s = t,

or to two significant figures, the elapsed time for the rubber ball to fall is 1.4 s.

(Response (A) is a_y/(2⋅y); response (B) is sqrt(y/a_y); response (D) is 2·y/a_y.)

Sections 70854, 70855
Exam code: quiz02Cs1o
(A) : 2 students
(B) : 17 students
(C) : 29 students
(D) : 6 students

Success level: 54%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.72

Physics quiz archive: kinematics, free fall

Physics 205A Quiz 2, fall semester 2019
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855
Exam code: quiz02Cs1o



Sections 70854, 70855 results

0- 6 :
7-12 :	******** [low = 9]
13-18 :	************
19-24 :	****************** [mean = 21.1 +/- 6.0]
25-30 :	**************** [high = 30]

Online reading assignment: forces and motion

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 forces and 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.

"I understood the motion flowcharts very well. By this, it represents when we should and when we should not use Newton's first and second law. It does this by asking whether the speed is constant and then whether the direction is constant. I understand the flowcharts and the seemingly easy two-step verification process for determining the specific Newton's law. I can observe how the flowchart could be used to decipher the correct Newton's law at play in a situation."

"I understand that in order for Newton's first law to apply, the speed and direction of an object must BOTH be constant. If either aren't, than Newton's second law applies. I also understand that Newton's first law is applicable when the net force of an object is zero, and the Newton's second law applies if the object's force is anything but zero."

"The basic postulates of Newton's first and second laws of motion. I know that the first law states that an object will continue to move at a constant velocity until an outside force causes that to change. The second law then goes on to say that acceleration of a moving object depends on the object's mass and the net force acting on the object."

"With constant direction and speed, Newton's first law is in motion; all other cases are second law. Similarly, if the net force is equal to zero, it is the first law. ΣF = m⋅a is the equation used and can end up being 0 = 0 (for the first law case)."

"Newton's first law is applied when an object is maintaining a constant velocity; Newton's second law is applied when an object’s velocity has changed and we have to take into account the net force acted upon the object and inertia of the object."

"An object will stay at rest or in motion at a constant speed unless it is compelled to a change in net force. The net force is also the vector sum of all forces acting upon an object.

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 was a bit lost when viewing the net force flowchart. It seems overly simplified. But after researching it more, I am slightly more comfortable using it for the questions."

"Something I found confusing was how vectors work in Newton's second law. If you are moving in a circle in constant motion, then would the forces be considered non-zero or zero because of the changing direction?"

"Where I get confused is when I overthink and take too many things into consideration with the object. Or when I forget to take things into consideration."

"The concept of net force. Honestly the pictures in the presentation made me more confused."

"I don't see how to calculate the net force of an object when two different forces occur in different directions, as well as the other factors such as air resistance and friction. I was confused by how to determine whether Newton's first law is being used or Newton's second law. I also was a bit confused by some of the equations for the vector components of the second law."

"I am having a difficult time understanding what we are getting to in class."

Identify whether a zero or non-zero net force corresponds to Newton's first law or Newton's second law. (Only correct responses shown.)

Zero net force (ΣF = 0): Newton's first law ("N1") [92%]
Non-zero net force (ΣF ≠ 0): Newton's second law ("N2") [88%]

For the rocket sled, Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.

first; zero.	************* [13]
second; non-zero.	******************************** [32]
(Unsure/guessing/lost/help!)	*** [3]

While the F-35B is descending at a steady rate (before it touches the ground), Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.

first; zero.	***************************** [29]
second; non-zero.	************** [14]
(Unsure/guessing/lost/help!)	***** [5]

For this car with a steady speedometer reading (and assuming the shaking is due to the person holding the camera, and not from the car itself), Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.

first; zero.	***************************** [29]
second; non-zero.	************** [14]
(Unsure/guessing/lost/help!)	***** [5]

For a person in the swinging chair ride moving along a circular trajectory at a constant speed, Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.

first; zero.	****************** [18]
second; non-zero.	************************* [25]
(Unsure/guessing/lost/help!)	***** [5]

For this car with a zero speedometer reading, Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.

first; zero.	****************************************** [42]
second; non-zero.	**** [4]
(Unsure/guessing/lost/help!)	** [2]

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

"Is there ever a situation when Newton's laws don't apply?" (No. Either motion is constant (Newton's first law), or is not constant (Newton's second law).)

"For the most part, this section was pretty straightforward. The only thing I would like touched upon would be the flowchart, in case I am not overthinking it. Is it as simple as it appears?" (Yes.)

Can you go over these in class with a couple of examples?"

"Will we go over the answers to the questions above in class?" (For some of these, yes.)

"Is there an answer key for these questions?" (Make sure you review these responses (with the correct answers in boldface) as part of the next reading assignment.)

"Definitely would appreciate discussion on the differences between Newton's first and second law in real physics problems."

"In the rocket sled example, is the speed constant even though the sled is accelerating when it first fires up?" (No, the speed is not constant once the rocket fires up, as it starts off stationary, and then begins to speed up, so its motion is then covered by Newton's first law (even though the direction is constant, though.)

"If an object is moving in a circular motion, is that 'constant?'" (Even if the speed is constant, the direction of the object's velocity vector is always changing, so its motion is then covered by Newton's second law.)

"What should we know about working with free body diagrams in this class?" (Nothing official, yet, but we'll get started on Wednesday.)

"what does is mean when it says forces don't 'cancel?'" (Two forces can 'cancel' in the sense that their contributions are opposite and as a result nothing happens, but it's not like they disappear or vanish--they're still there, its just that there is no net effect on the object.)

"When would vertical acceleration be negative and when would it be positive?" (If vertical acceleration is downwards/negative (say, only the force of gravity is acting on an object, and there is no air resistance), then an object moving downwards (in the direction of the net force) will speed up, and an object moving upwards will slow down. If vertical acceleration is upwards/positive (say, a rope is pulling upwards with a force greater than the weight force), then an object moving upwards (in the direction of the net force) will speed up, and an object moving downwards will slow down.)

"If Earth is spinning, how is it an inertial reference point?" (For our purposes, it's a "good enough" inertial reference frame given the situations we'll be dealing with in this course, especially in lab, where our scales, motion sensors, etc. are nowhere near sensitive enough to detect that Earth is not actually stationary. It's similar to neglecting air resistance for free fall, as that is also another "good enough" approximation to situations we'll analyze in lab.)

"Getting into forces. Things are now gonna get a little more overwhelming."

"Yee haw."

Online reading assignment: free fall, vector components

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 presentations on free fall and vector components.


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.

"How to find the right variables for a problem by analyzing which parts of the information are given and which are not. I also understand how to determine which of the kinematic equations to use by determining which of the five variables needs to be found and how to rule out a variable that doesn't need to be found."

"Things can be thrown upwards (start with positive velocity), thrown downwards (start with negative velocity), or dropped (start with zero velocity)."

"During free fall, air resistance is neglected and the acceleration is nearly constant. Because acceleration is constant, we can use kinematic equations. Using a right triangle we are able to define sinθ cosθ and tanθ which helps us solve problems that involve angles. Scalars are numbers with magnitude such as time and volume while vectors are quantities with magnitude and direction such as displacement and velocity."

"I understand that an object can have a different vertical distance traveled than magnitude of vertical displacement. This is dependent on if the object is thrown straight in the air and then free falls, dropped into free fall, or thrown downward. I believe that the vertical displacement is equal to the distance if the object is dropped or thrown down, but the distance traveled is GREATER than the displacement if the object is thrown up."

"The fact that the only force acting on a ball falling down is gravity which has a constant acceleration is consistent with my understanding of physics. I also understand that vectors are determined by the x and y components in a triangle bringing us back to trigonometry.

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 was confused about how an object thrown or shot upwards can reach the same speed when it comes back down to its initial starting point as when it is thrown or shot downwards from its starting point."

"I found the homework problems a little bit confusing just because we haven't gone over many of those sorts of problems together. It would be very helpful to go over one of those specific problems in lecture."

"Trigonometry, but just a refresher would be useful.

Explain what assumptions are made about the amount of drag (air resistance) on an object said to be in free fall.

"We assume that drag forces are negligible."

"Air resistance is ignored."

A boy steps off of a ledge (with no initial vertical velocity) and splashes into the water below.

Choose up to be the +y direction. The initial vertical velocity v_0y has a __________ value.

negative.	*********** [11]
zero.	********************************* [33]
positive.	*** [3]
(Unsure/guessing/lost/help!)	** [2]

For the boy, the vertical distance traveled is __________ the magnitude of the vertical displacement.

less than.	*** [3]
equal to.	************************************ [36]
greater than.	******** [8]
(Unsure/guessing/lost/help!)	** [2]

A ball is thrown and released downwards from the top of a building, and hits the ground below.

Choose up to be the +y direction. The initial vertical velocity v_0y has a __________ value.

negative.	************************* [25]
zero.	********** [10]
positive.	********* [9]
(Unsure/guessing/lost/help!)	***** [5]

For the ball, the vertical distance traveled is __________ the magnitude of the vertical displacement.

less than.	***** [5]
equal to.	**************************** [28]
greater than.	****************** [13]
(Unsure/guessing/lost/help!)	*** [3]

A hat is thrown and released upwards into the air and lands on the grass below.

Choose up to be the +y direction. The initial vertical velocity v_0y has a __________ value.

negative.	**** [4]
zero.	****** [6]
positive.	*********************************** [35]
(Unsure/guessing/lost/help!)	**** [4]

For the hat, the vertical distance traveled is __________ the magnitude of the vertical displacement.

less than.	***** [5]
equal to.	*** [3]
greater than.	************************************** [38]
(Unsure/guessing/lost/help!)	*** [3]

Mark the level of your exposure to trigonometry (triangles, unit circles, inverse functions, Pythagorean theorem):

None at all.	* [1]
Slight.	*** [3]
Some.	********** [10]
A fair amount.	************************ [24]
A lot.	*********** [11]

Indicate the following trigonometric relations between angle θ, the opposite leg o, the adjacent leg a, and hypotenuse h for a right triangle. (Assume that the angle θ is in the first quadrant: 0° ≤ θ ≤ 90°.)
(Only correct responses shown.)

sin θ: (o/h) [92%]
cos θ: (a/h) [88%]
tan θ: (o/a) [88%]
hypotenuse h length: √(o² + a²) [94%]

Describe what mnemonic device (if any) you use to memorize the right-triangle trigonometric relationships.

"Soh-cah-toa."

"Don't recall one."

"I've never heard one."

"I don't really use one."

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

"This is pretty cool. We are now moving into free fall which might start to make some things confusing and interesting when combining it with our horizontal motion knowledge."

"For the free fall examples above, would ground level be 0 or from when the ball left their hand?" (The convention used here is that we will always start at y = 0 at t = 0.)

"For all of the questions in this assignment, all of the initial velocities are zero? Because everything starts from not moving and is then thrown?" (If you throw a ball, it won't be in free fall (subject only to the force of gravity) until you let it go--so we can only start time t = 0 from the moment it was released with an initial velocity, as it leaves your hand.)

"Is there ever a setting where the free fall rules for falling objects don't apply?" (If air resistance (drag) is significant, then we can't say that acceleration is a constant value of 9.80 m/s² downwards.)

"In terms of this class are we to automatically assume drag/air resistance doesn't matter or will it be noted?" (On the quizzes and midterms, it will always be stated whether or not air resistance is negligible.)

"The last time I used SOH CAH TOA was 10th grade, so it's a little fuzzy."

"Will we only be using trigonometry to solve problems? I like calculus but I get lost when it comes to applying it to physics for some reason." (We'll use trigonometry to break down diagonal vectors such that we can analyze horizontal and vertical motion separately for projectile motion.)

"Nothing to ask here. Looking forward to the lecture!"