## 20170913

### Online reading assignment: projectile motion, identification of forces

Physics 205A, fall semester 2017
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 projectile motion and forces/interactions.

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 understand the basics and values of components in vertical, horizontal, and diagonal launches. Also the force interactions and their corresponding symbols."

"Gravity is a downward force that has an effect on a projectile's vertical motion. There is no horizontal force in a projectile motion, so it will have a constant horizontal velocity."

"When dealing with the vertical motion of an object you can use the same kinematic equations as the horizontal.However, when dealing with projectile motion you have to deal with both the vertical and horizontal motion."

"Everything is pretty much the same, same formulas just a little more added. We were doing one dimensions now we are doing two dimensions."

"A projectile is an object with only gravity as the force acting upon it. The trajectory is the path a projectile travels. Projectile motion depends on the independent horizontal and vertical motions. Trajectory motion is vertical free fall with horizontal constant added. Weight, normal, tension, static and kinetic are forces that can be described for an object through a free body diagram and when added together create net force."

"I understand the five kinematic equations and how to pick the one to solve, because the process of elimination is really easy, and I've dealt with it before."

"The perpendicular force of an object is the normal force being exerted. The magnitude of static frictional force is the coefficient of static force of a material multiplied by the magnitude of the normal force."

"I feel like I understand the individual forces themselves. They all play their own part in the mechanical aspects of reality in unique ways."

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 still think that the sine and cosines are confusing but I feel I will get it the more we practice. Besides that I think it makes sense if I know what each variable means."

"One thing I struggled with is trigonometry. I sometimes forget which function to use for a particular situation."

"Just setting up the problems; I can't get the steps right to pick the right formula. I read the problem and just freeze on the setup."

"How to choose the more straightforward equation when solving a problem; I'm usually really slow so I would like to be more efficient and not waste too much time doing more work than I should but I'm always taking the long path."

"Projectile motion equations are confusing in general."

"The equations are confusing, but I think once applied and talked about will make more sense. Can you better describe 'normal force?'"

"The difference between kinetic and static friction. It seems to me that they are mostly the same with a slight difference but I am ultimately unsure."

"I did not find the material too confusing. Working through the problems can be a little difficult though."

"I didn't find anything confusing or did not remember anything confusing."

Indicate the initial velocity components for the (ideally) vertically-launched anvil.
(Only correct responses shown.)
v0x: 0 [88%]
v0y: positive [71%]

Indicate the acceleration components after the anvil was launched.
(Only correct responses shown.)
ax: 0 [67%]
ay: negative [44%]

Indicate the initial velocity components for the car driven horizontally off the cliff.
(Only correct responses shown.)
v0x: positive [78%]
v0y: 0 [56%]

Indicate the acceleration components after the car was driven horizontally off the cliff.
(Only correct responses shown.)
ax: 0 [27%]
ay: negative [75%]

Indicate the initial velocity components for the car launched diagonally off the cliff.
(Only correct responses shown.)
v0x: positive [79%]
v0y: positive [48%]

Indicate the acceleration components after the car was launched diagonally off the cliff.
(Only correct responses shown.)
ax: 0 [21%]
ay: negative [54%]

Describe a situation with a negative starting angle of elevation θ for projectile motion.
"A car going down a ramp which angle is in the fourth quadrant."

"Releasing a bowling ball into the bowling lane; it has a fast positive horizontal velocity component and slow negative vertical velocity component, creating a negative angle when it leaves your hand."

"A gun is fired from a building to the street below."

"I am unsure of an example and could benefit from going over this."

"I don't understand the question. If you could address it in class that would be much appreciated!"

"When the Night's Watch is shooting an arrow from the wall down to the wildlings." (#winteriscoming)

"When Frodo threw the ring down into Mt. Doom, the ring hag a negative starting angle of elevation heading into the fiery pit of evil." (Don't forget Gollum also fell with a negative starting angle of elevation into the depths of Mt. Doom. #somethingsthatshouldnothavebeenforgottenwerelost)

Identify the type of interaction ("force") with its symbol. (Only correct responses shown.)
Weight ("gravitational force") : w [81%]
Surface contact force ("normal force"): N (or FN) [85%]
Tension ("rope/cable/string force"): T [94%]
Kinetic friction ("sliding force," or "sliption"): fk [85%]
Static friction ("sticking force," or "stiction"): fs [87%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Why do we take the square root of the sum of the squared horizontal components and vertical components to find a vector magnitude?" (Pythagorean theorem; the magnitude is the hypotenuse of a right triangle with the horizontal and vertical components as its sides.)

"I was wondering if there is an equation sheet we plug knowns and unknowns into like the past. Some of those formulas became very complex." (Yes, the horizontal and vertical kinematic equations will be given to you for the next quiz.)

The material seems easy, but the in-class approach can be confusing."

"I like how the process we go through to find givens, unknowns and what to solve for in the list of five."

"I would really like you to do more lecturing in class and less practice quizzes and worksheets."

"Possibly more review on projectile motion and being sure of which equation/which variables we want to solve for."

"Could we go over questions that use all of these variables in class?"

"Can you explain the angles and components a bit more in-depth or go over it with a few more examples? I feel like we didn't have enough time to get the idea in."

"Can we use the blog to find more related problems to practice on?" (I've pretty much linked to all the old quiz questions that are on the blog with worked-out solutions that are worth doing for homework. If you really to see all the quizzes posted from previous semesters (which don't all have answers given), then you can click on (or search for) the tag "physics quiz archive," and just keep scrolling down the page, backwards into time.)

"How useful is doing calculations in ideal situations when in real life air resistance gets in the way of things?" (Well, ignoring air resistance would get you numerical results that would only be approximately correct; but realistically bigger uncertainties would probably come from not being able to precisely measure the experimental direction and speed of the initial velocity vector, etc.)

"If we imagine that gravity doesn't exist on Earth and we launch something horizontally, would it keep going straight until it crashes on another object? But if there is no gravity in the space, why do shooting stars are falling?" ((1) Yes, if we could turn off gravity. (2) But Earth exerts gravity everywhere around it, even in space, so it can still pull in meteoroids (small rocky debris) into the atmosphere, where they'll burn up as meteors ("shooting stars"). Bonus fact: surviving fragments found on the ground are meteorites.)

"In an orbit, what do we consider our starting and ending points if the object is in continuous motion?" (For something like that, you would pick an arbitrary starting point, and then your ending point would be just a fraction of a second later from that, and analyze its change in motion. Then you would consider that your new starting point, and then your ending point would be just a fraction of a second later from that, so you would re-calculate its change in motion. And so on. So basically, this is essentially vector calculus, differentiating and integrating over time (step-by-step) over each part of the orbital path.)

"The 'launched diagonally' car was CGI, obviously."

"I really enjoy your positive attitude towards physics." (Let's see if I can maintain that for the next sixty semesters until I can retire from teaching. #sixtysemestersuntilretirement)