## 20160907

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

Physics 205A, fall semester 2016
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing a presentation on 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.
"For projectile motion, vertical direction differs from horizontal direction as vertical direction has an acceleration due to gravity. If there is no air resistance, there is no horizontal acceleration (making it zero)."

"How projectile motion can be divided into two parts, the horizontal motion of the object and the vertical motion. It makes sense to me how the acceleration would be zero in the horizontal direction and how it would be –9.8 m/s2 in the vertical direction."

"I am going to have a lot of questions tomorrow in class."

"I understand projectile motion and how it is the same as vertical free fall. However, it will have an added horizontal velocity. So, the ball and or object will be 'falling' but not in a straight line."

"Regarding projectile motion the initial horizontal and vertical velocity are considered individually. v0x is the initial velocity on the horizontal axis and v0y is the initial velocity on the vertical axis."

"Horizontal and vertical motions act independently. This means that the height reached by a bouncing ball is only dependent on the vertical velocity and height of its release, and not it's horizontal velocity."

"There are four different force types, weight, normal, tension/elastic, and friction. Each of these different forces can be calculated differently. Weight force is calculated by the mass times the graviational constant, the normal force which can vary from 0 to infinity depending on the surface exerting the force, and this is also true of a tension force, depending on the material. For the static friction force, it is also calculated the same by which can range form 0 up until it gets the object in motion."

"The different types of forces that were covered. There is gravitational force, normal force, static force, kinetic force, and tension force. These all have a way of affecting whether or not an object moves or how it stays in place."

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 still not very good with when and how to use sin, cos, and tan. So going over that would be very helpful!"

"I think this section is much easier to understand once you know exactly which equation(s) to use. The more practice with determining which equation to use, will help with projectile problems."

"What I found confusing was when adding forces together how that would equal the net force, especially if those forces were working in different directions."

"I still struggle with reading the math and would benefit from practicing in class."

"The concept of projectile motion and trajectory. I am a visual learner so the pictures help, but I would still need to explanation on the x and y components of projectile motion."

"I don't think I quite understand how to do all the math of everything quite yet, but all of the equations on paper do make sense."

"Not too much--if you understand trigonometry, you're good."

Indicate the initial velocity components for the (ideally) vertically-launched anvil.
(Only correct responses shown.)
v0x: 0 [76%]
v0y: positive [56%]

Indicate the acceleration components for the (ideally) vertically-launched anvil.
(Only correct responses shown.)
ax: 0 [69%]
ay: negative [44%]

Indicate the initial velocity components for the car driven horizontally off the cliff.
(Only correct responses shown.)
v0x: positive [73%]
v0y: 0 [56%]

Indicate the acceleration components for the car driven horizontally off the cliff.
(Only correct responses shown.)
ax: 0 [44%]
ay: negative [60%]

Indicate the initial velocity components for the car launched diagonally off the cliff.
(Only correct responses shown.)
v0x: positive [69%]
v0y: positive [47%]

Indicate the acceleration components for the car launched diagonally off the cliff.
(Only correct responses shown.)
ax: 0 [33%]
ay: negative [47%]

Describe a situation with a negative starting angle of elevation θ for projectile motion.
"If you threw a rock from a cliff down towards the lake below, that would have a negative starting angle."

"If you stood on the edge of a roof and launched a projectile towards the street, it would be moving positively to the right and down."

"A marble rolling off a slanted drafting table."

"I can't. Going to need help."

"Cyclops (from X-Men) standing tall, looking down to blast a big rat on the floor. His high energy beam projects at a negative angle of elevation."

Identify the type of interaction ("force") with its symbol. (Only correct responses shown.)
Weight ("gravitational force") : w [80%]
Surface contact force ("normal force"): N (or FN) [78%]
Tension ("rope/cable/string force"): T [89%]
Kinetic friction ("sliding force," or "sliption"): fk [87%]
Static friction ("sticking force," or "stiction"): fs [84%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I like when you do examples on the board in class. They are extremely helpful."

"I found the horizontal component of acceleration slightly confusing. Is ax always equal to zero for projectile motion when ignoring air resistance?" (Yes!)

"Were we supposed to neglect air resistance in the above examples?" (Yes.)