## 20190904

### Online reading assignment: projectile 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 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 that the horizontal component of projectile motion is constant when we disregard air resistance, but the vertical component changes because of gravity. I also understood the concept of the 'range' as being the horizontal distance traveled between launching and landing, assuming that the projectile returns to the same vertical level that it was fired at."

"Our approach to solving projectile motion problems will be to separate the horizontal motion and vertical motion parts of the problem and solve them separately. For the vertical motion, we will use the kinematic equations and rules of free fall. For the horizontal motion, we will use the kinematic equations for constant acceleration problems."

"After doing the assigned reading I learned that projectile motion depends on two independent ingredients: vertical (free fall motion) and horizontal (constant velocity motion). I also learned that vertical motion is described with the same set of free fall equations that we have seen before."

"I am slowly starting to understand where the symbols go to. Making a drawing for a problem that has a lot of information helps a lot, more than I thought."

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.
"Something that I found confusing is when trying to find two variables and which equation to use if you are trying to find two variables."

"I just need more practice with the different types of formulas."

"I didn't quite understand the explanation on how to apply the equations to the word problems."

"I am still struggling with trying to remember just little things like some of the symbols. And trying to find out the information that's giving but is not useful."

Indicate the initial velocity components for the (ideally) vertically-launched anvil. Choose up to be the +y direction.
(Only correct responses shown.)
v0x: 0 [74%]
v0y: positive [60%]
(The initial velocity (as the anvil is launched) is vertical, with no horizontal components.)

Indicate the acceleration components after the anvil was launched. Choose up to be the +y direction.
(Only correct responses shown.)
ax: 0 [70%]
ay: negative [51%]
(Neglecting air resistance, the only force acting on the the anvil after it was launched is due to gravity, such that there is a vertical acceleration (–9.80 m/s2, the negative denoting that it acts downwards). And since there are no forces acting horizontally to change the (zero) horizontal motion of the anvil, the horizontal acceleration is zero.)

Indicate the initial velocity components for the car driven horizontally off the cliff. Choose right to be the +x direction, and up to be the +y direction.
(Only correct responses shown.)
v0x: positive [81%]
v0y: 0 [47%]
(The initial velocity (as the car leaves the cliff) is horizontal, with no vertical component.)

Indicate the acceleration components after the car was driven horizontally off the cliff. Choose right to be the +x direction, and up to be the +y direction.
(Only correct responses shown.)
ax: 0 [37%]
ay: negative [60%]
(Neglecting air resistance, the only force acting on the the car after it was driven off the cliff is due to gravity, such that there is a vertical acceleration (–9.80 m/s2, the negative denoting that it acts downwards). And since there are no forces acting horizontally to change the (constant) horizontal motion of the car, the horizontal acceleration is zero.)

Indicate the initial velocity components for the car launched diagonally off the cliff. Choose right to be the +x direction, and up to be the +y direction.
(Only correct responses shown.)
v0x: positive [72%]
v0y: positive [64%]
(The initial velocity (as the car leaves the ramp) is diagonally upwards, with a positive (upwards) vertical component, and a positive (rightwards) horizontal component. Note that the the launch angle θ will have some value between 0° and 90°, in the first quadrant of the unit circle (as measured counterclockwise from the +x direction).)

Indicate the acceleration components after the car was launched diagonally off the cliff. Choose right to be the +x direction, and up to be the +y direction.
(Only correct responses shown.)
ax: 0 [32%]
ay: negative [58%]
(Neglecting air resistance, the only force acting on the the car after it was launched off the ramp is due to gravity, such that there is a vertical acceleration (–9.80 m/s2, the negative denoting that it acts downwards). And since there are no forces acting horizontally to change the (constant) horizontal motion of the car, the horizontal acceleration is zero.)

Describe a situation with a negative starting angle of elevation θ for projectile motion.
"When shooting a bullet downwards from the edge of a cliff."

"Throwing a rock down into a chasm."

"A cannon on top of a cliff is pointed at a downward angle θ from the horizontal at some initial velocity."

"A kid accidentally kicks his soccer ball to the roof of his house. Not wanting to make his parents mad, he climbs to the roof and finds the ball, then places it on the roof and kicks it off at a negative angle of elevation θ."

"A car rolling backwards down a hill (and off of a cliff)."

"I really have no idea."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Hope you had an awesome three day weekend." (I did, thank you for asking. Hope yours was awesome as well.)

"I really need to refresh my memory of the terminology as far as the subscripts, like v0 and whatnot." (That's okay, it's probably because you had an awesome three-day weekend.)

"Will Quiz 2 cover everything that we have done after the first quiz?" (Yes, but only up until free fall--so essentially constant acceleration motion that is either only horizontal, or only vertical. Quiz 3 will cover constant acceleration motion that is both horizontal and vertical (that is, projectile motion), and also forces and Newton's laws.)

"Are we supposed to memorize the equations that are given in this assigned reading assignment?" (No. All relevant equations are always given to you for the quizzes and exams.)

"Is air resistance always going to be ignored in these problems?" (For the purposes of this course, we will always neglect air resistance.)

"Will there ever be situations or problems where we will be starting to take air resistance into consideration?" (Only if you decide to major in physics.)

"When it comes to acceleration, is it always zero in the horizontal direction?" (Yes, because the horizontal velocity must stay constant if there is no air resistance.)

"I feel like I need to brush up on my trigonometry, it feels like it has been a while since I've worked with it."

"Are we going to use Excel again? (You will be using Excel for nearly every lab, except for when we use motion tracking software to analyze slow-motion videos and generate velocity versus time graphs.)

"Will we be working on any launch velocities and accelerations in lab? It would be helpful to be able to see this in person to visualize it better. (The next lab will have you using motion tracking software to analyze slow-motion videos of projectile motion.)

"I am slightly confused by the initial velocity for when a car is launched horizontally. I think that the car would have a (a) positive initial velocity for the x-direction, and a (b) negative initial velocity for the y-direction, but I'm looking forward to lecture to see if my assumption is true." (We'll cover this with an example in class, but: (a) yes; (b) no.)

"Please make sense of the examples and how to do the calculations with setup and finding solutions in hands-on problems to make the connections of how to complete the work." (Yes.)

"Parabolas make me think of quadratic equations, which makes me think about possibly getting to use the quadratic formula. I'm excited for the opportunity to refresh my mind on that!" (Be careful for what you wish for.)

"I'm really hoping I pass this class." (I hope so, too.)