## 20180905

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

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 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.
"Projectile motion combines both vertical free fall and a constant horizontal velocity."

"In projectile motion you focus not so much on the horizontal motion but the vertical motion because it takes gravity into account."

"Projectile motion has both a vertical and horizontal component, although they are independent of each other."

"Projectile motion is merely vertical free fall, with an added horizontal velocity component."

"If something is launched diagonally, it means there is an initial horizontal and vertical velocity component."

"Adding horizontal velocity to a vertical free fall will still cause the object to fall at the same speed as if it had no horizontal velocity. The horizontal motion and free fall motion are independent of each other. Net force is the sum of all forces acting on an object and will depict the changing (or non-changing) movement of the object."

"The differences between mass and weight such that the weight of an object exists because of the objects gravitational pull towards earth and mass is a quantitative measure of inertia. Mass is an intrinsic property and weight van vary."

"The different forces and what they mean. There is weight force (the force of gravity), normal force (the force exerted by a surface), tension force (the force exerted along a string, rope, or cable), and there is static/kinetic force (the forces used to unstick or slide an object along a surface."

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.
"Determining whether an object has zero or positive or negative initial velocity components in a specific direction is tough since it depends on the situation of a problem. If it's from rest it may be zero, however if the problem starts off with the ball in movement it's more difficult."

"The equations and variables. Would need typical example equations done in class."

"The trigonometry parts are still difficult for me."

"When and how to use the quadratic formula."

"I need a little help understanding how the force equations exactly work when more than one force is at work."

"I would like to know where the formulas are coming from."

"I am lost with the forces. Is weight a force? WHY?"

"I have a good amount understanding the new equations given to us and how to implement them algebraically into functions that I can solve. I'll mention this in class and personally seek help on the difficult ones."

"Nothing was particularly confusing, but it will take some practice to apply it all."

"I actually feel as though I understand this pretty well, I didn't feel as though there was one thing that was overwhelmingly confusing." Indicate the initial velocity components for the (ideally) vertically-launched anvil.
(Only correct responses shown.)
v0x: 0 [73%]
v0y: positive [51%]
(The initial velocity (as the anvil is launched) is vertical, with no horizontal components. This makes the launch angle θ = 90° (as measured counterclockwise from the +x direction).)

Indicate the acceleration components after the anvil was launched.
(Only correct responses shown.)
ax: 0 [47%]
ay: negative [43%]
(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.
(Only correct responses shown.)
v0x: positive [65%]
v0y: 0 [43%]
(The initial velocity (as the car leaves the cliff) is horizontal, with no vertical component. This makes the launch angle θ = 0° (as measured counterclockwise from the +x direction).)

Indicate the acceleration components after the car was driven horizontally off the cliff.
(Only correct responses shown.)
ax: 0 [20%]
ay: negative [74%]
(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.
(Only correct responses shown.)
v0x: positive [78%]
v0y: positive [59%]
(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.
(Only correct responses shown.)
ax: 0 [27%]
ay: negative [63%]
(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.
"We are rolling a marble down a ramp that ends at the edge of a table. Projectile motion begins after the marble leaves the ramp."

"If a car is driven off a cliff that originally sloped downwards."

"Throwing something downward off the top of a building."

"Kicking a ball of a cliff downward as opposed to kicking it upwards."

"Somebody throws a football down at a person from a roof top."

"I don't know."

"What?"

Identify the type of interaction ("force") with its symbol. (Only correct responses shown.)
Weight ("gravitational force") : w [90%]
Surface contact force ("normal force"): N (or FN) [82%]
Tension ("rope/cable/string force"): T [90%]
Kinetic friction ("sliding force," or "sliption"): fk [86%]
Static friction ("sticking force," or "stiction"): fs [82%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"One of the things that I don't quite understand is the concept of a projectile aving a constant horizontal motion. Does that mean that the horizontal acceleration is zero? I just don't see how a projectile has a constant horizontal motion." (If air resistance is negligible, then after the projectile is launched, there is nothing to slow down or speed up its horizontal motion. However, after the projectile is launched, there is still the force of gravity that acts on it to cause a vertical acceleration.)

"Horizontal acceleration is zero because we neglect air resistance?" (Yes.)

"Horizontal velocity remaining constant if there is no air resistance?" Wouldn't the object eventually slow down or would that be caused by a different factor?" (Realistically, yes, an object would eventually slow down horizontally because of air resistance; but since we'll be neglecting it for the purposes of this class (which may or may not necessarily be a reasonable assumption), then for this idealized case the horizontal motion will remain constant.)

"If a vertical launch has no horizontal motion, then we can ignore the horizontal component of velocity and acceleration, right? (Yes.)

"Could you please clarify the initial velocity and acceleration components in class, please?" (Part of the next reading assignment is to review those launch examples again, but with specific comments on how to deduce those horizontal and vertical components for velocity and acceleration.)

"The use of the quadratic equation was thrown in on the presentation preview but didn't really explain how we use it. Hopefully there will be clarification in class?" (There will be. But it won't be pretty.)

"I did not understand free-body diagrams. Can all of the forces be added into a free-body diagram amongst each other?" (There can be. But it won't be pretty.)

"Will we have to memorize any of the equations, or will they be given to us on a quiz?" (All necessary equations will be given on a quiz. The only "equations" not given on a quiz are definitions (such as trigonometry, etc.).)