## 20160831

### Online reading assignment: free fall, vector components

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 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 the constant acceleration equations we use are the same for either horizontal or vertical direction."

"Free fall is basically what would happen if there was only gravity working as a force on falling objects. However, since there is air resistance and other factors free fall can only happen in a vacuum."

"In the reading assignment examples, the boy going cliff-jumping begins with a zero initial velocity. An object thrown up begins with a positive initial velocity and when thrown down, a negative initial velocity."

"How the acceleration for a free fall object is known, due to it being the constant acceleration of gravity."

"Trigonometric functions sine, cosine and tangent of angle θ are used in an equation with the lengths of the opposite or adjacent side, and the hypotenuse. Adding together vector components gives the overall vector magnitude."

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 didn't find too much confusing, maybe just need a refresher on the trigonometry."

"I really did not understand vectors. I think I need to brush up on my trigonometry."

"The only thing I found sort of confusing was on the presentation review the questions where it asked if the vertical distance traveled is less than, equal to, or greater than the magnitude of displacement. None of those ones really made sense to me."

"The part that I found a little confusing in this section is the difference between free falling and when the object is being thrown up or down. I am unsure if there is a difference in initial velocity and if that changes the equation that would be used."

"I don't understand how a pellet fired in the downward direction from the edge of a cliff would strike the ground with the same speed as a pellet fired in the upward direction. Fired in the downward direction, the acceleration and velocity would be acting in the same direction, while acting in opposite directions if fired upward. Are the speeds the same at ground impact because the pellet shot upward gains enough speed due to acceleration on the way down to match the acceleration and velocity of the one fired downward?" (Yes.)

"The vertical distance traveled versus the magnitude of vertical displacement of an object thrown downwards. It's still traveling the same distance as something falling from rest, but the velocity is negative so does that mean the magnitude of vertical displacement is different just because it was falling faster?" (No, vertical distance traveled would be equal to the magnitude of vertical displacement, as there is no back-and-forth motion.)

Explain what assumptions are made about the amount of drag (air resistance) on an object said to be in free fall.
"In free fall motion, air resistance is neglected, such that vertical acceleration is constant."

"That the drag forces are negligible and need not be recorded."

"Air resistance is almost insignificant to where it is negligible when gravity is the main force pushing down on an object."

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

The vy(t) graph has __________ initial velocity v0y.
 a negative. ***** [5] zero. ***************************************** [41] a positive. [0] (Unsure/guessing/lost/help!) * [1]
For the boy, the vertical distance traveled is __________ the magnitude of the vertical displacement.
 less than. ****** [6] equal to. ************************************ [37] greater than. * [1] (Unsure/guessing/lost/help!) *** [3]

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

The vy(t) graph has __________ initial velocity v0y.
 a negative. ************************* [25] zero. ********* [9] a positive. *********** [11] (Unsure/guessing/lost/help!) ** [2]
For the ball, the vertical distance traveled is __________ the magnitude of the vertical displacement.
 less than. ***** [5] equal to. ************************ [34] greater than. ** [2] (Unsure/guessing/lost/help!) ****** [6]

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

The vy(t) graph has __________ initial velocity v0y.
 a negative. *** [3] zero. ******* [7] a positive. ********************************** [34] (Unsure/guessing/lost/help!) *** [3]
For the hat, the vertical distance traveled is __________ the magnitude of the vertical displacement.
 less than. ****** [6] equal to. ********** [10] greater than. **************************** [28] (Unsure/guessing/lost/help!) [3]

Mark the level of your exposure to trigonometry (triangles, unit circles, inverse functions, Pythagorean theorem):
 None at all. [0] Slight. ***** [5] Some. ************ [12] A fair amount. ****************** [18] A lot. ************ [12]

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) [91%]
cos θ: (a/h) [91%]
tan θ: (o/a) [94%]
hypotenuse h length: √(o2 + a2) [91%]

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

"I burned an image of a right triangle into my brain over the course of a summer trigonometry class."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Can you please go over more math in class?"

"Can you go over the trigonometric functions real quick? Thanks."

"I honestly can't think of a question right now."

"Will there be any material on the quizzes/midterms that we will not/do not cover in class? I'm assuming everything from the blog examples and the assigned problems in the textbook can be asked on quizzes and exams." (Your assumption is correct--your quizzes and midterms should reflect the level of understanding asked for from your in-class work, blog examples, and textbook homework.)

"Can we go over the EMAS stopping distance of a Boeing 727 final exam question?"