## 20160912

### Online reading assignment: forces and motion

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 forces and motion.

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.
"The net force for Newton's second law cannot be cancelled out to equal to zero. The net force of the first law does equal zero."

"Newton's first law is applied under constant speed and direction. If either are not constant, newton's second law is applied."

"How Newton's first two laws differ. And I understand how to tell most of them apart for each situation but not all of them."

"There are two ways to determine whether or not Newton's first or second law applies. We either consider the motion or net force of an object. If an object has a constant speed and direction we can use the first law. If either of those is not constant then we use the second law. As for net force, if it is equal to zero, then we use the first law, and if not then we use the second law."

"The only thing that seemed simple to me was the flowchart on which Newton's law to use. It is easy to follow a chart like that to understand that concept."

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 understood most of the reading and don't have many questions on this particular portion of reading."

"Nothing in the text was confusing."

"At first, I was having problems understanding how to solve the problems with two-dimensional kinematics. When I reviewed your blog though and started actually drawing out the pictures it started to make more sense to me."

"It would be rad for you to elaborate on what we just read."

"As for things that were confusing, were some of the animated motion examples, and determining whether or not the speed and/or direction was changing, seeing as it was hard to decipher at times."

Identify whether a zero or non-zero net force corresponds to Newton's first law or Newton's second law. (Only correct responses shown.)
Zero net force (ΣF = 0): Newton's first law ("N1") [88%]
Non-zero net force (ΣF ≠ 0): Newton's second law ("N2") [86%]

For the rocket sled, Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.
 first; zero. ******** [8] second; non-zero. ********************************* [33] (Unsure/guessing/lost/help!) ** [2]

While the F-35B is descending (before it touches the ground), Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.
 first; zero. ************************** [26] second; non-zero. ************* [13] (Unsure/guessing/lost/help!) **** [4]

For this car with a steady speedometer reading (and assuming the shaking is due to the person holding camera, and not from moving the car itself), Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.
 first; zero. **************************** [28] second; non-zero. ************ [12] (Unsure/guessing/lost/help!) *** [3]

For a person in the swinging chair ride moving along a circular trajectory at a constant speed, Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.
 first; zero. **************** [16] second; non-zero. ************************* [25] (Unsure/guessing/lost/help!) ** [2]

For this car with a zero speedometer reading, Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.
 first; zero. ********************************** [34] second; non-zero. ***** [5] (Unsure/guessing/lost/help!) **** [4]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I really like that you give practice quizzes in class for us to work on with other classmates--thank you. :)"

"Excited for this quiz on Monday!"

"What's the difference between the second and first law of motion?"

"I am very very lost on this concept! I tried reading through the text twice and through the blog, but I am not understanding the net force concept as well as which newtons law to use. That is why I clicked unsure/lost/guessing/help for all of them!"

"I can't wait to see these answers for clarification!"

"Will we be going over how to use that Newton's second law equation in class?" (Before we get to use Newton's second law, I think we really first need to discuss when to use Newton's second law (or first law), based on the motion of an object.)

"I would like help with the textbook problems on projectile motion because that is where I am struggling still." (Go to office hours or tutoring hours for clarification. Those were some of the more difficult ones you've seen so far; keep in mind that the quiz will have relatively straightforward questions on projectile motion; while the midterm (which comes much later) will then have full-on complete projectile motion problems like those in the textbook.)