20161003

Online reading assignment: impulse and momentum

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 impulse and momentum.

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 product of the average force and the time of contact is equal to the impulse of force. Impulse is vector quantity and has the same direction as the average force. The SI unit of impulse is equal to newton times second."

"Momentum is equivalent to mass multiplied by velocity. An impulse is the product of a net force acting on an object and the duration of time is acts on that object, and causes a change in momentum."

"The impulse-momentum theorem--how impulse causes a corresponding initial-to-final change in the momentum of the object."

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 not really sure still what impulse is or when to use it. The equation is easy to use but I'm having trouble grasping the concept."

"How impulse changes momentum--is impulse just change in momentum over time?"

"The difference between momentum and the impulse is a little confusing for me. They involve the same things, so its a little tough to see how they are different."

"What impulse is, or how it correlates with momentum."

"Sometimes analyzing what needs to be found in a problem or what cancels out isn't too clear to me."

"I think the concepts are pretty easy, but relating all the equations to some real life situations and problems would be great. That way I can have a better understanding of it all."

"I actually didn't find this section that confusing! Yay!"

For the child hitting the tee ball with a bat, if the bat is swung such that it exerts the same net force on the tee ball for a longer time (by giving the bat more "follow-through"), the impulse on the tee ball will be __________, and the change in momentum of the tee ball will be:
 less; less. [0] less; greater. **** [4] greater; less. ******** [8] greater; greater. ***************************** [29] (Unsure/lost/guessing/help!) **** [4]

Indicate which crash-test dummy (if any) has the greater magnitude quantity in the process of coming to a complete stop. (Only correct responses shown.)
Greater magnitude momentum change ∆p: (there is a tie). [22%]
Greater magnitude impulse "J" exerted on it: (there is a tie). [18%]
Longer stopping time ∆t: seatbelt and airbag. [33%]
Greater magnitude net (stopping) force ΣF: no seatbelt/airbag. [36%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I do not understand why the momentum equation is the same for translational kinetic energy. How are they different?" (They are different because although they both have mass and velocity in them, momentum is a vector quantity (using velocity direction and magnitude); while translational kinetic energy is a scalar quantity (using only velocity magnitude). Also they have different conservation laws as well.)

"Too many letters to remember. 'p' for momentum and 'J' for impulse? Come on! (We already used "m" for mass, and "I" will be used for "moment of inertia." We need more letters in the alphabet--"t" is used for time, "τ" is for torque, "T" is for temperature, and "T" is for period. #justsaying)

"Can we crash some cars?" (Yes. In lab. Sort of. Don't get too excited. Maybe)

"Should we be memorizing the equations in the reading for the quizzes we take for them?" (If the equations show up on the practice quizzes, then they'll show upon on your quizzes this semester as well.)

"Will we be talking about impulse more in class? Will it be on our first midterm?" (Yes. No. The study guide for the upcoming midterm has already been posted, it covers only up through uniform circular motion. Energy and momentum conservation will be on the quiz after the midterm.)