Day 133: Right Answer. Wrong Physics?


The student calculated the initial momentum of the cart no problem (0.8 kg m/s). But rather than calculate the impulse (3 N•s) and add it to the initial momentum,   the student added the force multiple times — in other words, the student treated the impulse product (0.3 N)(10 s) = 3 N•s as repeated addition.

What do you think? Right or wrong? 


About Frank Noschese

HS Physics Teacher constantly questioning my teaching.

5 responses to “Day 133: Right Answer. Wrong Physics?”

  1. Anonymous says :

    By this you mean they added .3 N, 10 times? If so, the magnitude might be correct, but the units wouldn’t (unless they added 0.3 Ns, 10 times)…

  2. almeidaphysics says :

    Right Physics. Maybe.Student simply "integrated" over 1s intervals, in which an equal impulses were imparted. Each second, Delta p = j = (0.3 N)(1s). Makes perfect sense, especially if you’ve been relating impulse to a bowling ball being struck by a mallet, as I remember you doing.However, if it’s not consistent with the way you’ve been discussing it in class then there are two possibilities: (1) Student came up with it himself and it made sense to him (which is OK although I’d discuss with him/her the value of expressing your thought process) (2) Student just kind of stumbled into it without much regards to what he/she was doing. This is why written quizzes are imperfect – the lack of communication/possibility for clarification from student. Such is the game, though.I’d either a) not grade it and ask the student to come in and clarify thought process in person or b) not give full credit for that objective because they can redo quizzes under your system, and give him/her a question with a non-integer time interval to see if his/her thinking is consistent.Sorry for the LaTeX up there, by the way.

  3. Anonymous says :

    I was thinking the same this as almeidaphysics in regards to the bowling ball lab. It seems like it is a model that they have in their head. Break the model like he says with non-integer time.

  4. Conor says :

    Might be worth a conversation, but it’s pretty hard to fault the physics.

  5. Brian Frank says :

    I think this is just as good or bad as a student who does the math as you describe (if they didn’t show units and didn’t explain in words). I’ll say that I encourage student to think through problems this way… I want students to be able to think through that velocity changes about 10 m/s every second, or that pressure changes about about 0.1 atm every meter, or that PEg changes about 10 J every 1m per kg… Of course, challenging student to think through non-integer situations is important, but if we were wanting here to assess whether the students can reason about problems with non-integer times, it’s on us to write question that require students to do that. To me, this is only wrong if you would mark students wrong for doing your math method without units and/or with our verbal reasoning.

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