Re: [Phys-L] Conservation of Momentum Quesion

[John Denker <jsd@av8n.com>]

On 10/13/2016 12:24 PM, Folkerts, Timothy J wrote:
> Here is a paraphrase of a question from a physics text.
>
> A ball falls, hits the ground, and rebounds at nearly the same
> speed. a)      Is the momentum of the ball conserved throughout this
> time? b)      Is the momentum of the (ball+earth system) conserved
> throughout this time?
>
> [...] then I
> started thinking about what conservation means.  Certainly the
> momentum of the ball is not *constant*, while the momentum of the
> (ball+earth) *is* constant.

Yes.

> Now my position is that this is a poorly worded, misleading question.

Yes.

> Conservation of momentum still applies to the ball by itself.

Yes.

> Conservation laws mean the quantity only changes to the extent that
> there is some flow into/out of the system.  

Yes.

> This is still true for
> the ball by itself – the change in momentum is equal to the impulse
> due to external forces.

Yes.

> So I now would  to reword this this: 
> a)      Is momentum conserved if we consider the system to be the ball by itself?
> b)      Is momentum conserved if we consider the system to be the (ball+earth)?
>
> Then the answers would be “yes” and “yes” but require slightly
> different reasoning in each case. 

The question is fine as it stands.

As a separate matter, there is also another version that might better
serve the pedagogical purpose by including the concept of constancy:

a')      Is momentum conserved if we consider the system to be the ball by itself?
b')      Is momentum constant  if we consider the system to be the ball by itself?
c')      Is momentum conserved if we consider the system to be the (ball+earth)?
d')      Is momentum constant  if we consider the system to be the (ball+earth)?

Both versions have merit, depending on the objective (optimized for
assessment versus optimized for making a point).

> But it is too late because the
> students already turned in their answers.

There is no law against "review" questions, and especially not
against asking similar but much-improved questions.


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For the next level of detail, with possibly useful discussion and diagrams
  https://www.av8n.com/physics/conservation-continuity.htm