Re: [Phys-l] Student Misconceptions

["Anthony Lapinski" <Anthony_Lapinski@pds.org>]

It is a conceptual question. I also do similar ones with numbers they
can't do in their head, and then it becomes more of a "math" problem.

Like... You must average 40 mph on a trip. When you've gone half the
distance, you've averaged only 30 mph. What must be your average for the
remainder of the trip? Answer is not 50 mph...

These tend to be the most difficult motion problems for my students, even
my "brightest" ones.


Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
> I might be missing something (which I often do)  but this is a conceptual
> question.  
>
> Joel has illustrated the numerical answer is 40mph... 
>
> However I look at this problem as a conceptual one and NOT one for the
> calculator. 
>
> The driver spends MORE TIME at 30 mph than at 60 mph...(twice as much at
> 30 than 60 since they are covering the same distance)..  hence the
> average time traveling cannot be 45... it must be less that than 45 since
> the dominant weight would be at 30mph...  so quickly answer the question
> and get on to the next problem on the test.  Don't fret about getting the
> exact answer no one wants that... there is where the student wastes
> time...  we are looking for a back of the envelope response. 
>
> Just my thoughts.
> Steve
>
>
>
> Steve Bailey
> The Gunnery
> Science Chair
>
> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu
> [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Rauber,
> Joel
> Sent: Thursday, September 29, 2011 6:25 PM
> To: Forum for Physics Educators
> Subject: Re: [Phys-l] Student Misconceptions
>
> Average speed = Distance/time
>
> As defined in most textbooks, which I presume would be precursor material
> to asking the question.
>
> Which implies a unique answer to your question provided reasonable
> assumptions such as "return the same way" means retracing your path on
> the way home that you took originally.
>
> I.e. avg speed = (2x)/{(x/60)+(x/30)}= 60*60/90 = 40 mph
>
> Where x equal the one-way distance (the delta odometer reading)
>
> I use this as an example of follow what the definitions tell when
> performing a calculation; rather than jumping to intuition.
>
>
>
> > -----Original Message-----
> > From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> > bounces@carnot.physics.buffalo.edu] On Behalf Of Robert Cohen
> > Sent: Thursday, September 29, 2011 5:15 PM
> > To: Forum for Physics Educators
> > Subject: Re: [Phys-l] Student Misconceptions
> >
> > - - - - -
> > Anthony Lapinski wrote:
> > <snip>
> > If you drive somewhere with an average speed of 60 mph and return the
> > same way with an average speed of 30 mph, then is your average speed
> > for
> > the entire trip less than, equal to, or more than 45 mph?
> > <snip>
> > - - - - -
> >
> > Average over time or over space?
> >
> > Robert A. Cohen, Department of Physics, East Stroudsburg University
> > 570.422.3428   rcohen@esu.edu    http://www.esu.edu/~bbq
> > _______________________________________________
> > Forum for Physics Educators
> > Phys-l@carnot.physics.buffalo.edu
> > https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
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