[Phys-L] Re: Aristotelian thinking among modern students

[herbgottlieb at JUNO.COM (Herb Gottlieb)]

I would have had difficulty in selecting any of the given choices that
you offer for the problem.  Because the problem does not specifically
state
that we are to neglect the presence of air,  or air currents, during the
flight of the rock none of the given choces are necessarily true or
false.

Good multiple choice questions  are very difficult to prepare.

See my comments in bold type interspersed below:

On Thu, 20 Oct 2005 11:28:11 -0500 "David T. Marx" <marx@PHY.ILSTU.EDU>
writes:
> I am presently teaching a general ed course for mostly non-technical
> undergraduates (mostly > freshmen).  Two-thirds of the students had
> high school physics.  One  half had calculus in high school.  The
> course is taught as primarily conceptual with examples and practice
> problems that use algebra and trig.  I gave a written exam last week
> covering Newton's  laws, gravity, projectile motion, work and energy.
> I give written tests rather than multiple choice tests so that I can
see
> student's thought processes at work.  Sometimes, as below, I give
> them choices and then ask for an  explanation of their answer.
>
> The first question on the exam was the following:

> A rock is thrown straight up from the Earth's surface.  Which one of
> the following statements  concerning the net force acting on the rock
> at the top of its path is true?

> (a) The net force is instantaneously equal to zero newtons.

*** Choice (a) could be correct if there happened to be an upward current
of air
at the time that the rock was at the top of its path.  Nothing in the
question indicates
that the air was still during the flightof the rock.

> (b) The net force is greater than the weight of the rock.

***Choice (b) could be correct if there happened to be a downward current
of air
at the time that the rock was at the top of its path.

> (c) The net force is less than the weight of the rock, but greater than
zero newtons.

***Choice (c) could be correct because the net force on the rock at the
top of its
path is less than the weight of the rock at the surface of the earth.

> (d) The direction of the net force changes from up to down.

***Choice (d) is not likely to be correct because the pull of gravity is
always
down towards the earth.


> (e) The net force is equal to the weight of the rock.

***Choice (e) would not be correct if there happens to be upward or
downward air
currents acting on the rock when it is at the top of its path.


Herb Gottlieb from New York City
Where we like multiple choice questions principally  because they are
relatively easy to grade
by overworked  and underpaid teachers.


----------------------------- --------------------------------
----------------------------  ------------------------
> The majority of students answered the question incorrectly.  Here is
> a sampling of some of the  incorrect explanations:

> Student #1: choice a
> "The force pushing the rock up equals the force of gravity pulling
> it down so at the top its
> acceleration is zero and the net force is zero."
>
> Student #2: choice b
> "The net force is greater than the weight of the rock because in
> order for the force to push the
> rock up into the air, the net force has to be greater than the net
> weight."
>
> Student #3: choice a
> "At the top of the path at that instant both the velocity and
> acceleration of the rock is 0.  It is
> not until the rock starts its fall towards the ground that the net
> force downward is greater than
> the weight of the rock."
>
> Student #4: choice e
> "There is always an equal an opposite force acting on an object.
> Therefore the weight of the rock
> which is pulling it down is equal to the force upward."
>
> I can understand students' misunderstanding with regard to velocity
> being zero and acceleration not
> being zero, but I am amazed that students would think that some
> force other than gravity was acting
> on the object to make it move upward, even after it left the hand
> that threw it.  It very much
> reminded me of Aristotelian thinking.  Any comments?