[Phys-l] That controversial motion question.

[Bernard Cleyet <bernardcleyet@redshift.com>]

People!

"If we weigh something in air, the result on the scales is the  
gravitational force downwards minus whatever buoyant force is acting  
upward. Since answer A says "weight of the rock", it implies to me  
that this weight which would be the same at the top of its motion as  
it would be on a scales set on the ground.   posted on PTSOS.



There was a long thread on phys-l on the definition of weight --  
IIRC, the general agreement was weight is not (for physics) what a  
scale reads.  Therefore, buoyancy is not included in weight.  If this  
is so, E is the answer.

This thread, I think, proves it a horrifying question, unless, as I  
suggested, it be used for discussion.

bc


a previous post:




HI Jason

A is the answer the quiz wants you to give.
Ask the kids what are the forces on the rock.
 Gravity, which direction is it? Down...always down before during and
after the reversal of direction.

There also might be air resistance but! at the top it
is not moving so the air drag s zero!  so it is a good choice of  
position
to ask the question about.

Alas the real answer is E because there is yet another
upward force independent of the motion of the rock..... buoyancy!  So  
the
net force is slightly less than the weight (defined as mg) of the  
rock. However
this answer is more confusing than illuminating so I wouldn't have  
included it
in the set of questions, but I'd be forced to give full credit to  
students who
answered E.


Paul D



On Oct 28, 2009, at 1:24 PM, Jason Westfall wrote:

> >
> Hi All,
>
>
> > I had this as a question on an exam, and had a hard time  
> > convincing the kiddos
> of the correct answer.  Could someone help me to explain it in a  
> different
> way?
>
> > Thanks.
> > J. Westfall, Sonora High
>
>
> > A rock is thrown up from the earth's surface.  Which one of the  
> > following
> statements concerning the net force acting on the rock at the the  
> top of it's
> path is true?
>
>
> > A.  The net force is equal to the weight of the rock
> > B.  The net force is instantaneously equal to zero newtons
> > C.  The net force's direction changes from up to down
> > D.  The net force is greater than the weight of the rock
> > E.  The net force is less than the weight of the rock, but greater  
> > than
> zero newtons.