Re: [Phys-l] Buoyancy (was: motion problem)

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

This is a great thought-provoking demo. I do a similar activity in class.
Attach a small pocket level to a cart, attached to a string which has a
mass at the other end hanging over a pulley. The typical N2L activity. I
then ask the students when the hanging mass is released and the cart
accelerates, which way will the bubble move: forward or backward? No
buoyancy knowledge needed. My "honors" students have trouble with this
question. Some even say the bubble will remain in the middle. It's really
about inertia. Easy to set up. Simple idea but counterintuitive result.

I then ask the helium balloon situation as a bonus test question.


Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
> I like this Minds on Physics approach.
>
> In Robotics some years back I had a student who was going off to MIT soon.
> He had finished all our intro and advanced physics courses. (But THAT 
> intro was AP-C level so he hadn't 'done' buoyancy. While carting the 
> team around on the Short Bus for a meal - I did my usual schtick with 
> a helium balloon. If we take this balloon back on the bus, tie it to 
> a seat back, close all the windows - what will happen when I slam on 
> the brakes? Will it move forward, backward or stay still?
>
> Everybody has to chime in with their answer AND try to explain it to 
> the other students.
> I was hoping to eventually lead the MIT-bound math geek into using 
> his multi-variable calculus knowledge to 'explain' buoyancy 
> analytically. Turned out to be a lost cause, 'cause he didn't 
> understand it qualitatively enough to get started.
>
> Buoyancy was just another arrow to tack on to the FBD.
>
> He was quite good at explaining either the forward or the backward
> possibility.
>
>
> > Buoyancy is taught in the series Minds on Physics, and it is included in
> > many college texts.  Actually all common mechanical forces should be
> part of
> > the student's toolkit from the time the concept of force is introduced. 
> So
> > buoyancy should be introduced early on, but not necessarily in detail.
> >
> > MOP is the only book that to my knowledge does this.  They also have
> > activities where the students have to do ranking tasks which make them
> think
> > about all common forces, and which expose common misconceptions about
> > forces.  So one ranking asks about the normal force on various sized
> blocks.
> > One is very large, two are large, and the rest are all the same mass. 
> Then
> > various combinations of springs, strings, tables which bend are used to
> > explore misconceptions.  One in particular puts the block inside an
> > evacuated bell jar.  Students are supposed to recognize that the normal
> > force then goes up.  They studiously avoid the term weight at this point
> > because it is defined in so many different ways.  They also studiously
> say
> > ignore buoyancy and air resistance in most problems.  So they continually
> > alert students to its presence.
> >
> > But it also does fun things like asking how high a block will float in a
> jar
> > on the Moon compared to the Earth.  This is done before any principles
> > involving floating objects have been explored.
> >
> > McDermott has a classic evaluation question involving objects of various
> > densities most of which are greater than 1, but a couple are less than 1.
> > Students are asked to draw a picture of them in a bottle of water.  They
> > tend to draw them floating at different levels depending on their density
> > with maybe one at the bottom.  So buoyancy is a known problem with
> student
> > misconceptions.
> >
> > John M. Clement
> > Houston, TX
> >
> > _______________________________________________
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>
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