Re: [Phys-L] [SPAM] Re: Physics, Errors and Different Teaching Styles

["John Clement" <clement@hal-pc.org>]

The basic idea that something displaces its volume when submerged is
supposed to click in before age 10, but sadly it does not.  I think that
once this is established then students have an easier time.  Usually only a
small percentage of advanced students do not understand this.  But sometimes
you even get students in a calculs based college course who do not
understand this.

Buoyancy is even harder.  MOP has a great ranking task on normal forces
where it challenges many preconceptions.  It has a block on a flat table,
and the same block on a table that is sagging in the middle with of course
the same normal force.  Then it has the same block inside an evacuated bell
jar.  Students hardly ever pick up on the fact that the normal force is just
slightly higher.

One of the challenging, but good activities is to ask how high does
something float in water on the Moon compared to the Earth.  Actually having
students translate situations to the moon is very helpful in getting them to
straighten out some thinking.

Then of course there is air pressure which is related to buoyancy.  Most
students think suction cups work by pulling up on the object, but are really
flummoxed when they realize there is a vacuum inside the suction cups so
there can be no interactions there.

John M. Clement
Houston, TX

> Great discussion! Archimedes' Principle is the most 
> challenging topic in
> fluids. Even my honors students struggle with it. I could do an entire
> test on it, but I don't. You really need many in class demos, 
> questions,
> and peer instruction to help students understand these ideas.