[Phys-L] causality(was Buoyancy question)

[kphox at CHERRYCREEKSCHOOLS.ORG (SSHS KPHOX)]

Forum for Physics Educators <PHYS-L@list1.ucc.nau.edu> writes:
> With regard to the idea that a floating object must displace an amount
> of water equal to the weight of the floating object, Ken Fox asked (I
> assume rhetorically) why it is true.  Ken called this an artifact of the
> [true] cause of floating which is a pressure difference.
>
> It seems to me that "artifact" is too strong because it almost makes it
> sound like "displaced water equal to the weight of the floating object"
> is an accident or serendipity.

I think I know why objects float and am often troubled by explanations I
get from students that attribute "cause" where I am not sure it belongs.

Trying to keep to introductory Physics (such as bouyancy) is the
displacement of a weight of water the "cause" of floating or a good tool
to predict whether it will float or not? ("artifact" is probably not the
best word, but it did get a discussion.) My mind needs an upward force to
balance the gravitaonal pull (either mg or GMm/r^2) to "cause" the
equilibrium we call floating.

A student wrote the other day that two collliding balls behaved as they
did "because" of the law of conservation of momentum. In my mind the law
is not the "cause" of anything. It helps me predict the outcome of an
interaction. The cause is the nature of the forces of interction.

Maybe I am whistling in the wind, but when I ask "why" I want to go to as
fundamental level as I can. "What is there about density that tells us
...."

Ok. Back to grading. Yuch!

Ken Fox