Re: [Phys-L] Nice question on buoyancy and balance

[John Denker <jsd@av8n.com>]

On 01/29/2014 08:21 AM, Anthony Lapinski wrote:
> For those who may remember, I posed a similar question on Tap-L back in
> February, 2004. It was the string and ping pong ball in a beaker of water
> on a scale. If the string were cut and the ball then floated, would the
> scale reading change?
>
> These are great thinking questions that are very challenging for students
> -- even my brightest ones. And when physics teachers argue back and forth,
> you can only imagine what students think about these questions. I made
> them bonus questions on tests. Few got them correct. Maybe better as a
> demo or something.

Not meaning to sound like a stuck record, but this is
super-easy from the momentum-flow viewpoint.

Draw a dotted line around the beaker.  Momentum is flowing
in via gravity.  Momentum is flowing out via contact with
the scale.  There are no other flows crossing the boundary.

Therefore, it doesn't matter whether you cut the string or
not, because that cannot possibly change the flow across
the boundary.

I just don't see any way to mess up an argument like this.

==============

When people ask me "what is physics" I usually duck the question.

I can however tell you that /conservation laws/ are super-important
to physics, and have been since Day One.  A good conservation 
argument is very easy and very powerful.

Pedagogically speaking, if you wanted to, you could start with
conservation of energy on the first day of class and go from
there.  Start with things like Galileo's interrupted pendulum.
It gives an interesting result, using nothing but conservation
of energy.