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



I agree with all of this for the variation I suggested with the ping pong
ball held down by a dowel. But in the case presented, the ball is held
down by a string. NL3 says that if the string pulls down on the ball, the
ball pulls up on the string. The string is in equilibrium, so the pan must
be pulling down on the string. NL3 says the string is pulling up on the
pan. Pan rises. What am I still missing?


On Wed, Jan 29, 2014 at 8:43 AM, Paul Lulai <plulai@stanthony.k12.mn.us>wrote:

"And from this you conclude...?"

If the spheres have the same volume they displace the same weight of water
and experience the same buoyant force. The buoyant force pushes the sphere
up. The N3L interaction has the sphere push the water (and the pan) down
with the same magnitude as the buoyant force.

I think this is one of those questions that has a few factors of which we
should be aware (aside from physics):
- these can be fun
- if we don't build to these, these can be damaging to student thought
processes.

If we build to these (how do you find Fb of the ping pong ball? How does
the buoyant force affect the force the table experiences? What happens if
another ping pong ball is used with a slightly larger Volume? What happens
if a metal sphere is used with the same volume?) we can build problem
solving skills for kids on these topics.

If we have physics puzzle day where we ask the scale problem without
anything building to this (on the multiple levels of Fb depending on Volume
and on analyzing the scale or the bottom of the beaker to determine what
happens to the scale) then we are always asking them what they find to be
"trick questions." They develop the sense that whatever they think, the
answer will be opposite. Dangerous.

It is easy to say we are teaching them to think or to analyze. It is more
effective to build that process up to these questions than to give them
these sorts of questions in isolation , resulting in making students think
physics doesn't make sense.

Sorry to preach.

Have a good Wednesday.

Paul (where the temp is up to -4deg F and school is on)

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