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

A barge loaded with a stack of steel plates is approaching a low bridge.

Would their clearance be helped or hindered by tossing a plate or two overboard?

(It's a river/canal, not a closed swimming pool/lake.)

explain your reasoning.


It really IS a shame that 'Real HS Physics' (by this I mean the AP-C curriculum) totally leaves out so much of the important good stuff.

We sent a VERY intelligent, advanced student off to MIT, but he couldn't predict/explain how a He balloon would behave in an accelerating van. His excellent math skills let him argue EITHER side of the discussion. When asked for a Force Diagram he could only come up with the 'Buoyant' force with no idea of the pressure thing.

Later he was fascinated by the pressure area integral relating to the weight volume integral.
He had the math tools (Cal-3) - just not the intro ideas.
.
At 1:18 PM +0000 6/27/12, Philip Keller wrote:
>
On 6/26/2012 6:15 PM, Jeffrey Schnick wrote:
>> #3. "A boat carrying a large rock Is floating on a lake.
The boulder
>> is thrown overboard and sinks. The water level in the lake (with
>> respect to the
>> shore)
>>
>> 1) rises
>>
>> 2) drops
>>
>> 3) remains same"


My favorite solution to this old puzzle said it this way: while on the boat, the boulder displaces an amount of water equal to it in weight. Thrown overboard, the boulder now displaces water equal to it in volume. Since the boulder is denser than water, we know that the first volume is bigger. So it displaces less water submerged than when it was in the boat.

One advantage of this solution is that it doesn't depend on the normal force from the lake bottom. After all, the water level drops even before the boulder reaches the bottom.

Also, this solution method handles a common variation on the problem: throwing wooden blocks overboard instead. Now, they continue to displace a volume equal to their weight, so no change in water level.
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