csmorris,
In your experiment with a rock in a boat then thrown into the pool, the water level in the pool will go down. This is because the boat had displaced the water's volume equivalent to the weight of the rock, which is more volume than the dense rock will displace when tossed out of the boat. When the rock is tossed into the water, the boat floated up to stop displacing the water weight of the rock, and the rock displaced less water when it sank to the bottom. In fact the pool water went down by an amount equal to the difference between the specific gravity of the rock and the SG of the water times the weight of the rock.
Your points about the box are right on! Guys, regarding the box experiment, remember that the helo flies by exchanging momentum with the earth system. Inside the box, the rotor allows the helo to fly, but the entire sealed box/helo system is constant weight, forever. Even as the engine and pilot consume the oxygen, it is mearly recombined in the H2O and CO2 expelled in the exhaust. The weight of the entire system stays constant (conservation of energy and mass.)
A great science show 20 years ago performed this exact experiment. "The Ring of Truth" with Philip Morrison weighed the box and then ran little cars, rockets and such inside the box, then re-weighed it, to the gram. Here is a reference to the video tape, which some libraries might have (there are 13 copies in Connecticut, for example):
Ring of Truth, Morrison, Program 2: Change
Summary: Using everyday examples that are dramatic, exciting & entertaining, Philip Morrison devotes an entire program to this subject, demonstrating in simple experiments that it is true that matter is equivalent to energy. 060 Min. VIDEO 1987 Subject: PHYSICS, ASTRONOMY Series: RING OF TRUTH, THE
Last edited by NickLappos; 6th Dec 2004 at 02:06.