johnf@apollo.uucp (John Francis) (12/05/85)
When the iron is taken out of the barge, the barge rises out of the water so that the mass of water displaced is decreased by an amount equal to the mass of the iron. This lowers the water level in the lock, since the total volume below the water level is now less. Is the barge now higher or lower with respect to the lock? Answer : It must already be higher. If it was at the same level then the water level would be the same as before, and so the barge would NOT have risen with respect to the water. If the barge was lower (WRT the lock) than before then more of the barge would be below the original water level, and thus the extra water displaced would have caused the water level in the lock to rise. So even before we throw the iron into the lock the barge has risen. After throwing the iron into the lock the barge rises even more!