A man is in a boat floating in a swimming pool. He is holding a large solid (heavy) iron ball. If he drops the ball into the water, what happens to the level of water in the pool? In case it isn’t obvious, does it increase, decrease, or stay the same? [Answer after the break]
The water level will decrease. This is due to Archimedes principle. In short, it states that when an object is floating, the water displaced is proportional to the mass of the object. When submerged, the water displaced is proportional to the volume.
Since the ball is made of iron, you can assume that the objects mass is much larger than the volume. This means that when the ball is submerged, is displaces less water, so the level decreases.
Puzzle courtesy of Professor Siva.
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