Answer: The water level falls when the brick is thrown into the water – Consider the brick in the boat: according to Archimedes’ principle the upward buoyant force is equal to the weight of water displaced (this is why anything more dense than water sinks). So when the brick rests in the floating boat, it displaces the volume of water equal to the brick’s weight. Now consider the brick in the water. This is easy, it displaces the volume of water equal to the brick’s volume. The brick is more dense than water so a larger volume of water must be displaced to equal the weight of the brick. Hence the water level then falls when the brick is thrown off the boat.
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A person tells you that they have 2 children, and they say that at least one of them is a boy. What is the probability that the other child is a girl?
TOM ROCKS MATHS 