Answer to puzzle #21: The Brick, The Boat and The Lake
This used to be used as a question at the Oxbridge university interviews for science candidates to gauge how people think. For all I know it still is... Oh sometimes people ask about throwing an anchor overboard. It's the same.
There is maybe a little physics needed here...
When an object is floating be it because it is something less dense than water, say polystyrene, or because it is in a boat, say a brick and the whole boat is less dense than water, then it displaces in the water it's own mass. For example a 10 Kg lump of lead on a boat will force the boat to sink by a volume equivalent to 10Kg of water. Hence displacing 10Kg of water. Or indeed forcing the water level to raise by an amount equivalent to 10Kg of water. (Like when you get in the bath.)
When an object sinks in water it necessarily displaces it's own volume.
So when the brick is in the boat it is displacing it's own mass equivalence in water. When the brick is thrown over the side it is displacing it's own volume in water.
So which of these is greater?
Well we know the brick to be more dense than water because it sinks. So the volume of water equivalent to the mass of the brick is greater than the volume of the brick. And so less water is displaced after than before. Hence...
The Water Level Goes Down!!!!If that is not clear then consider the following example: you need to know that 1 litre of water has a mass of 1Kg:-
Say the brick weighs 2Kg and is 1 litre in volume.
When the brick is in the boat it is forcing the water to be displaced by 2Kg or as we know 2ltr.
When the brick is over the side and sat on the bottom of the lake it is displacing just it's own volume or one litre.
Bard and ChatGPT got this right and wrong respectively. Although Bard's explanation was wrong too. 5/10 for Bard, 0/10 for ChatGPT
If you're curious what Bard made of this puzzle...
This is a common misconception, as many people believe that the water level will stay the same or even go up. However, this is not the case. The water level will always decrease when an object is thrown into water, regardless of whether the object sinks or floats.
If you're curious what ChatGPT made of this puzzle...
When the man throws the brick over the side of the boat and it sinks to the bottom of the lake, the overall water level of the lake does not change.
This is due to the principle of displacement, which states that when an object is placed in a fluid, it displaces an amount of fluid equal to its own weight. When the brick is in the boat, it displaces an amount of water that is equal to its weight. When the brick is thrown into the water, it displaces an amount of water that is equal to its weight. Therefore, the overall amount of water displaced by the brick remains the same, regardless of whether it is in the boat or in the water. As a result, the water level of the lake remains the same.
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