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Rock in a Pond

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A man sits in a small boat in a pond. He has a large rock in the bottom of the boat, which he picks up and throws overboard (the rock sinks in the pond). How does doing this affect the water level in the pond; does it rise, fall, or remain the same?

By the way, the pond has no outlet or inlet which would affect the water level otherwise.

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Posted · Report post

It falls.

While it's in the boat, the rock displaces its weight of water.

When it's on the bottom of the pond, the rock displaces its volume of water.

Since the rock's density -- weight/volume ratio -- is greater than that of water, it displaces more water while it's in the boat.

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Posted · Report post

I think it remains the same.

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The water level decreases. Because, when the stone is in the boat, the larger surface area of boat displaces larger amount of water. which in turns results water level to rise relatively high (remember archimedis?). But when the stone is thrown into water causes a lower amount of water displacement.

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The water level decreases. Because, when the stone is in the boat, the larger surface area of boat displaces larger amount of water. which in turns results water level to rise relatively high (remember archimedis?). But when the stone is thrown into water causes a lower amount of water displacement.

Well, my understanding is that when an object is immersed in water (or any other fluid for that matter), the water rises to a height proportional to mass (or weight) of the object. Then my argument is it makes no difference whether the rock is immersed into the bottom of the water or it is being on the boat.

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Well, my understanding is that when an object is immersed in water (or any other fluid for that matter), the water rises to a height proportional to mass (or weight) of the object.

I think if you check, you'll find that water rises proportional to volume. When immersed, an object displaces the water by it's size (volume), not it's weight.

One way to look at this is by example: Consider a rock of 1 cubic meter inside of a boat. Since rock generally weighs roughly 3 times water, the weight of the cube is around 3 tons. Since a boat displaces it's weight in water, the amount of displacement added by the rock is 3 tons of water or 3000 liters.

When the rock is totally immersed in the water, it will displace exactly 1000 liters of water.

Therefore, when the rock is removed from the boat and put into the water, the pond level will fall by the amount equivalent to removing 2000 liters.

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I think if you check, you'll find that water rises proportional to volume. When immersed, an object displaces the water by it's size (volume), not it's weight.

One way to look at this is by example: Consider a rock of 1 cubic meter inside of a boat. Since rock generally weighs roughly 3 times water, the weight of the cube is around 3 tons. Since a boat displaces it's weight in water, the amount of displacement added by the rock is 3 tons of water or 3000 liters.

When the rock is totally immersed in the water, it will displace exactly 1000 liters of water.

Therefore, when the rock is removed from the boat and put into the water, the pond level will fall by the amount equivalent to removing 2000 liters.

I 've got it now ... thanx

Edited by brhan
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Water level of the pond rises if it is the ice rock (Iceberg). But initially it will float on the water surface.

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