Does dropping a rock or catching a fish affect water displacement in a pond?

In summary, the argument is about whether dropping a rock or catching a fish will change the water level in a pond. The correct answer is that dropping a rock will decrease the water level, while catching a fish will increase it. This is because the rock, when thrown in, displaces more water than when it is floating in the boat. Similarly, a fish that is denser than water will also displace more water when it is out of the water, causing the water level to increase.
  • #1
My friend and I are arguing over this question. If you're in a boat on a pond and drop a large rock into the pond will the water level change? If so will it increase or decrease? What will happen if you catch a fish that has swallowed a lot of water? I said that the rock will increase the water level b/c it's like dropping pennies in a cup full of water, is displaces the same amount of water that's equal to the mass of the penny. In the case of the fish that water is taken out so the level decreases. The difference is very small though. My friend says that the water level will stay the same. Who's right?
 
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  • #2
You're both wrong: the water level goes down if you drop a rock over the side. Don't feel bad; this famous puzzle has tripped up a lot of people. See:

http://physics.about.com/cs/puzzles/a/160603.htm [Broken]
(the solution is on pages 2 and 3)

http://www.rain.org/~mkummel/stumpers/07sep01a.html
 
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  • #3
Neither of you.

When the rock is in the boat (which is floating), it displaces its weight in water.

When it is thrown in & sinks, it displaces its volume of water.

Since the rock weighs more than an equivalent volume of water, it is displacing more water when floating than when it is at the bottom of the lake. So when it is thrown in, the level of the lake falls.
 
  • #4
What happens then when a fish is caught that has swallowed a lot of water?
 
  • #5
If you mean the fish is denser than water, then it's equivalent to a rock: pulling it out of the water will be the reverse of throwing it in, so the water level will increase.
 

What is displacement of water?

Displacement of water refers to the phenomenon of an object being submerged in water and causing the water to move and take up space, resulting in a change in the water's level or volume.

How is displacement of water calculated?

The displacement of water can be calculated by measuring the volume of water before and after an object has been submerged in it. The difference between the two volumes is the amount of water displaced by the object.

What is the principle behind displacement of water?

The principle behind displacement of water is Archimedes' principle, which states that the buoyant force on an object submerged in a fluid is equal to the weight of the fluid that the object displaces.

What is the significance of displacement of water?

Displacement of water is important in understanding the concept of buoyancy and how objects float or sink in water. It is also used in various applications such as shipbuilding, designing submarines, and measuring the volume of irregularly shaped objects.

How does displacement of water relate to density?

The displacement of water is directly related to the density of an object. Objects with higher density will displace more water and sink, while objects with lower density will displace less water and float. This is because the density of an object determines its weight and, therefore, its ability to displace water.

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