After the previous Sink and Float post, my dad told me about a puzzle he had heard years ago (maybe from Car Talk).
A man is in a boat on a lake. He is transporting large boulders. When he gets to the middle of the lake, he tosses the boulders into the lake. Does the water level in the lake go up or down when he drops the boulders into the lake?
I asked my kids what they thought. They had a variety of responses. At this point I could have just explained the correct answer, but we must experience science. So I pulled out a lake, a boat, and a bunch of boulders (also known as: a 5 gallon bucket, a plastic container, and a cupful of coins).
We loaded the “boat” with as many coins as it could hold and then marked the water level.
After the level was marked, we dumped the coins into the water.
As you can see the water level went down.
This seems counter-intuitive. We added something to the water and the level went down. But we have to remember where the coins came from. They were already under the water level when they were inside the cup. There was also a lot of air inside the cup that was under the level of the water. So when the coins went into the water, the boat floated higher and all of that air came out of the water.
When the coins are in the water, they displace an amount of water equal to their volume. When they are floating, they displace an amount of water equal to their mass. Because the coins are denser than water, the amount of water that equals the mass of the coins is greater than the amount of water that equals the volume of the coins.
If this explanation did not make things clear, there is only one thing you can do; find a lake, a boat, and some boulders…
Many of you are probably asking some questions about the puzzle, so I would like to give some possible answers here:
How was the man able to toss large boulders?
- He used some levels and pulleys
- He is on a planet with significantly less gravity
- His name is Sampson
Why was the boat on a lake?
- It was a large lake
- The lake had a quarry on one side and a need for boulders on the other
- The puzzle would not have worked well in a river or the ocean. Stop being so picky.
Why did he throw the boulders into the lake?
- He didn’t like them.
- He found out that the boulders were sandstone and the people on the other side needed granite.
- He had heard of this puzzle and wanted to try it, but couldn’t find his 5 gallon bucket.