Stupid science question

Okay, I did a few searches and couldn’t find any post on this topic. Here’s the question. I remember doing an experiment once, a long time ago in school, were we put rocks in a set amount of water in a plastic tub. Before we put the rocks in, we measured the hight of the water. Then, we put the rocks in the tub. After that, we measured the water level again, and it lowered. My question is: Why? Rocks have mass, therefore, they should take up some of the space of the water making the water level rise.

And, please note that I have a major difficulty understanding science. It’s just not my thing, so don’t gang up on me too much :slight_smile:

You got yourself some magic rocks there, SK! Are you sure you’ve got all the details right? I’ve never heard of such a demonstration, and it seems impossible the way you describe it.

(BTW, forgive the nitpick, but it’s not the “mass” of the rocks that displaces – or in this case, doesn’t displace – the water, it’s the “volume.”)

The way you’ve described this experiment, you’re right, the water should go up. So you’ve probably forgotten a key part of the experiment.

Before putting the rocks in the water, they were almost certainly floating on a small boat in the water. You measured that water height and then took the rocks out of the boat and dropped them in the water. Then the water level will go down.

Reason: Rock is denser than water. What that means is that the amount of water that weighs the same as the rock takes up more space than the rock. When floating, the rocks displace an amount of water that’s equal to their mass which is more water than their volume. But when submerged, they only displace the amount of water equal to their volume.

You all are only half right.

The initial problem or goal is to determine the density of the rock. Density is a fairly easy concept, it is simply the concentration of mass, and stated as mass per unit volume.

Typical units would be grams/liter, pounds per cubic foot and such. So to find the density of an object, you need to know its mass and its volume. Mass is usually easily determined on a scale or a balance. But measuring dimensions to obtain volume can be tricky with an irregularly shaped object.

So the easy way is to sink the rock in water and measure the volume of water that was displaced. If it is a small rock, drop it into a large graduated cylinder with water and measure the milliliters of displacement.

honkytonkwillie, it helps if you read and understand previous postings. What you are saying may be true but quite irrelevant.

SilentKnight says he dropped a rock in a tub with water and the water level went down. I’d like to see you do that and later we can talk about “cubic foots and such”.

did anyone handle the rocks? Maybe they were those fake rocks that are made of sponge, that would do it the way the person is describing I suppose.

Sorry, even a sponge will not make the water go down.

The scenario I posted earlier with the rocks in a boat is a puzzle so old it’s grown moss. Usually it’s just given verbally (and is about a boat in a lake), but there’s no reason they couldn’t have high school kids do it for real.

I like the sponge suggestion (even if it’s not the right explanation) because it suggests trickery. The OP doesn’t say that the poster put the rock in personally but says that “we” did it.

Here’s my thought: what if the tub was sitting on a ring frame, and the bottom was covered with stretchy plastic (like a balloon)? If you dropped rocks in that, the water level would go down a bit. I think.

There’s one thing I’m sure of about the experiment. It wasn’t rigged. Everything was done with our own equipment, even the rocks. And only I handled the equipment and rocks, no one made any switches. I’m sure it’s purely science related, not trickery of any sort.

Thanks for the helpful posts too, guys. There is one part I forgot (well, didn’t think it mattered) to tell you about. We measured the water with the rocks in a “boat” in the tub first. I really didn’t think this would matter, but now that I think about it, it probably has something to with the results of the experiment.

Yes, that would make sense. If you just throw the rock in, the water volume incrases because the volume of the rock is added to it. But if you put the rock on a boat, the rock drags the boat+air into the water, displacing even more water.

Aha! So now the sordid details come out. And I was right. It was Col. Mustard in the gazebo with the penknife after all!

Seriously, did my explanation in the third article of this thread answer your question or do you still not understand?

If the rocks had sharp edges and punctured a hole in the tub…

Maybe the rocks were made out of neutronium. That would lower the level of the water.

You did an experiment of one of my favorite physics puzzle questions, which is: You have a toy boat, floating in a bucket of water. A develops in the boat, and it sinks. Does the water level go up, down, or stay the same?

Even pretty smart people usually have to think about this one a while (unless they had your science teacher).

Answer: The level goes down. When the boat is on top of the water, it’s displacing its mass in water. When in sinks, it’s displacing its volume. You know that its mass is lower than the volume of water it displaces, because this means that the density of the boat is greater than that of water. You know this has to be the case because the boat sank.

I said that bass-ackwards. You know that its mass is greater than the mass of the volume of water it displaces…

That is one of the classic basic physics lessons, Archimedes’ Law. Archimedes sat in a full bath and saw it overflow. In a flash of insight, he realized the amount of water he displaced was equal to his body’s volume. He could determine his mass with a scale, and could thus determine his body’s density.

There’s some second part of the story, I can’t quite recall the specifics. There was some dispute over a gold crown, whether it was made from solid gold, or an amalgam of lighter cheaper metals. He dunked and then weighed gold and got its density. Then he dunked the crown and weighed it, and got the density. I forget whether the crown was judged real or not, I wish I could remember, its the punchline to the physics lesson.

So anyway, your experiment is obviously a measurement of the rocks’ volume.

Here’s a solution to your riddle - if the rocks were superheated, they could quickly evaporate a quantity of water, causing the water level to drop. Of course… this is a detail not likely to be forgotten or overlooked.

so if there are sponges in the ocean, how come they have’nt soaked up all the water?:smiley: :rolleyes:


“You screw up just this much, and you’ll find yourself flying a cargo plane full of rubber dogshit outta Hong Kong!”

More possibilities:

  1. The water is frozen and a few pebbles are added. Then the water melts and the level is lower.

  2. The rocks were tossed into the tub, with obvious results from the splashes.

  3. The placement of the rocks in the tub tilted the tub, causing water to spill out.

  4. The second measurement was done long after the rocks were added to the tub, and water evaporated.

  5. The experiment was done in space.

  6. The experiment was done in such a way that a huge federal “star wars” grant would be forthcoming if indeed water levels were reduced.