# what makes stuff float? **URL:** \ **Category:** Factual Questions\ **Created:** [September 22, 2004, 10:14pm UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572 "2004-09-22T22:14:57Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![vinniepaz](https://avatars.discourse-cdn.com/v4/letter/v/b4bc9f/32.png) [@vinniepaz](https://boards.straightdope.com/u/vinniepaz)\ **Post date:** [September 22, 2004, 10:14pm UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/1 "2004-09-22T22:14:57Z")
this is one of those things ive always accepted but never knew how it worked. i always assumed it was density, but a boat weighs more than a rock. so whats the deal? ---
**Author:** ![zev\_steinhardt](https://avatars.discourse-cdn.com/v4/letter/z/97f17d/32.png) [@zev\_steinhardt](https://boards.straightdope.com/u/zev_steinhardt)\ **Post date:** [September 22, 2004, 10:21pm UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/2 "2004-09-22T22:21:03Z")
> [@vinniepaz](#): > > this is one of those things ive always accepted but never knew how it worked. i always assumed it was density, but a boat weighs more than a rock. so whats the deal? True, a boat weighs more than a rock. But OTOH, a boat has a lot of empty space in it filled with air (which is lighter than water). As such, the total density is lower than water and the boat floats. The rock, OTOH, is (mostly) solid and therefore has a greater total density than water. So it sinks. The ultimate example of this is the planet Saturn. Saturn (which presumably weighs more than a boat) has a density of less than water. As such, if there was an ocean big enough to hold it, the planet would float. Zev Steinhardt ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [September 22, 2004, 10:21pm UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/3 "2004-09-22T22:21:13Z")
It \*\*is \*\*density. Your analysis neglects the density of the air in the boat. If you dismantled the (presumed) metal boat so that it can’t trap\* air, those pieces would not float. If something has a \*\*net \*\*density less than water it will float. \*or try to force air below the water level ---
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [September 22, 2004, 10:28pm UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/4 "2004-09-22T22:28:52Z")
You’re almost there, Hoss. You just need to revisit your idea of density. As opposed to _weight_, I mean. Us regular folks need to think of density as a combination of weight and volume. The short answer is that if you put something into a liquid, it will sink into that liquid. When it is sinking, it is \*displacing \* the liquid. Fill a pan with water. Carefully place an object into the water and catch every drop that overflows. As long as the object is floating, the \*weight \* of that water will equal the weight of the block of wood. The \*volume \* of the displaced water will be less than the wood. Because the wood has equal weight but greater volume, it is said to be less dense. If you could squeeze that same piece of wood into a chunk that took up less space than the water it displaces, it will sink like…a stone. So if you take 1,000 tons of sheet steel and configure it in such a way as to create a space that would hold 1,000 tons or more of water–but don’t let any water in that space, it will float on water. Because the density (weight / volume) is less than the density of water–it is capable of displacing more than its own weight in water. (is this horse dead yet?) ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [September 22, 2004, 10:29pm UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/5 "2004-09-22T22:29:06Z")
Or looking at it another way, an object displaces its own volume of water, but that water may weight more or less than the object itself; if the displaced water weighs more than the object, the object will float. ---
**Author:** ![633squadron](https://avatars.discourse-cdn.com/v4/letter/6/ee59a6/32.png) [@633squadron](https://boards.straightdope.com/u/633squadron)\ **Post date:** [September 22, 2004, 10:51pm UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/6 "2004-09-22T22:51:16Z")
> [@vinniepaz](#): > > this is one of those things ive always accepted but never knew how it worked. i always assumed it was density, but a boat weighs more than a rock. so whats the deal? It is density, but it’s hard to see the difference between the boat and the rock because you don’t see the air within the boat hull as being part of the boat. Technically, a boat floats because of displacement: Take a 25 lb rock and a 25 pound boat. The rock is the same weight, but much more dense. Because of that, it displaces (takes up volume) that is _less_ than its weight in water. That is, a 25 pound rock takes up as much volume as say 8 pounds of water. Drop the rock in water, it sinks. Now take a 25 pound boat. That’s a pretty lightweight boat; let’s say it’s a canoe made of carbon fiber. It displaces a LOT more water than the rock, probably 5 gallons which is roughly 40 pounds of water. Since it displaces MORE weight in water than its own weight, it floats. If you put the canoe in a car crusher and smash it to roughly the size of the rock, it will sink. Back to density: That this is related to density is not so obvious. You have to count in the air that’s inside the canoe. We don’t do that because the air doesn’t seem to be part of the boat, but it is; you’re just not used to seeing it that way. Another way to look at this is to imagine a carbon-fiber ball filled with air, floating in the water. Remember, it floats because the whole ball, including the air, is less dense than the equivalent volume of water. Fill the think with liquid mercury and it will sink! The air is not “holding up the ball.” Now cut the top off the ball. It still floats, because it’s still less dense than the equivalent volume of water. The air has been “let out,” but that doesn’t matter. Air means nothing, volume means everything. Asides: Some rocks DO float. Pumice, a volcanic rock, floats in water. Pumice is like a rocky sponge, filled with air vesicles. This leads to a great camping trick: making camp coffee. Pour ground coffee into boiling water and cook. If someone asks you when you know it’s ready, tell them “when a rock floats in it.” Good for a laugh. Reinforce the idea by tossing pebbles in, then when you think it’s ready, palm your trusty piece of pumice and toss it in… The planet Saturn has a density less than that of water, according to our most reliable calculations. If you could find enough water, you could float Saturn in it. One heck of a beach ball! ---
**Author:** ![Ringo](https://avatars.discourse-cdn.com/v4/letter/r/779978/32.png) [@Ringo](https://boards.straightdope.com/u/Ringo)\ **Post date:** [September 23, 2004, 12:07am UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/7 "2004-09-23T00:07:53Z")
While the horse appears very tired, we ought to at least give a nod to [Archimedes](http://www.crystalinks.com/archimedes.html). ---
**Author:** ![hajario](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hajario/32/171_2.png) [@hajario](https://boards.straightdope.com/u/hajario)\ **Post date:** [September 23, 2004, 12:11am UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/8 "2004-09-23T00:11:33Z")
A canoe that floats can be made out of concrete as a matter of fact. It’s a classic Mechanical Engineering student project. Haj ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [September 23, 2004, 12:38am UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/9 "2004-09-23T00:38:36Z")
> [@Inigo Montoya](#): > > You’re almost there, Hoss. > > You just need to revisit your idea of density. As opposed to _weight_, I mean. Us regular folks need to think of density as a combination of weight and volume. Not sure if your post was directed at mine or at the OP… Actually, [density](http://en.wikipedia.org/wiki/Density) is **Mass** /Volume. People often confuse mass and weight. They aren’t the same, since weight depends on the strength of the gravitational field you are in. Most of the time this is OK, since we’re usually talking about stuff that happens on the surface of the earth, in which case you can pretty much ignore the difference (just a multiplicative factor). ---
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [September 23, 2004, 12:48am UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/10 "2004-09-23T00:48:49Z")
> [@John Mace](#): > > Not sure if your post was directed at mine or at the OP… > > Actually, [density](http://en.wikipedia.org/wiki/Density) is **Mass** /Volume. People often confuse mass and weight. They aren’t the same, since weight depends on the strength of the gravitational field you are in. Most of the time this is OK, since we’re usually talking about stuff that happens on the surface of the earth, in which case you can pretty much ignore the difference (just a multiplicative factor). Not directed at you, sir. I figured I’d get something wrong if I started adding details. 😃 ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [September 23, 2004, 1:48am UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/11 "2004-09-23T01:48:38Z")
> [@Inigo Montoya](#): > > I figured I’d get something wrong if I started adding details. 😃 And that’s all you need to know to have a future as a political adviser! 🙂 ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [September 23, 2004, 2:52am UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/12 "2004-09-23T02:52:44Z")
Eureka! \ ---
**Author:** ![danceswithcats](https://avatars.discourse-cdn.com/v4/letter/d/9e8a1a/32.png) [@danceswithcats](https://boards.straightdope.com/u/danceswithcats)\ **Post date:** [September 23, 2004, 6:38am UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/13 "2004-09-23T06:38:41Z")
So if she’s a witch, then she floats? Or is she a duck, or a piece of wood? 😕 ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [September 23, 2004, 7:38am UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/14 "2004-09-23T07:38:20Z")
> [@hajario](#): > > A canoe that floats can be made out of concrete as a matter of fact. It’s a classic Mechanical Engineering student project. > > Haj Concrete pontoons and barges are also quite commonplace. ---
**Author:** ![Small\_Clanger](https://avatars.discourse-cdn.com/v4/letter/s/9fc348/32.png) [@Small\_Clanger](https://boards.straightdope.com/u/Small_Clanger)\ **Post date:** [September 23, 2004, 9:06am UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/15 "2004-09-23T09:06:49Z")
> [@Mangetout](#): > > Concrete pontoons and barges are also quite commonplace. And when steel was scarce [concrete ships](http://www.concreteships.org/ships/ww2/) too. ---
**Author:** ![Canadjun](https://avatars.discourse-cdn.com/v4/letter/c/76d3ee/32.png) [@Canadjun](https://boards.straightdope.com/u/Canadjun)\ **Post date:** [September 23, 2004, 1:04pm UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/16 "2004-09-23T13:04:38Z")
> [@Mangetout](#): > > Or looking at it another way, an object displaces its own volume of water, but that water may weight more or less than the object itself; if the displaced water weighs more than the object, the object will float. It only displaces its own **volume** of water if it sinks. ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [September 23, 2004, 1:20pm UTC](https://boards.straightdope.com/t/what-makes-stuff-float/265572/17 "2004-09-23T13:20:09Z")
Stuff float? Two scoops of stuff and some soda water…