How does a boat float??

Floating- Floating is a lot like a shoving match.  The boat is pushing down on the water, and the water is pushing up, holding the boat up.  If the water pushes up harder than the boat pushes down, the boat floats.  If the boat presses down harder than the water pushes up, the boat sinks.  Which ever pushes harder, wins.

A steel boat is not as heavy as the amount of water that the boat pushes away, or displaces.  Imagine the boat as making a hole in the water.  If the boat weighs less than the water it would take to fill the hole it floats.  So if a boat weighs 1000 pounds, it will sink into the water until it has displaced 1000 pounds of water.  Provided that the boat displaces 1000 pounds of water before the whole thing is submerged, the boat floats.

Shaping- It is not very hard to shape a boat in such a way that the weight of the boat has been displace before the boat is completely under water.  The reason that this is so easy is because a good part of the inside of any boat is air, unlike a cube of steel or a nail, which is solid steel throughout.  The average density of the boat which is the combination of steel and air is very light compared to the average density of water.  Therefore a very little part of the boat actually has to submerge into the water before it has displaced the weight of the boat. 

Buoyancy- Referring back to the boat that weighed 1000 ponds, how do the water molecules know when 1000 pounds of them have gotten out of the way?  Well, when thinking about floating and sinking you must think about something that is called buoyancy.  This is the force that helps objects float.  The actual act of floating has to do with pressure rather than weight.

Pressure- The upward force exerted by a fluid is equal to the weight of the fluid that is displaced, this is known as Archimedes' principle.  For example, consider a gallon-sized beach ball pushed all the way under water.  The gallon of water it displaces weighs about nine pounds, this is the upward force that is felt if you try to hold the ball under water.  When the ball is floating on top, it is displacing just a little bit of water-just the amount that weighs the same as the ball. 

Now, you can see that if the ship were to spring a leak below the water line, it would no longer be displacing as much water.  Or , to look at it another way, it would be displacing as much water but would be much heavier due to the water inside of it.  Either way, the ship would sink. 

Experiment- Floating vs. Sinking,

    Put the little foil ball in the water.  What happens?

The little foil boat floated.  That is because it was not as heavy as the water that it pushed away.  Then you made the foil smaller and it could not push away enough water to float.  The ball weighed exactly the same as the boat.  It got so small it was heavier than the water it could push away.  So it sank.

Even though a ship is very big and very heavy, it is not as heavy as the water it pushes away.   That is why a big ship made of steel can float.

www.eecs.umich.edu/~coalitn/sciedoutreach/funexperiments/agesubject/lessons/beakman/float.html     good, had interesting experiments

www.howstuff works.com/question254.htm      good, kind of confusing

http://physics.about.com/education/physics       great, lots of information