How do ships made of steel float

When applying this principle to ships, it is natural to question how a ship that has a hull made of steel, which has a density eight times greater than that of water, can float. A steel bar would sink, so why don't ships? Archimedes In the third century BC, the Greek mathematician and philosopher Archimedes discovered the principle of buoyancy while relaxing in a bathing pool. When he entered the pool he noticed that water spilled over the sides and that he felt lighter.

Archimedes realised that the amount of water that spilled was equal in volume to the space that his body occupied, and concluded that an object in a fluid experiences an upward force equal to the weight of the fluid displaced by the object.

Because the upward force equals the weight of the fluid displaced, an object must displace a greater weight of fluid than its own weight in order to float. That means that in order to float an object must have a lower density than the fluid. If the object's density is greater than that of the fluid, it will sink. The density of ships Although ships are made of materials that are much denser than water, the density of a ship itself is its total weight including, cargo, bunkers, stores, crew, etc.

This means that the hull must have an external volume that is big enough to give the whole ship a density that is just less than that of the water in which it floats.

Ships are therefore designed to achieve that. Much of the interior of a ship is air compared with a bar of steel, which is solid , so the average density, taking into account the combination of the steel, other materials and the air, can become less than the average density of water. When the metal hull of a ship is breached, water rushes in and replaces the air in the ship's hull. As a result, the total density of the ship changes and depending on the extent of the change, the ship may sink.

If you take a column of water 1 inch square and 1 foot tall, it weighs about 0. That means that a 1-foot-high column of water exerts 0. Similarly, a 1-meter-high column of water exerts 9, pascals Pa. If you were to submerge a box with a pressure gauge attached as shown in this picture into water, then the pressure gauge would measure the pressure of the water at the submerged depth:. If you were to submerge the box 1 foot into the water, the gauge would read 0.

What this means is that the bottom of the box has an upward force being applied to it by that pressure. This just happens to exactly equal the weight of the cubic foot or cubic meter of water that is displaced! It is this upward water pressure pushing on the bottom of the boat that is causing the boat to float.

Each square inch or square centimeter of the boat that is underwater has water pressure pushing it upward, and this combined pressure floats the boat. Sign up for our Newsletter! If you crumple it too much, just carefully pull apart some of the aluminum foil to get the desired size. What percentage of the ball is below the top of the water? Remove it, shake out any water and dry it. Keep testing smaller diameters until the ball completely sinks.

Try testing these diameters or ones roughly similar : 4. If it is too hard to squeeze the ball smaller by hand strength alone, then carefully use the hammer or mallet to gently pound the foil into a smaller ball or as close to a ball-shape as you can make it. For each diameter you test, what percentage of the ball is submerged? Do you think that the ball that sank had the lowest or highest density? At which diameter did the ball have a density that was approximately equal to that of water?

When was the ball almost completely submerged or fully submerged but not quite sinking to the bottom? Do you get the same results with all the aluminum squares you test, or is there a lot of variation? Calculate the volume of the spheres for each diameter, using the fact that the volume of a sphere is equal to four thirds times pi 3. Using the mass and the volumes, compute the average density of the aluminum sheet for each diameter by dividing mass by volume.

At what density did the aluminum ball sink? At what density was the aluminum ball approximately equal to that of water? For each diameter of the sphere, what is the mass of the water that was displaced? For more accurate results, continue testing additional cm aluminum squares. Observations and results Did more and more of the ball end up below the top of the water as the ball's diameter decreased? Was about half of the ball below the water when the ball had a diameter of about 2.

If an object is floating in water, the amount of water that gets displaced weighs the same as the object. Consequently, while it was floating, the ball should have displaced the same amount of water as it decreased in diameter, and so the buoyant force should have remained the same. However, the density of the ball was changing—it increased as the ball's diameter decreased. Density is the mass per unit volume—it describes how much "stuff" is packed into a volume of space.