Why is it that ships float in the sea

To understand why this happen in first place; we need to understand principle of flotation or Archimedes principle in particular. In this post we will discuss displacement, buoyancy, draft and Archimedes principle; explaining how the same iron ship afloat easily at sea but float deeper in fresh water. In day to day life we use weight and mass interchangeably to such extent that even we sometimes seems to be confused of their distinguished meaning.

Mass is the amount of matter a object or substance possess independent of its location or position in this universe. While weight is the force of gravity acting on that mass. Mass is fixed but the weight change from planet to planet and place to place; depending upon the change in force of gravity acting upon that mass. I do remember doing such calculations without knowing the science behind it. Similarly i use to know the term volume without what it really is! Volume is the amount of space a object or substance occupies; and the relation between the mass and its volume is called density.

The density of fresh water is 1. With average salinity of 35 gram per ml of water it weights 1. This upward force is equal and opposite to the weight of the water it displaced.

When a ship displace water volume of more or equal weight in comparison to its own weight; the ship will float and this concept is called principle of flotation.

So, a , tonne ship must displace at least , tonne of water to stay afloat. There is two key force acting on a floating body, weight and buoyancy. For a ship to be stable and stay in equilibrium the center of gravity and buoyancy must be in a straight line. Suppose the ship weight is distributed uneven and is tilted on one side along the longitudinal axis; then the point at which original vertical line meets the line passing through center of gravity and buoyancy is called metacenter.

Now the ability of ship to return to its equilibrium position depends upon the center of gravity and metacenter. If the metacenter lie above the center of gravity; then the ship will regain its equilibrium.

But when metacenter lies below the center of gravity; the weight and buoyant force leads the ship to topple overturn causing the ship to sink. Density describes how much something weighs relating to its size, or mass per unit volume.

In technical terms, the density of a body is defined as the weight mass of the body in kilograms kg divided by its external volume in cubic metres m 3. From the above, only oil and wood naturally float in water. This is because fluids and materials that float in water have densities that are less than the density of water. In other words, an object's buoyancy is determined by its density in relation to the density of the surrounding liquid. 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.

Ah, but will that object float? So a large hollow object might float because large means more water displaced — so more buoyant force — and hollow means relatively little weight. A small solid object might not float, however. Less water displaced results in a smaller buoyant force. But the weight of the water it displaces is more than the weight of the aircraft carrier, so it floats.

Me, I weigh more than the water I displace, so I sink. The keel of the boat is deep under the water, and the ship is quite wide for most of its length.

If you roll a blob of clay or putty into a ball and drop it into a pot filled with water, it sinks. But it floats after you flatten it and curl it up into a cup shape. Try it! Image link to full profile By Phil Kesten.