07 May 2011

Why Ships Float?

Have you ever thought why a small stone sinks in water while a big ship can stay afloat in the sea? Well, physics can explain this.

In 212 BC, Archimedes who was a Greek mathematician, physicist, engineer, inventor and astronomer discovered a principle, now known as the Archimedes’ Principle which states that “for a body immersed wholly or partially in a fluid, the upward force (known as the buoyant force) acting on the body is equal to the weight of fluid displaced by it.”

This means that if a wooden block of 600g is placed in water and only half the block sinks in water, the weight of the water displaced is equal to the weight of block immersed in water which is 300g.

Weight of the object immersed in water = weight of water displaced

Therefore, this principle of floatation is used to explain any objects that float in a liquid. Questions like why ice float in your drink, why stone sinks in water and why a ship floats on water can be explained using this principle.

You see, a stone sinks in water because its density is much higher than water which has a density of 1000 kg per meter cube. However, even though a ship is heavy and has a much higher density than water, it can float due to its hollow shape which displace large amount of water as a result which the buoyant force is enough to support its weight.

When more loads are added to the ship, it must sink more so as to displace more water. To prevent overloading, a plimsoll line is marked on its body to show the maximum depth the portion of ship that can be immersed in water. The plimsoll line functions as an indication to the level of danger as the density of sea water varies with location.

A ship will be submerged deeper in fresh water because its density of fresh water is less than sea water. For this reason, ship will have to displace more water to obtain sufficient buoyant force to support its weight. A ship can float higher in cold seasons as cold water had a higher density.

So, now you know why a ship can stay afloat compared to a small stone that sinks. Basically, the shape and overall density of an object are important factors in determining its ability to float.

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