why do ships float?

“Why does a massive ship made of iron float, where as a nail made of iron sink in water?”. Rashmi looked at me inquisitively as soon as I asked this question.

We were on a boat cruise, taking a short break at Thames River at Windsor. I never miss a chance to showoff. “I never thought about that” she replied. I cleared my throat as I set about explaining it in one sentence.

“you see” I blurted out, “any object in water will float, if the weight of the water displaced by the object is less than the weight of the object itself”. She innocently stared at me. I was feeling like a High priest of Egypt, explaining the mystery of the pyramids to a common man.

“well, in simple terms” I continued, “say you are falling into the river now and your weight is 95 Kilos..” she threw a sharp look as soon as she heard this. “and if the water you displace in the river by falling into it is either 95 Kilos or more than that, you will float”. I continued ignoring her sharp glance. “In case of ship, the weight that ship displaces is much greater than the weight of the ship and thats why the ship floats in the water”, I concluded.

Obviously, million times smarter than me was Archimedes, who found this principle. Archimedes was an ancient scientist, who is famous for the episode of Eureka! He is supposed to have run naked to palace, when he found the principle of buoyancy.

ps: Many might think this as a silly physics thing, a thing which they have learnt in school. I also have learnt the same principle during my school days. But understanding of the principle is miles apart from having learnt the principle. Unfolding of the principles in natural worlds gives your mind a real boost. You begin to appreciate nature in a new way.

7 thoughts on “why do ships float?”

  1. is there anything to do with the way ships are designed?
    why are ships left hollow at the bottom?
    is it the size of ships that help it float whereas the nail being small as compared to its volume sinks?

  2. Its simple amreen, If any object displaces more water than its own weight, the object will float. So, say the ship weighs 10 kilo, and if on water it displaces 11 kilo of water, it will float. (in case of water, 11 kilo = 11 liter). If you are not still clear, do mention, I will explain in detail.

  3. When a body floats, the weight of water it displaces equals its own weight, and is not less than its own weight. Only then can you have equilibrium. If the weight of water displaced exceeds the weight of the body, the body will rise upward. Important point.
    A ship is hollow, and much of its volume is air, which has negligible weight. The weight of the ship is concentrated in the iron portion which occupies a very small space compared to the volume of the entire ship. Hence a part of the ship’s volume can displace sufficient water to equal the weight of the ship, and the ship therefore floats partly submerged.

  4. pls help me why an iron nail sinks water but a ship which made of iron floats on water ? red all of things but ? didnt understand completely so who can help me?????

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