What is Archimedes principle? How does a ship float?

Definition: Archimedes principle states that when a body is wholly or partially immersed in a fluid, an upward force acts on it which is equal to the weight of fluid displaced.The upward force acting on the body is called Buoyant force which helps in the floating of an object.

Weight of a body showing Archimedes principle.What is Archimedes' principle?
Weight of a body showing Archimedes principle

When a 10kg mass of body is kept on a water body, downward force will act on it along with buoyant force. The buoyant force acting on the body is equal to the weight of the liquid that gets displaced.

Archimedes Principle.What is Archimedes' principle?
Archimedes Principle
  1. If the downward force is more than the buoyant force the body will sink.
  2. If the downward force is less than the buoyant force then the body will float in a semi-submerged position.
  3. If the downward force is equal to the buoyant force then the body will float at water level in a submerged position.

Due to the displacement of water, it feels like it weighs less underwater.

The buoyant force=weight of liquid displaced

    =mass of liquid displaced gravity


Therefore, buoyant force= volume of the liquid density of liquid g

Density of fresh water=1000 kg/m^3

Density of salt water=1025 kg/m^3

Applications of Archimedes’ principle

1. Archimedes’ principle is used in designing ships and submarines. They are required to be made of specific dimensions so that in spite of having a large weight of steel involved in building the ship,  the ship would float in water.

Fig: ship in freshwater on the left and ship in seawater on the right side.
ship in freshwater on the left and ship in seawater on the right side.

There’s a difference in the ship’s hull submerged in water due to the difference of the density of water in which the ship is floating. The same ship when floating in freshwater will be submerged more than when the same ship is floating in seawater.

2.  Archimedes’ principle also applies on ships draft, draft is the shortest distance between the waterline and the bottom end of the ships hull, in a ship when there is more cargo the ships hull is intended to submerge more into the water which leads to more draft and when cargo is less in the ship the draft is less.

Fig: draft of ships.
Draft of ships
Fig: draft of ships. What is Archimedes' principle?
Draft of ship
Fig: draft of ships. What is Archimedes' principle?
Fig: Draft mark

3.  A lactometer is a device used to measure the density and impurity of milk. It works on the principle of Archimedes’ principle. The lactometer typically consists of a glass cylinder with a weighted bulb at the bottom and a graduated scale on the side. To use the lactometer, a small sample of milk is poured into the cylinder, and the lactometer is floated in the milk. The density of the milk can be determined by reading the point where the surface of the milk intersects with the graduated scale on the side of the cylinder.

        The lactometer works because the density of milk is related to its fat content. Milk with a higher fat content without any water will have a higher density than milk with a lower fat content mixed with water. Thus it can be found out that how much water has been mixed with the milk. The deeper it sinks, the more impure the milk.

Lactometer. What is Archimedes' principle?

How does the ship float?

Ships float due to Archimedes’ principle, buoyant force acts on it which is the upward force exerted on the ship. When a ship is placed in water, the weight of the ship displaces an amount of water equal to its own weight. This creates an upward buoyant force that supports the weight of the ship and allows it to float.

Overall, the buoyant force acting on a ship is determined by the weight of the water it displaces, and the ship will float as long as this buoyant force is greater than the weight of the ship.


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