Gravitation refers to the force of attraction between any two objects due to their masses. This force of attraction can be given by universal law of gravitation, which states that the force of attraction between any two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. Force of gravitation due to the earth is called gravity. Whenever an object falls under the force of gravity alone, it is said to be in free fall. The force with which the earth attracts the object towards it gives the weight of the object. It is different from the mass of the object, which refers to the total quantity of matter contained in the object.

The force acting on an object perpendicular to the surface is called thrust. The thrust acting per unit area is called pressure. All fluids have weight, so they exert pressure on the base and the walls of the container in which they are enclosed. This pressure is transmitted undiminished in all directions. If an object is immersed in a fluid, it experiences an upward force called buoyant force. According to Archimedes Principle, this buoyant force is equal the weight of the fluid displaced by the object. The objects with density less than that of a liquid float and the objects with density greater than that of a liquid sink. Relative density, which is the ratio of density of a substance to the density of water, is a convenient way to compare the densities of different substances

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