A body floats in a liquid if

  • The density of material of the body is less than or equal to the density of liquid.
  • When the density of material of the body is equal to density of liquid, the body floats fully submerged in liquid in neutral equilibrium.
  • When body float in neutral equilibrium, the weight of the body is equal to the weight of displaced liquid. The centre of gravity of the body and centre of gravity of the displaced liquid should be in one vertical line for this condition.


  • Density is defined as mass per unit volume.
  • Relative density is measured by hydrometer.
  • The density of sea water is more than that of normal water.
  • When a ship enters is a sea from a river, it raises a bit because the density of saline water (salt water) is higher.
  • The density of iron is more than that of water but less than that of mercury. So, a solid chunk of iron sinks in water but float in mercury.
  • If ice floating in water in a vessel melts, the level of water in the vessel does not change.

Surface Tension

  • It is the force (F) acting normally on unit length (l) of imaginary line drown on the surface of liquid i.e., T=F/L’ its unite is N/m.
  • The property of a liquid by virtue of which it tries to minimize its free surface area is called the surface tension
  • Surface tension decreases with rise in temperature and becomes zero at the critical temperature.
  • The surface tension of clean water is higher than that of a soap solution.
  • Formation of lead shots, spraying result in coldness, floatation of needle on water, dancing of camphor on water are based on surface tension
  • Rain drop form spherical shape due to surface tension.
  • When kerosene oil is sprinkled on water, its surface tension decreases, due to which the excess of mosquitoes floating on the surface of water die due to sinking.

Cohesive and Adhesive Forces

Force of attraction applied between molecules of same substance is called cohesive force while attractive force between molecules of different substances is called adhesive force


The phenomenon of rise or depression of liquids in a capillary tube is called capillarity.

  • A piece of blotting paper soaks ink because the pores of the blotting paper serve as capillary tubes.
  • The oil in the wick of a lap rises due to capillary action of threads in the wick.
  • The root hairs of plants draw water from the soil through capillary action.


Viscosity is the property of a fluid by virtue of which an internal frictional force acts between its layers, when it is in motion.

Bernoulli’s Theorem

When an incompressible ad non-viscous liquid (or gas) flows in streamlined motion from one place to another, then at every point of its path, the total energy per unit volume (pressure energy + kinetic energy + potential energy) is constant.

Venturimeter, Atomizer, filter pump, motion of aeroplane are based upon the Bernouli’s theorem.

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