Science: Gravitation: Weight, Weightlessness, Buoyancy and Archimedes Principle (For CBSE, ICSE, IAS, NET, NRA 2022)

Get unlimited access to the best preparation resource for ICSE/Class-10 : get questions, notes, tests, video lectures and more- for all subjects of ICSE/Class-10.


The weight of an object is the force with which it is attracted towards the earth.

If Weight of an object is denoted by W, then

  • Its SI unit is the same as that of the force (NEWTON) .
  • This force (weight) acts vertically downwards.
  • It has both magnitude and direction.
  • The weight of an object is generally measured by a spring balance.
  • The weight of an object will be different on different parts of the earth as the value of g is different on different parts of the earth.


  • Weightlessness is a phenomena which refers to the absence of apparent weight of an object, so an object would possess no weight.
  • This generally occurs in the absence of gravitational forces.
  • The weight of a body is felt due to the attractive force of earth՚s gravity.
  • If this force of attraction of the earth is somehow cancelled, or nor present then the body experiences no force towards the center of the earth. This condition is called weightlessness.
  • e. g. Weightlessness is experienced by astronauts in the orbits around the earth because the gravitational force of attraction of the earth is cancelled by the force that appears because of the circular motion of the astronaut around the earth. Weightlessness is also observed during free fall that is if a lift allowed to fall; the people inside the lift will experience no weight.


  • Whenever an object is immersed in a liquid, the liquid exerts a buoyant force or up thrust in the opposite direction of the gravitational force. This is also called the Force of Buoyancy.
  • It depends upon the density of the fluid.
  • Therefore, an object is able to float in water when the gravitational force is less than the buoyant force.
  • Similarly, an object sinks into the water when the gravitational force is larger than the buoyant force.
Force of Buoyancy

Why Does an Object Sink or Float on Water?

  • An object can sink or float on water based on its density with respect to water. The density is defined as mass per unit volume.
  • Objects having a density less than water float in it.

For Example, Cork flows in water because its density is lower than that of water.

Objects that have a density higher than water sink in it.

For Example, Iron nail sinks in water because the density of iron is more than water՚s density.

  • Thus, we can conclude that buoyancy depends upon:
  • The density of the liquid
  • The volume of the object (as the volume of object increases, its density decreases and vice-versa)

Archimedes Principle

  • According to the Archimedes principle, whenever an object is immersed in a liquid (fully or partially) , the liquid exerts an upward force upon the object. The amount of that force is equivalent to the weight of the liquid displaced by the object.
  • This means that if the weight of an object is greater than the amount of liquid it displaces, the object will sink into the liquid. However, if the weight of an object is less than the amount of water it displaces, the object will sink.
  • Submarines have a tank called Buoyancy Tank. Whenever the submarine needs to be taken inside water the tank is filled which thus increases the weight of the submarine. Similarly, when the submarine is to appear above water the tank is emptied, and the weight of the submarine becomes lighter and it rises above the water.
  • Ships are heavier than water, but their unique shape gives them a large volume. Their volume is larger than their weight and hence the water displaced by a ship provides it with the right up thrust so that it can float on water.

Applications of Archimedes Principle

  • In evaluating relative density
  • In designing ships and submarines
  • In making lactometers and hydrometers

Developed by: