Science: Force and Motion: Newton՚s Third Law of Motion and Friction (For CBSE, ICSE, IAS, NET, NRA 2022)

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Newton՚s Third Law of Motion

According to this law, to every action there is an equal and opposite reaction. The action and reaction act on two different bodies if action and reaction are on same body, they will constitute a balanced force and body will not move.

There are three significant features of third law of motion:

  • We cannot say which force out of the two forces is the force of action and which one is the force of reaction. They are interchangeable.
  • Action and reaction always act on two different bodies.

The force of reaction appears so long as the force of action acts. Therefore, these two forces are simultaneous.

e. g. While jumping out of the boat, your foot exerts a backward force on the boat. This force is called action. At the same time a force is exerted by the boat on your foot, which makes you move forward. This force is known as reaction.

Newton՚s Third Law of Motion

Conservation of Momentum

According to this law, if two or more objects collide with each other, their total momentum remains conserved before and after the collision provided there is no external force acting on them.

If p1 = initial momentum and p2 = final momentum after time t, then

Now, if F = 0, then we have p1 = p2


  • The force that always opposes the motion of objects is called force of friction.
  • Friction exists between the surfaces of all materials which are in contact with each other.
  • The direction of the frictional force is always in a direction opposite to the motion.
  • The resistive force, before the body starts moving on a surface is called static friction.
  • Once a body starts moving on a surface the friction between them is called sliding or kinetic friction.
  • The sliding friction is slightly less than the static friction.

Factors Affecting Friction

  • Smooth glass surface offers less friction compared to a rough sand bed. Thus, smoothness of the surfaces is one of the factor on which friction depends.
  • Friction also depends upon the normal reaction.

Advantages and Disadvantages of Friction

Advantages of Friction

  • Friction enables us to walk freely.
  • It helps to support ladder against wall.
  • It becomes possible to transfer one form of energy to another.
  • Objects can be piled up without slipping.
  • Breaks of vehicles work due to friction.

Disadvantages of Friction

  • It always resists the motion, so extra energy is required to overcome it.
  • It causes wear and tear of machines.
  • It decreases the life expectancy of moving parts of vehicles.
  • Since friction is very useful in some cases while harmful in some cases, friction is called a necessary evil.

In most of the machines, to reduce friction ball bearings are used between the moving parts. By using the ball bearing the sliding friction is replaced by rolling friction. As the rolling friction is less than the sliding friction, therefore, the friction between the moving parts is reduced

Disadvantages of Friction


The force acting upon the surface of a body perpendicular to it is called thrust.

Pressure = thrust … (10.5) area

  • The SI unit of pressure is Nm – 2.
  • This unit has also been given a specific name pascal (Pa) in honor of the scientist named Blaise Pascal.
  • Force acting on a smaller area exerts a larger pressure and a smaller pressure on a larger area. This is the reason why cutting tools like knives and axes always have sharp edges.

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