CBSE – 9th Standard Science

Class 9 Chapter 9 – Force and laws of motion

  • The concept of force is based on push, hit or pull.
  • Pushing, hitting and pulling of objects are all ways of bringing objects in motion.
  • They move because we make a force act on them.
  • We also know that a force can change the shape and size of objects.
  • When an object is pulled from both the sides with equal forces this forces are called balanced forces and do not change the state of rest or of motion of an object.
  • If an unbalanced force is applied on the object, there will be a change either in its speed or in the direction of its motion.
  • Thus, to accelerate the motion of an object, an unbalanced force is required.


  • An object remains in a state of rest in a straight line unless compelled to change that state by an applied force.
  • In other words, all objects resist a change in their state of motion.
  • In a qualitative way, the tendency of undisturbed objects to stay at rest or to keep moving with the same velocity is called inertia.
  • This is why, the first law of motion is also known as the law of inertia.



  • Heavier or more massive objects offer larger inertia.
  • Quantitatively, the inertia of an object is measured by its mass.
  • Inertia is the natural tendency of an object to resist a change in its state of motion or of rest.
  • The mass of an object is a measure of its inertia.


  • In other words, there appears to exist some quantity of importance that combines the object’s mass and its velocity.
  • One such property called momentum was introduced by Newton.
  • The momentum, p of an object is defined as the product of its mass, m and velocity, v. That is, p = mv
  • Momentum has both direction and magnitude.
  • Its direction is the same as that of velocity, v.
  • The SI unit of momentum is kilogram-meter per second (kg m s-1).
  • The second law of motion states that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.
  • The unit of force is kg m s-2 or Newton, which has the symbol N.
  • The second law of motion gives us a method to measure the force acting on an object as a product of its mass and acceleration.





  • The third law of motion states that when one object exerts a force on another object, the second object instantaneously exerts a force back on the first.
  • These two forces are always equal in magnitude but opposite in direction.
  • These forces act on different objects and never on the same object.
  • In other words, there is a pair of forces and not just one force.
  • The two opposing forces are also known as action and reaction forces.
  • When a gun is fired, it exerts a forward force on the bullet.
  • The bullet exerts an equal and opposite reaction force on the gun. This results in the recoil of the gun.



  • The sum of momenta of the two objects before collision is equal to the sum of momenta after the collision provided there is no external unbalanced force acting on them.
  • This statement can alternatively be given as the total momentum of the two objects is unchanged or conserved by the collision.




  • All conservation laws such as conservation of momentum, energy, angular momentum,
  • charge etc. are considered to be fundamental laws in physics.
  • These are based on observations and experiments.It is important to remember that a conservation law cannot be proved.
  • It can be verified, or disproved, by experiments.
  • An experiment whose result is in conformity with the law verifies or substantiates the law; it does not prove the law.
  • On the other hand, a single experiment whose result goes against the law is enough to disprove it.