Class 9 Science

Newton’s First Law of Motion : Explained


Newton’s first law ( law of inertia) :

  • The famous Newton’s laws of motion are basically always the starting point in learning mechanics (a branch of physics) but the observations of the first law were actually made by scientist Galileo Galilei; Newton just studied those observations and presented three laws of motion and are thus named as Newton’s laws of motion.


  • In the above pictures we observe a simple experiment in which glass is mounted by a sheet of paper.
  • We place a metal coin on that sheet of paper.
  • When we apply slight force to the sheet of paper the paper moves as expected but the surprising observation was that the coin instead of moving with paper simply falls perpendicular into glass with respect to its original position.
  • Why does this happen?
  • Galileo observed that when a marble rolls down an inclined plane it’s velocity increases.
  • One would expect the ball to keep moving horizontally forever trying to achieve the same height from which it was released but we don’t find this happening in our practical lives because of frictional or other forces acting on that marble.
  • Thus, after studying all these observations Newton formulated three laws of motion.
  • The first law states that :

An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force.


In simple words, all objects resist a change in their state of motion which means that than object at rest would prefer to stay at rest and an object in motion would prefer to stay in uniform motion unless acted upon by an external force.

  • The property that we observe here is called inertia which is defined as the natural tendency of an object to resist a change in its state of motion or of rest.

Some common day to day examples that we observe regarding this law are:

  1. When a car is moving with uniform velocity and suddenly brakes are applied then we experience a push in the forward direction i.e., the direction of motion of car. This is because our body in was in uniform motion and thus is resisting to change its state of motion. In such situations the seat belt worn by us exerts force on our bodies in opposite direction thus preventing our bodies to move forward in case of sudden brakes. This is also the reason why seat belts are compulsory.

  1. Now, suppose a train is at rest and suddenly it starts. The passengers in train will experience a jerk backwards due inertia.Therefore the train always first starts at very slow speed and then step by step increases it speed.
  2. You are sitting in a merry go round and it is moving with high speed. In such case you tend to experience a force pulling you outside. This is again due to inertia but instead of jerk you feel a constant force pulling you outside because merry go rounds are always circular and the direction of force changes at every point in a circle (reason is that the direction of force is given by a tangent in circle and the tangent changes at every point).
  • Another important point to note is that inertia depends on mass because the force required to stop a bicycle moving in uniform motion is definitely much lesser than force required to stop a moving train.

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