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Newton's First Law of Motion

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First Law of Motion

“An object will remain at rest or in uniform motion in a straight line unless acted upon by an external force.”

Also called the Law of Inertia, an object will stay at rest or in motion indefinitely, unless an external or unbalanced force acts upon this object.
Inertia of an object is dependent upon the mass of an object. Greater the mass of an object greater is the inertia.

Eg: Water in a bottle continues moving forward even though the bottle has come to a halt.



Newton’s First law of motion

A body at rest tends to remain at rest and a body in uniform motion tends to remain in the state of uniform motion until & unless an external force is applied on it.

For example, a ball lying on the table at rest will remain at rest until an external force is applied on it.

Balanced & Unbalanced forces :-Balanced Forces:

  • Equal & Opposite forces

  • Do not cause any change in motion

Unbalanced Forces:

  • Unequal forces

  • Can be in the same or opposite direction

  • Causes a change in motion

For example, in Tug of war, If teams 1 & 2 apply equal forces in opposite directions, there would be no net force. This is Balanced force.
However, if Team 1 exerts more force than Team 2, then there would be a net movement towards Team 1 and Team 1 would win. This is unbalanced force.

Newton’s first law was for scenarios where net force = 0. The second law is for scenarios with net force not equal to 0. Momentum plays a crucial role in Second law.

  • Momentum is the product of mass of a body & its velocity

  • It is a Vector quantity

  • It is denoted by p = mv

For example, A ball of 1 kg moving with 10m/sec has a momentum 10kg m/sec.

Momentum of a system remains conserved. Therefore,

  • Greater force is required to set heavier bodies in motion


  • Greater force is required to stop bodies moving with higher velocities

    Greater the change in momentum in a given time, greater is the force that needs to be applied. In other words, greater the change in momentum vector, greater is the force applied.


  • Newton’s first law of motion
  • Force
  • Types of forces
  • Resultant Force
If you would like to contribute notes or other learning material, please submit them using the button below. | Aristotle and Galileo's law

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