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What is Inertia In Physics? Definition, Formula & Examples

The Inertia in physics refers to the property of an object to resist changes in its state of motion. More specifically, an object tends to stay at rest or in uniform motion unless acted upon by an external force. This concept is one of the fundamental principles described by Newton’s First Law of Motion.

What is Inertia In Physics

What is Inertia In Physics? Meaning And Definition

Inertia, in physics, is the property that bodies have of opposing a change in their state of rest or movement in which they are found. As such, inertia is the resistance offered by a body to alteration of its state at rest or movement.

Newton’s first law , also called the law of inertia or principle of inertia, states that an object will remain at rest or in uniform motion in a straight line, as long as its state is not altered by the action of an external force. Therefore, the greater the mass of the object, the greater the inertia, that is, the greater the resistance that the body offers to the alteration of its state.

In chemistry , inertness is the quality of some chemical substances of not reacting chemically in the presence of elements of other chemical species. An example of chemical inertness is the noble gases and molecular nitrogen, whose formula is N2.

On the other hand, in geometry, moment of inertia or second moment of area is a geometric property that is related to the addition of the products obtained by multiplying each element of mass by the square of its distance to the axis. The greater the distance between the mass and the center of rotation, the greater the moment of inertia.

Inertia Formula

I = L / {\omega}


I = Inertia

L= Angular Momentum

{\omega}Angular velocity

Rest Inertia Vs Motion Inertia

  1. Rest Inertia: An object at rest tends to stay at rest unless a force is applied to it. This property is often called rest inertia.
  2. Motion Inertia: An object in motion tends to stay in motion with the same speed and in the same direction unless a force is applied to change its motion. This property is often called motion inertia.

Inertia Examples

  1. Car Seat Belt: When a car comes to a sudden stop, the passengers tend to keep moving forward due to their inertia. Wearing a seat belt is essential to prevent the passengers from continuing in motion and colliding with the car’s interior.
  2. Sliding on Ice: If you try to slide a book or any object across a smooth surface, it may not come to a stop immediately. Its inertia keeps it moving for a certain distance even after you stop applying force.
  3. Opening and Closing Doors: When you push a door, it doesn’t immediately start moving at the same speed as your hand. The door resists the change in its state of rest (or slow motion) due to its inertia.


The inertia is crucial for explaining various phenomena in mechanics and is a foundational concept in classical physics. It plays a key role in Newton’s laws of motion and contributes to our understanding of how objects behave in response to external forces.

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