Last updated on May 19th, 2023 at 10:48 am

In this post, we will briefly cover the topic of **inertia for class 9** students. We will define, classify, and present examples of each type of inertia. We will also see how mass relates to inertia, how Newton’s first law relates to inertia, the formula of inertia, and the unit of inertia.

**Definition of Inertia**

Inertia: The tendency of a body to remain in its state of rest or of uniform motion along a straight line is called inertia.

Inertia is that property of a body due to which it resists a change in its state of rest or of uniform motion.Newton’s first law recognizes that every body has some inertia.

It is due to inertia that an external, unbalanced force must be exerted on the body to change its state of rest or of uniform motion.

**Types of Inertia**

Inertia can be of 3 types: Inertia of rest, Inertia of motion, and Inertia of direction.

Let’s define and understand these with examples.

**Inertia of Rest**

The **inertia of Rest** is the reluctance of a body to change its state of rest.

Example: When a bus starts suddenly, the passengers of the bus are thrown backward. This happens because the body tends to stay at rest even after the vehicle has started moving. This is the **Inertia of Rest**.

**Inertia of Motion**

The **inertia of Motion** is a tendency of a moving body to continue its motion along a straight line.

Example: Our bicycle continues to move forward for some time even after we stop pedaling. This is due to the **Inertia of motion** of the bicycle.

**Inertia of Direction**

The **inertia of Direction** is the inability of a body to change its direction of motion along a straight line.

Example: When the car takes the sharp turn to the right, it changes its direction of motion but the person sitting in it tends to move in the original direction due to inertia and is pushed towards the left.

**Mass and Inertia**

The greater the inertia of a body, the greater will be the force required to bring a change in its state of rest or of uniform motion. The force depends on the inertia of the body. And, inertia depends on the mass of the body. A body of greater mass has larger inertia. Therefore,** mass is a measure of the inertia of a body.**

If a body has more mass, it has more inertia. That is, heavier objects have more inertia than lighter objects. Thus, all bodies do not offer the same resistance to change their state of rest or of uniform motion.

**Examples showing mass and Inertia are equivalent:**

1] A larger force is required to move a loaded truck from rest than an unloaded truck.

2] For example, if a body has a mass of 10 kg and another body has a mass of 20 kg, then the body having 20 kg mass will have more inertia. It is easier to move a body of mass 10 kg by pushing it (because of its small inertia) but it is much more difficult to move a body of mass 20 kg by pushing it (because of its very high inertia).

**Newton’s First Law and Inertia**

Newton’s first law recognizes that everybody has some inertia.

This First Law of Newton law states that everybody continues to be in its state of rest or the state of uniform

motion along a straight line unless it is acted upon by some external force. It gives the definition of force as well as inertia.

**Formula of Inertia**

Inertia i = mass m

=> i =m

The unit of inertia is Kg in SI.