## An Introduction to Inertial Force

Physics is technically analogous to the contributions of Sir Isaac Newton. He is the man who revolutionised classical physics with his laws of motion. He propounded three laws of motion, and the first of these is related to inertia. But first, let us first understand the meaning of inertia.

The term 'inertia' comes from the Latin word 'iners', which translates to lazy or idle. Johannes Kepler coined the term. The meaning of inertia is related to the fixed characteristic of an object made of matter. Inertia is a quality found in all things made of matter that have mass. An object made of matter keeps doing what it is doing until there is a force that changes its speed or direction. A ball on a table will not start rolling unless someone or something pushes it. It is noteworthy that if you toss a ball in a frictionless vacuum space, the ball will keep moving at the same speed and direction forever unless there is some action created by gravity or collision.

The measure of inertia is mass. Objects with a greater mass resist a change in their motion or rest more than objects with lower mass. For example, moving a truck will require more forceful pushes. On the contrary, moving a bike will require less aggressive impulses. This difference in force is because the truck and bike have different masses. A truck has more significant inertia than that of a motorcycle.

The law of inertia is also known as Newton's First Law, it forms the basis of physics, it postulates that if an object is at rest or moving at a constant speed in a straight line will keep remaining at rest or will keep moving at the same speed unless it is acted upon by a force. This object will keep moving and less force or friction causes it to come to rest. The Law of inertia is the first law of the three laws of motion. This law was first experimented by Galileo Galilei and was later deduced by René Descartes.

### Definition of Inertia

We can define inertia as the property of an object by which it cannot change its state of rest along a straight line on its own unless acted upon by an external force. Inertia increases with an increase in the mass of the body and vice-versa. We experience a jerk while suddenly using the brakes of a moving car because of inertia.

### The Law of Inertia

Sir Isaac Newton utilised and expanded the principles shown in Galileo's observations into his first law of motion. He gathered that it requires force for a moving ball to stop rolling once it is in motion. It takes force to change the ball's speed and direction. In Newton's Principia Mathematica, he defined the Law of Inertia as "the motion of bodies included in a given space are the same among themselves, whether that space is at rest or moves uniformly forwards in a straight line without circular motion."

Thus, Newton's First Law of Motion asserts that an object will continue to be in the state of rest or a state of motion unless an external force acts on it.

Newton's Second Law of Motion defines the relationship between acceleration, force, and mass.

Newton's Third Law of Motion states that an equal force acts back on the original object any time a force acts from one thing to another. This law means that every action has an equal and opposite reaction. If you pull on a rope, therefore, the rope is pulling back on you as well.

### Fictitious Force

A fictitious force acts on all masses whose motion we can describe using a non-inertial frame of reference. Fictitious force comes in effect when the frame of reference has started acceleration compared to a non-accelerating frame. This force arises when there is no physical interaction between two objects. But, instead, the acceleration of the non-inertial reference frame leads to the formation of fictitious force. On account of the arbitrary nature of a reference frame, the fictitious force can also be arbitrary. The leading fictitious forces are the Centrifugal force, Coriolis force and Euler force. Fictitious force is also known as Inertial force or Pseudo force.

We can understand the fictitious force with an example. If a person standing at a bus stop is watching an accelerating car, he infers that a force is exerted on the vehicle. Hence, there is no fictitious force in this scenario. But, if the person inside the moving car is looking at the person standing at the bus stop, he realises that person is accelerating with respect to the car, although no force is acting on it. Here, the concept of fictitious force is necessary to convert the non-inertial or still frame of reference to an equal inertial frame of reference.

### Types of Inertia

The inertia of Rest refers to the inability of a body to change its state by itself. For example, when we shake the branches of a tree, the leaves fall because the components they are attached to come into motion. On the other hand, the leaves tend to be at rest and hence, get detached.

The inertia of motion refers to the inability of a body to change its state of uniform motion by itself. For example, when a moving car suddenly stops, the person sitting in the car falls forward because the lower portion of the body contact with the vehicle comes to rest. In contrast, the upper part tends to remain in motion due to the inertia of motion.

The inertia of direction implies that the body cannot change its direction of movement by itself. For example, when a car takes a curve, the person sitting inside is thrown outwards to maintain his direction of motion. This phenomenon happens due to the inertia of direction.

### Formula of Inertia

We can understand the moment of inertia as a quantity that decides the amount of torque needed for a specific angular acceleration in a rotational axis. The moment of inertia is alternatively called angular mass, and its SI is kg.m2.

In General form, we can express the Moment of Inertia in the following way

I = m x r2

Here,

m = sum of the product of mass.

r = distance from the axis of rotation

I = Integral form

M1 L2 T0 gives the dimensional formula of the moment of inertia.

The moment of inertia is the calculation of the resistance of a body required to bring change in its rotational motion. It is constant for a particular rigid frame and a specific axis of rotation.

### Topics Related to Inertial Force

A few topics related to inertial force are as follows:

Aristotle’s fallacy

The law of inertia

Newton’s first law of motion

Newton’s second law of motion

Momentum

Impulse

Newton’s third law of motion

Conservation of momentum

Equilibrium of a particle

Common forces in mechanics

Friction

Rolling friction

Circular motion

Solving problems in mechanics

### Important Laws Concerning Inertial Force

Some of the important laws can be stated as follows:

Aristotelian Law of Motion- An external force is necessary to keep a body in motion.

Law of Inertia- An object is in a state of rest or of uniform motion in a straight line, it only moves when it is compelled by an outer force to act otherwise.

Newton’s First Law of Motion- If the net force of an object is zero, then the acceleration is zero. Acceleration can change only after there is a change in net force.

Newton’s Second Law of Motion - The rate of change of momentum of an object is directly proportional to the force applied and takes place in the direction in which the force acts.

Newton’s Third Law of Motion - To every action, there is always an equal and opposite reaction. Or, Forces always occur in pairs. Force on a body A by B is equal and opposite to the force on body B by A.

### Conclusion

The concept of inertia is one of the most critical topics in Physics. Understanding inertia may seem challenging. But, the topic becomes manageable with regular revision and thorough understanding. You can take the help of our study materials available on our website to get a firm grasp on such complicated subjects.

## FAQs on Inertial Force

**1. What are the types of inertia?**

There are three types of inertia – the inertia of rest, the inertia of direction and the inertia of motion.

**Inertia of rest -****Inertia of direction -**The body cannot change its direction of movement by itself.**Inertia of motion -**It is the inability of an object to change its state of uniform motion by itself.

**2. Give examples of the law of inertia.**

The inertia of motion facilitates the movement of a satellite around a larger object

The falling of fruits and leaves

The beating of the carpet causes the dust particles to drop

Continuous swirling of milk

Athletes running and preparing themselves for a long jump

An object thrown outside a vehicle

**3. How is Galileo's free-fall experiment related to the law of inertia?**

Prior to the renaissance, the most accepted form of the theory of motion was the Aristotelian theory. That states- external force is necessary to keep a body in motion. Galli Leo litre refined the theory of inertia and the Aristotelian theory was disregarded by various philosophers.

**4. What is the formula for inertia?**

Inertia is basically the quantity that decides how much torque is needed for angular acceleration in a rotational axis.

The moment of inertia is alternatively called angular mass, and its SI is kg.m2.

Generally, the moment of inertia can be expressed as follows:

I = m x r2

Here, m = sum of the product of mass.

r = distance from the axis of rotation

I= Integral form

M1 L2 T0 = the dimensional formula of the moment of inertia.

**5. State the law of inertia.**

The law of inertia states that - an object is in a state of rest or of uniform motion in a straight line, it only moves when it is compelled by an outer force to act otherwise.