Inertia is a concept in Physics that is defined as the tendency of an object to resist a change in its motion. In other words, Inertia acts as a resistance to any change in the velocity of any physical object.
What is Inertia?
Inertia is defined as the force which keeps stationary objects to stay still (remain at rest) or moving objects in motion at the same speed and in the same direction unless it is stopped by an external force. It can also be defined as the resistance of an object to resist change by remaining in its state of rest or in motion.
Law of Inertia and Newton’s First Law of Motion
The concept of inertia lies in Newton’s first law of motion, which is also termed as the “Law of Inertia.” According to this law-
An object will continue to be in the state of rest or in a state of motion unless an external force acts on it.”
Let’s look at an example to understand the concept of inertia in a better way.
Let’s say a person is pedaling a bike and suddenly hits the front brakes. In this case, his/her body continues to move, and they fly over the handlebars. Here, the force applied on the front brakes acts as an external force.
Types of Inertia
There are three types of Inertia namely:
A. Inertia of Rest
Inertia of Rest can be defined as the inability of an object or body to change its state of rest by itself. For example, when a car is suddenly started, the passengers in it fall backward. Ever wondered why that happens? Well, the lower part, which is in contact with the car, comes in motion, whereas the upper part remains in rest due to inertia of rest.
B. Inertia of Motion
Inertia of motion can be defined as the inability of an object or body to change its state of motion by itself. For example, if we reverse the first example, we will understand the inertia of motion. Let’s say that you are in a moving car, and when it suddenly stops, all the passengers fall forward. That’s because the lower part of the body, which is in contact with the car is at rest, whereas the upper part tends to remain in motion due to inertia of motion.
C. Inertia of Direction
Inertia of direction can be defined as the inability of any physical object to change its direction of motion by itself. For example, when a car moves along a round curve, the passengers sitting inside are thrown outwards. It happens due to the inertia of direction in order to maintain the direction of motion.
Examples of Inertia in Daily Life
(i). Satellites
A satellite is an object in space that circles or moves around a larger object. Do you know how it keeps moving around the bigger object without getting stopped? It’s due to the inertia of motion, which keeps it moving in a circular motion.
(ii). Falling of fruits and leaves
Have you ever wondered why the fruits or leaves fall down when the branches of a tree are shaken? It’s due to the inertia of rest. Before shaking, the branch is at rest, but when the tree is shaken, the branch starts moving, whereas the fruits and the leaves remain in their initial state of rest. Thus, they fall down.
(iii). Dusting a carpet
Do you know what makes the dust particles fall off the carpet? It’s all due to the magic of inertia. When you beat the carpet with a stick, the carpet starts moving but the dust particles in it remain in their initial state of inertia of rest. The repeated beating of the carpet causes the dust particles to drop, thus, giving you a clean carpet.
(iv). Falling forward while getting down from a moving bus
Have you ever tried getting down from a moving bus? Well, if you have then you must know the result, but did you ever wonder why you fall forward and not backward? This is again due to the inertia of motion. When a passenger gets down from a moving bus, their upper body is still in motion, and when it comes in contact with the ground which is at rest, their body topples forward due to the inertia of motion.
(v). The continued swirling of milk after the stirring is stopped
Ever noticed the continued swirling of milk even after you have stopped stirring a spoonful of sugar or cocoa in your milk? Magic, isn’t it? It would not be wrong to say that it is due to the magic of inertia of motion, which keeps the milk moving inside the glass.
(vi). Running of an athlete before taking a long jump
You must have seen athletes running before taking a long jump but have you ever pondered on why they do so? Well, they do it to bring themselves in the inertia of motion from the inertia of rest. It makes it easier for them to take a long jump. Isn’t it fascinating?
(vii). An object thrown outside a moving train
Have you ever noticed why an object, when thrown outside a moving train, moves in the opposite direction and with an equal velocity to that of the train? Well, it’s again due to the concept of the inertia of rest.
(viii). Driving along a curved road
While driving along a curved road, you must have felt like you are flying out of the car. It’s because of the inertia of direction, which tends to keep your motion in a straight line.
(ix). Falling of a coin into the glass
You must have experienced this trick of putting a coin into the glass without actually touching it. It happens because when you suddenly pull the cardboard on which the coin is placed, it falls into the glass as it tends to remain in the state of rest.
(x). Seatbelts
Well, car seatbelts are there to keep you safe during an accident. It acts as an external force to keep your body intact when your vehicle collides with any external object. In this case, the upper part of your body, which is in the state of motion, tries to move forward due to the inertia of motion, but the seatbelt comes into your rescue and cancels the effect of inertia.