Inertia is the property of a body that resists any change in its state of motion or rest.
- If an object is at rest, it tends to stay at rest.
- If an object is moving, it tends to keep moving in the same direction and speed unless an external force acts on it.
This idea comes from Newton’s First Law of Motion, often called the Law of Inertia.
Relation Between Mass and Inertia
Mass is the measure of inertia. Objects with greater mass possess greater inertia. For example, it is easy to stop a bicycle due to its small mass, but difficult to stop a moving bus or truck because of its large mass. Similarly, starting a heavier vehicle requires more force than starting a lighter one.
Law of Inertia
The law of inertia, also called the first law of motion is the fundamental law of physics that was proposed by the famous English Scientist Sir Issac Newton. The law of inertia states that,
"An object will continue to be in the state of rest or in a state of motion unless an external force acts on it."
Law of Inertia Examples
- A ball rolling on a frictionless table in a vacuum rolls for infinity until an external force is applied to it.
- A person standing on a moving bus tilts forward when the brakes of the bus are applied.
- Shaking the branches of the tree with fruits makes the fruits fall on the ground because they are in a state of rest, etc.
Types of Inertia
1. Inertia At Rest
A body at rest tends to remain at rest unless acted upon by an external force. This tendency to resist change from rest is called inertia of rest.
2. Inertia In Motion
A body in motion tends to continue moving with the same velocity (same speed and direction) unless acted upon by an external force such as friction or brakes.
3. Inertia Of Direction
A body moving in a particular direction tends to continue in the same direction unless an external force acts on it to change its direction.
Galileo’s Free Fall Experiment and Inertia
Before Galileo, Aristotle believed that a continuous force was required to keep an object in motion. Galileo disproved this idea through experiments and introduced the concept of inertia.
Since free fall was too fast to measure, Galileo used an inclined plane to slow down motion. By rolling a ball down a smooth slope and measuring time, he found that the distance travelled is proportional to the square of time:
\boxed {s \propto t^2}
Using two inclined planes, Galileo observed that a ball rolling down one plane climbs to nearly the same height on the other. When the second plane was made horizontal, the ball continued moving for a longer distance. Under ideal conditions without friction, it would move indefinitely with constant velocity.
From these observations, Galileo concluded that no external force is needed to maintain uniform motion; a force is only required to change the state of motion. This established inertia as an inherent property of matter and later formed the basis of Newton’s First Law of Motion.
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Examples of Inertia and Moment of Inertia
1. When sitting in a bus, brakes are applied, should people in the bus move forward, backward, or remain intact?
When brakes are applied on a bus in motion. People will move forward with a jerk. This is because people along with the bus are in motion, and when brakes are applied on the bus, people are still in motion due to the inertia in motion. Therefore, they still want to move forward and be in motion, and hence they move forward with a jerk.
2. Why a Carpet is beaten to remove dust?
The concept of inertia plays a role in the most minute things done in daily life. A carpet is beaten to remove dust because the carpet and the dust sticking to the carpet are both at rest, and when the carpet is beaten, the dust still remains at rest and falls down.
3. When sitting in a Train and the Train suddenly starts, why do people sitting in it move Backward?
The people initially were at rest along with the train and when force is applied on the train to move it forward, people in the train are still at rest, and therefore due to the inertia at rest, they want to be at rest and hence, they move backward.
4. There are two different objects of different Mass, one is 50kgs and the other is 30kgs. Which object will have more Inertia and why?
The inertia of an object depends on the mass it has, therefore the greater the mass, the more will be the inertia. Hence, the object having more mass will have more inertia. The object having a mass of 50kgs will have more inertia.
5. What is Moment of Inertia?
Moment of Inertia is a physical quantity analogous to the inertia with a difference that Moment of Inertia resist the rotational motion of an object. Moment of Inertia is also referred as Rotational Inertia.
6. What is Moment of Inertia Formula?
Moment of Inertia Formula or Equation is given as I = Σmiri2 where i = 1 to n and m is the mass of the particles and r is the distance of the particles from the rotational axis.