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The law of inertia, also referred to as Newton's first law, is a physics postulate that states that if a body is at rest or travelling at a constant speed in a straight line, it will remain at rest or continue to move in a straight path at constant speed until acted upon by an external force.
- Galileo Galilei developed the law of inertia for horizontal motion on Earth.
- Before Galileo, it was assumed that all horizontal motion needed a direct cause.
- However, he concluded from his experiments that a body in motion would stay in motion until a force (such as friction) forced it to come to rest.
According to the law of inertia, Inertia is an intrinsic property of a body that has the inability to change its state of rest, uniform motion, or direction of motion on its own. Mass is a measure of inertia; the more the mass, the greater the inertia.
There are three types of inertia, they are
- Inertia of Rest: The inertia of rest states that if a body is at rest, it will remain at rest.
- Inertia of Motion: The inertia of motion states that if a body moves at a constant velocity, it will continue to move at that velocity.
- Inertia of Direction: The inertia of direction states that if a body moves in a certain direction, it will continue to move in that direction and cannot change its direction on its own.
Very Short Answers Questions [1 Mark Questions]
Ques. What is the law of inertia?
Ans. According to the law of inertia, Inertia is an inherent property of a body that has the inability to change its state of rest, uniform, or direction of motion by itself.
Mass is the measure of inertia i.e. more is the mass, more is the inertia.
Ques. Who developed the theory of inertia?
Ans. The theory of inertia was developed by Galileo Galilei. He proposed that a falling body gains the same velocity at equal time intervals. This means that the speed increases at a constant rate as it drops.
Ques. What are the main types of inertia?
Ans. The different types of inertia are
- Inertia of rest
- Inertia of motion
- Inertia of direction
Ques. Which Newton’s law is connected to the law of inertia?
Ans. Newton’s first law of motion is connected to the law of inertia. According to Newton’s first law, everybody continues in its state of rest or uniform motion in a perpendicular direction until and unless an external force is applied to change that state.
Ques. What is Aristotle’s fallacy?
Ans. Aristotle observed practical experiences and concluded that an external force is essential to keep a body in uniform motion. This is referred to as Aristotle's fallacy.
Short Answers Questions [2 Marks Questions]
Ques. Explain the Inertia of rest with an example.
Ans. The inertia of rest states that if a body is at rest, it will remain at rest.
When a bus starts to move suddenly, the person moves backwards due to the inertia of the rest of the passengers. This is because, due to their inertia, the passengers tend to stay in a condition of rest when the bus suddenly starts.
Ques. Explain the Inertia of motion with an example.
Ans. The inertia of motion states that if a body is moving with constant velocity, it will keep moving with that velocity.
When a running bus comes to a sudden stop the passengers are jerked forward because, due to inertia of motion, passengers tend to continue in motion even after the vehicle stops moving.
Ques. Explain the Inertia of direction with an example.
Ans. The inertia of direction states that if a body is moving in a particular direction, it will keep moving in that direction and cannot change its direction by itself.
If a bus takes a turn along a curved road, the passenger experiences a force acting away from the center of the curved road. This is due to the inertia of direction, which tends the passenger to move in a straight path.
Ques. What misled Aristotle to conclude that bodies would be in motion only if continuous force was exerted on them?
Ans. On the Earth's surface, inertia is usually masked by gravity and the effects of air resistance and friction, both of which tend to reduce the speed of moving objects (usually to a state of rest).
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Long Answers Questions [3 Marks Questions]
Ques. What is the difference between momentum and inertia?
Ans. The following are the differences between momentum and inertia
| Momentum | Inertia |
|---|---|
| Momentum is a measure of an object's motion and is calculated as the product of its mass and velocity. | Inertia is a quality of matter that resists changes in its state of motion, whether at rest or in a uniform motion. |
| Momentum is classified into two types: linear and angular. | There are three types of inertia: inertia of rest, inertia of motion, and inertia of direction. |
| Momentum is determined by mass and velocity. | Inertia is a measure of mass only. |
| If the net external force is zero, the total momentum of the body is conserved. | If the net external force is zero, a body at rest remains at rest and a body in motion moves at a constant velocity. This is the law of inertia. |
Ques. Why is it impossible to determine absolute motion?
Ans. Rest and motion are consistently related to one another.
- A body can never be in absolute rest or absolute motion.
- For example, if two people are travelling on a bus, there will be no change in their positions.
- The two persons seated are then at rest with one another.
- However, if someone is standing outside the bus, the two individuals inside are in motion in relation to the person standing outside.
- When we view trees and buildings, we say they are at rest, yet they are in motion.
- Because the earth is always in motion, everything on the planet is in motion as well.
- Since everything is relative to each other, there is no such thing as absolute motion.
Ques. What is the physical significance of inertia?
Ans. Inertia is the characteristic of matter that allows it to maintain its state of rest or motion in a straight line in the absence of an external force.
- Certain other forces, such as constant velocity, require inertia to be maintained.
- An orbiting body, for example, without its own propulsion, depends on inertia to maintain its constant velocity, which counteracts gravity.
- The fuel of the rocket would run out in a very short time.
- The inertia they generate during the ascent is what maintains them in orbit.
Very Long Answers Questions [5 Marks Questions]
Ques. How did Galileo measure the speed of the falling object?
Ans. To compensate for his technical disadvantage, Galileo attempted to slow down the speed of the body by replacing the descending body with a ball rolling down an inclined path.
- Galileo believed that because free-falling motion is basically comparable to a totally vertical ramp, a ball rolling along a ramp would accelerate in the same manner that a descending ball would.
- He determined the time it took the ball to go a certain distance down the slanted plane using a water clock.
- After several attempts, he discovered that the time it took the ball to roll the whole length of the ramp was similar to double the time it took the identical ball to roll a quarter of the length.
- He reasoned from the experiment that if a body is in motion, it moves with a uniform velocity if no external force acts on it.
Ques. What are the two quantitative measures of the inertia of an object?
Ans. There are two primary quantitative measurements of the inertia of an object: mass and moment of inertia.
- The resistance of mass to the application of a force on an object is controlled by mass.
- The moment of inertia along a certain axis determines the object's resistance to torque exertion along that axis.
- In general, inertia refers to a body's resistance to changes in speed or velocity.
- In simple terms, it is the opposition to a change in the state of motion (or, in some cases, the body's momentum).
- The law of inertia is one of the fundamental concepts of classical mechanics that is still used to explain the motion of objects and how they are influenced by applied forces.
Ques. Explain some common scenarios of inertia of rest.
Ans. Some common examples of inertia of rest are
- Passengers may feel as if they are traveling backward when a vehicle rapidly accelerates. As the vehicle moves forward, inertia causes the passengers to remain seated.
- Shock may experienced by the driver if a stationary vehicle collides with a moving vehicle from behind. This happens because, during the collision, the driver's body moves forward while the driver's head remains at rest due to inertia.
- When a balloon is in a car, it appears to move as the vehicle moves forward. In actuality, the balloon is attempting to maintain its previous position. The only thing moving is the vehicle.
- If a tablecloth is pulled rapidly from underneath the plates and utensils, it can be taken out. As long as the motion of the tablecloth isn't overpowering, plates and utensils will tend to stay stationary.
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