Law of Inertia - Definition, Types, Examples, FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 04:31 PM IST

The Principle of Moment of Inertia or the first law of motion, may also be referred to as Newton’s first law. It states that an object, once at rest or in straight uniform motion, will continue to do so unless some force comes in from outside to disrupt this status quo. This is a basic reason for understanding the concept of inertia – how do objects oppose any change in motion? Inertia is a phenomenon that is experienced in everyday situations, from the sudden gripping of the seat when a car stops suddenly to the movement of the solar system, which all exist even without the intercession of any motor force. This makes it one of the primary features in the field of physics.

Law of Inertia - Definition, Types, Examples, FAQs

Law of Inertia Definition?

A body, either in a state of rest or in a state of motion, always tries to resist change. So if an external force is applied to the body, the body will try to resist the force. This inbuilt ability of the body is equal to the mass of the body. It is called inertia. Hence, inertia can be defined as the resistance provided by the body to a change of state, directly proportional to its mass.

Example:

1. When you travel in a vehicle, if the driver presses the brake suddenly, you feel a jerk. This jerk is felt because your body tries to resist the sudden change.

2. Athletes run a certain distance before taking a long jump.

3. When the horse runs full and suddenly stops, the man falls forward.

4. When you move a mango tree, the mango falls from the tree.

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Types of Inertia

Inertia is the resistance of the body to any change in its velocity.

There are Three Types of Inertia Force

1. Inertia of Rest: The body's tendency to stay in a state of rest. The body usually stays at rest until external energy is used to move. Therefore, inertia of rest is the body's inability to move and remain in a state of rest.

Example: When we hit the carpet, it comes in motion, with dust particles in the resting state. This is because dust particles tend to remain restless, which is why they are separated.

2. Inertia of Direction: The tendency of the body to sit on one side. The body usually stays on the same side until the external force acts on it to make a change in its movement.

Examples

1. A bicycle usually travels in a straight line unless it is turning the handle of the bike.

2. When you turn one side of the rope tied to the stone. Suddenly the rope breaks and the stone flies along the tangent to a circle.

As soon as the rope breaks, the pull disappears and the stone flies away in a lovely way.

3. Inertia of motion: The inclination of the body to remain in the same state of motion. A body continues to move in the same direction unless acted by an external force.

examples

Here are a few real-life examples of defining inertia of movement:

1. When the train suddenly stopped, it collapsed. With the inertia of movement, the upper part of the body contacts the seat and begins to move, while the lower part tries to sit still.

2. A man jumps off a train and falls ahead. This is because his feet touch the ground and they stay at rest while the rest of the body moves

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Solved Example Based on Inertia

Example 1:A man is standing in a moving bus. Now he is getting down, he falls forward because -

1) due to the inertia of the rest road is left behind and man reaches forward.

2) due to inertia of motion, the upper part of the body continues to be in motion in the forward direction while the feet come to rest as soon as they touch the road.

3) He leans forward as a matter of habit.

4) None of the above

Solution:

Inertia is the property of a body by which it continues to remain in its existing state of rest or uniform motion in a straight line unless an external force acts on it. So due to inertia of motion, the upper part of the body continues to be in motion in the forward direction while feet come to rest as soon as they touch the road.

Hence when he is getting down from the moving bus, he falls forward.

Hence, the answer is the option (2).

Example 2: A particle is in straight-line motion with uniform velocity. A force is not required:

1) To increase the speed

2) To decrease the speed

3) To keep the same speed

4) To change the direction.

Solution:

Inertia is the property of a body by which it continues to remain in its existing state of rest or uniform motion in a straight line unless an external force acts on it.

A body cannot change its state on its own. So to keep the same speed, a force is not required.

Hence, the answer is the option (3).

Example 3: Given below are two statements: one is labeled as Assertion A and the other is labeled as Reason R

Assertion A: An electric fan continues to rotate for some time after the current is switched off.

Reason R: The fan continues to rotate due to the inertia of motion.

In the light of the above statements, choose the most appropriate answer from the options given below:

1) A is not correct but R is correct

2) Both A and R are correct and R is the correct explanation of A

3) Both A and R are correct but R is NOT the correct explanation of A

4) A is correct but R is not correct

Solution:

Inertia is the property of mass due to which the object continues to move until any external force does not stop it. In the case of the rotation of the fan, if we switch it off then also it moves for some time as air resistance takes time to stop it, and due to the inertia of the fan it moves for some time.

Hence, the answer is the option(2).

Example 4: Why do passengers in a moving train suddenly feel a jerk in the forward direction when the train stops abruptly?

1) The upper body of the passenger continues to be in a state of motion while the lower part of the body which is in contact with the seat remains at rest.

2) The back of the seat pushes passengers forward

3) The inertia of rest stops the train and takes the passengers forward

4) None of these

Solution:

Inertia is the property of a body by which it continues to remain in its existing state of rest or uniform motion in a straight line unless an external force acts on it.

So the passengers in a moving train suddenly feel a jerk in the forward direction when the train stops abruptly because The upper body of the passengers continues to be in the state of motion while the lower part of the body which is in contact with the seat remains at rest.

Hence, the answer is the option (1).

Frequently Asked Questions (FAQs)

1. What is Physical Inertia?

Inertia is the natural tendency of the body to withstand any change in its condition. For example, while you are asleep, you are in a good mood. If someone tries to wake you up. He continues to fall asleep and tries to resist the person in waking up.

2. What Is the Importance of Inertia?

Inertia has a real app for our daily activities. Let's talk about them:

Understanding the unwillingness of any material object to make a change in its condition.

For example, on Mars' machines, we just needed fuel to escape the rocket from gravity, and then we slowed down when we reached Mars.

Inertia played a major role here, carrying a rocket of about 54.6 million Km from the earth to Mars without fuel.

3. Is Inertia Power?

In the ideal case, the object will not have inertia in an area without gravity.

Inertia, therefore, is a natural tendency for the physical object to withstand movement, and ultimately brings the body into a state of relaxation.

4. State and explain the law of inertia.

The law of Inertia states that the body in a state of rest or movement similar to sitting remains in the same state until and again without the external force acting on it.

5. Describe inertia.

Inertia is defined as the property of the body where it remains in a state of relaxation or in the same motion in the same straight line unless something is done by an external force.

6. Describe the types of inertia

The following are three types of inertia

1. Inertia of Rest: When resistance is provided by the body to remain in a state of rest unless the external force is working through it.

2. Inertia of Direction: When resistance is provided by the body to continue to move in the same direction without the external force acting on it.

3. Inertia of Motion: When resistance is provided by the body to continue to be in the same direction without the external force acting on it.

7. What is the significance of inertia?

Inertia is the force that holds the universe together. Really. Without it, things would be different. And that's what keeps us trapped in destructive habits, and resisting change.

8. What is the law of inertia class 11?

The law of inertia, also called Newton's first law, writes in physics that, if the body is at rest or moving at a steady pace in a straight line, it will always tend to rest or continue to move in a straight line and at a constant speed unless it is made by external force.

9. How does the Law of Inertia relate to everyday life?

The Law of Inertia is observed in many everyday situations. For example, when a car suddenly stops, passengers tend to move forward due to their inertia. Similarly, when you're standing in a bus that starts moving, you feel a backward push because your body wants to remain at rest. These examples demonstrate how objects resist changes in their motion.

10. Why do we feel a "push" when an elevator starts moving up?

When an elevator starts moving upward, we feel a "push" or increased pressure on our feet due to inertia. Our body wants to remain at rest (inertia of rest), while the elevator floor pushes up against our feet, causing us to accelerate upward with the elevator.

11. Why is it harder to start pushing a heavy shopping cart than to keep it moving?

This phenomenon is directly related to the Law of Inertia. Initially, the cart is at rest, and its inertia of rest must be overcome to start moving. This requires more force. Once the cart is moving, it has inertia of motion, and you only need to apply enough force to overcome friction and maintain its motion, which is typically less than the initial force needed.

12. How does the Law of Inertia explain the importance of seatbelts?

Seatbelts are crucial because of the Law of Inertia. In a collision, a car comes to a sudden stop, but passengers tend to continue moving at the same speed due to their inertia. Seatbelts prevent passengers from continuing their forward motion, potentially saving them from serious injury or ejection from the vehicle.

13. How does friction relate to the Law of Inertia?

Friction is a force that opposes motion and can overcome inertia. While the Law of Inertia states that objects in motion tend to stay in motion, friction is often the force that eventually brings moving objects to a stop. Without friction, objects in motion would continue moving indefinitely, as described by the Law of Inertia.

14. How does inertia affect car safety design?

Inertia is a crucial consideration in car safety design. Features like crumple zones, airbags, and seatbelts are all designed to manage the effects of inertia during a collision. These safety measures help to gradually decelerate passengers, reducing the force they experience and minimizing injuries caused by sudden stops.

15. What are the two types of inertia?

The two main types of inertia are:

16. Can inertia be overcome?

Yes, inertia can be overcome by applying an unbalanced force to an object. The force must be sufficient to change the object's state of motion. For example, pushing a stationary car requires overcoming its inertia of rest, while applying brakes to a moving car overcomes its inertia of motion.

17. How does the shape of an object affect its inertia?

The shape of an object doesn't directly affect its inertia. Inertia is primarily determined by an object's mass, not its shape. However, shape can indirectly influence inertia by affecting how the object interacts with forces like air resistance, which can impact its motion.

18. How does inertia affect the motion of planets?

Inertia plays a crucial role in planetary motion. It's the reason why planets continue their orbital paths around the sun. The inertia of motion keeps them moving forward, while the sun's gravitational pull constantly changes their direction, resulting in their elliptical orbits.

19. Why do figure skaters spin faster when they pull their arms in?

This phenomenon, known as the conservation of angular momentum, is related to rotational inertia. When skaters pull their arms in, they decrease their moment of inertia (rotational inertia). To conserve angular momentum, their angular velocity must increase, causing them to spin faster. This demonstrates how changing the distribution of mass affects rotational motion.

20. What is the Law of Inertia?

The Law of Inertia, also known as Newton's First Law of Motion, states that an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. This law describes the tendency of objects to resist changes in their state of motion.

21. What would happen to a ball thrown in space according to the Law of Inertia?

In the absence of external forces (like gravity or air resistance) in space, a ball thrown would continue moving in a straight line at a constant velocity indefinitely. This is a perfect illustration of the Law of Inertia, where an object in motion stays in motion unless acted upon by an external force.

22. What's the relationship between inertia and Newton's Third Law of Motion?

While the Law of Inertia (First Law) describes an object's resistance to changes in motion, the Third Law states that for every action, there's an equal and opposite reaction. Inertia explains why objects resist changes in motion, while the Third Law describes the forces objects exert on each other during interactions. Together, they help explain many physical phenomena.

23. Why do objects in a car seem to "fall backwards" when the car accelerates forward?

This is due to inertia. When a car accelerates forward, objects inside the car tend to remain at rest relative to the Earth (inertia of rest). This makes it appear as if they're falling backwards relative to the car. In reality, they're maintaining their original state of motion while the car moves forward around them.

24. How does the concept of inertia apply to spacecraft navigation?

In space, where there's virtually no friction, inertia is crucial for spacecraft navigation. Once a spacecraft is set in motion, it will continue moving in that direction due to inertia. This allows for efficient travel over long distances. However, it also means that precise calculations are needed for course corrections, as the spacecraft will keep moving in its current direction unless acted upon by a force (like thrusters).

25. Is inertia a force?

No, inertia is not a force. It is a property of matter that describes an object's resistance to changes in its state of motion. Forces cause changes in motion, while inertia is the tendency to resist those changes.

26. Can an object have zero inertia?

No, an object cannot have zero inertia. All objects with mass have inertia. Even objects with very small mass, like subatomic particles, have inertia. The only theoretical entity with zero inertia would be something with no mass, such as a photon (a particle of light).

27. How does the Law of Inertia relate to Newton's Second Law of Motion?

The Law of Inertia (First Law) and the Second Law of Motion are closely related. The First Law describes what happens when there's no net force acting on an object, while the Second Law quantifies how an object's motion changes when a net force is applied. The Second Law (F = ma) shows that the acceleration of an object is inversely proportional to its mass, which is a measure of its inertia.

28. How is mass related to inertia?

Mass and inertia are directly related. Objects with greater mass have more inertia, meaning they are more resistant to changes in their motion. This is why it's harder to start or stop a heavy object compared to a lighter one.

29. What's the difference between inertia and momentum?

While both concepts relate to motion, inertia is an object's resistance to changes in motion, while momentum is the product of an object's mass and velocity. Inertia is a property of matter, whereas momentum is a measure of motion. An object can have inertia without momentum (if at rest), but it always has inertia, whether moving or not.

30. How does inertia affect the tides on Earth?

Inertia plays a role in tidal forces. As the Earth rotates, the ocean water tends to "bulge" outward due to its inertia. This inertial force, combined with the gravitational pull of the Moon and Sun, contributes to the complex pattern of Earth's tides.

31. What role does inertia play in the functioning of a gyroscope?

Inertia is fundamental to how gyroscopes work. The rapidly spinning wheel of a gyroscope resists changes to its axis of rotation due to rotational inertia. This property allows gyroscopes to maintain their orientation and is why they're used in navigation systems, stabilization devices, and even some children's toys.

32. How does inertia affect the movement of tectonic plates?

The inertia of tectonic plates plays a role in plate tectonics. Once in motion, the massive tectonic plates tend to continue their movement due to inertia. This, combined with other forces like mantle convection, contributes to the ongoing movement of continents and the formation of geological features over millions of years.

33. Why is it easier to balance on a moving bicycle than a stationary one?

This is related to rotational inertia. When a bicycle is moving, its wheels have rotational inertia that helps keep the bike upright. The faster the wheels spin, the more they resist falling over. This gyroscopic effect, combined with other factors like the caster effect of the front wheel, makes it easier to balance on a moving bicycle.

34. How does inertia affect blood flow in the human body?

Inertia affects blood flow, particularly in the arteries. When the heart pumps, it creates a pressure wave that moves blood through the arteries. Due to inertia, the blood tends to keep moving even between heartbeats. This helps maintain a more continuous flow of blood throughout the body.

35. What is the role of inertia in the design of maglev trains?

Maglev (magnetic levitation) trains take advantage of inertia for efficient high-speed travel. Once the train is set in motion, inertia helps it maintain its velocity with minimal additional energy input. The lack of wheel friction allows the train to capitalize on its inertia of motion, enabling very high speeds with relatively low energy consumption.

36. How does inertia relate to the concept of centrifugal force?

Centrifugal force is often described as an "apparent" or "fictitious" force that seems to push objects away from the center of a rotating system. In reality, this is the effect of inertia. Objects in circular motion constantly want to move in a straight line (due to inertia) but are constrained to a circular path by centripetal force. What feels like an outward "centrifugal force" is actually the object's inertia resisting the change in direction.

37. Why do trucks carrying liquids often have baffles inside their tanks?

Baffles in liquid transport tanks help manage the effects of inertia on the liquid cargo. Without baffles, the liquid would slosh around due to its inertia when the truck accelerates, brakes, or turns. This sloshing can make the truck difficult to control. Baffles divide the tank into smaller compartments, reducing the movement of the liquid and making the truck more stable and safer to drive.

38. How does inertia affect the formation of planetary rings?

Planetary rings, like those around Saturn, are influenced by inertia. The individual particles in the rings orbit the planet, and their inertia keeps them in motion. The balance between this inertial motion and the planet's gravity helps maintain the ring structure. Collisions between particles, influenced by their inertia, also play a role in shaping the rings over time.

39. What is the significance of inertia in the functioning of an accelerometer?

Accelerometers, used in devices like smartphones and airbag systems, rely on inertia to measure acceleration. They typically contain a small mass suspended by springs. When the device accelerates, the mass resists this change in motion due to its inertia. The displacement of the mass relative to the device's frame is measured and used to calculate the acceleration.

40. How does inertia affect the movement of air masses in the atmosphere?

Inertia plays a significant role in atmospheric dynamics. Large air masses, once set in motion, tend to continue moving due to inertia. This contributes to the persistence of weather patterns and the movement of air currents. The Coriolis effect, which influences the direction of air movement on a rotating Earth, is also a manifestation of inertia.

41. Why do figure skaters start spinning with their arms out and then pull them in?

This technique, related to the conservation of angular momentum, takes advantage of rotational inertia. By starting with arms extended, skaters increase their moment of inertia, allowing them to control their initial spin. When they pull their arms in, they decrease their moment of inertia. To conserve angular momentum, their angular velocity must increase, resulting in a faster spin.

42. How does inertia affect the design of roller coasters?

Inertia is a key consideration in roller coaster design. The inertia of the cars and passengers is used to maintain motion through loops and turns. The initial climb and drop convert potential energy to kinetic energy, and then inertia (along with gravity) helps carry the train through the rest of the track. Designers must carefully calculate how inertia will affect the coaster's motion to ensure a thrilling yet safe ride.

43. What role does inertia play in the functioning of a pendulum clock?

In a pendulum clock, inertia is crucial to maintaining consistent timekeeping. Once set in motion, the pendulum wants to keep swinging due to its inertia. The pendulum's period is determined by its length and gravity, not the amplitude of its swing, thanks in part to this inertial property. This allows the clock to keep time accurately even as the pendulum's swing gradually decreases due to friction.

44. How does inertia affect the movement of ocean currents?

Ocean currents, once established, tend to persist due to the inertia of the massive volumes of water involved. This inertia contributes to the stability of major ocean currents like the Gulf Stream. However, it also means that changes to these currents (due to factors like climate change) can take a long time to manifest and can have long-lasting effects.

45. Why do objects appear to float in free fall?

In free fall, objects appear to float because everything is falling at the same rate due to gravity. The apparent weightlessness is a result of inertia - all objects are moving with the same acceleration, so there's no relative motion between them. This is why astronauts appear to float in the International Space Station, which is actually in constant free fall around the Earth.

46. How does inertia affect the design of vehicle suspensions?

Vehicle suspensions are designed with inertia in mind. When a vehicle hits a bump, the inertia of the vehicle body wants to keep it moving in its original direction. The suspension system works to absorb this motion, providing a smoother ride. Additionally, the inertia of unsprung mass (wheels, axles) affects ride quality and handling, which is why reducing unsprung mass is often a goal in vehicle design.

47. What is the relationship between inertia and the concept of escape velocity?

Escape velocity is the minimum speed an object needs to escape a planet's gravitational field without further propulsion. This concept is related to inertia because once an object reaches escape velocity, its inertia (tendency to continue moving) will be sufficient to overcome the planet's gravitational pull. The greater the mass (and thus gravity) of a planet, the higher the escape velocity needed to overcome its gravitational inertia.

48. How does inertia affect the movement of glaciers?

Glaciers, despite their massive size, move slowly downslope due to gravity. Once in motion, the inertia of the ice mass contributes to its continued movement. This inertia, combined with factors like the weight of the ice and the slope of the land, determines the glacier's flow rate. The inertia of glaciers also means that changes in their movement (due to climate changes, for example) can take a long time to manifest.

49. Why do some objects, like a raw egg, spin differently than a hard-boiled egg?

This difference is due to the internal structure and how it affects the object's moment of inertia. A raw egg's liquid interior can slosh around, changing its distribution of mass and thus its moment of inertia as it spins. A hard-boiled egg, with its solid interior, has a fixed distribution of mass and a constant moment of inertia, allowing it to spin more uniformly and for a longer time.

50. How does inertia affect the formation and behavior of tornadoes?

Inertia plays a role in tornado dynamics. The rotating air in a tornado has angular momentum due to its rotational inertia. As the tornado's diameter decreases (often due to rising air in the center), its rotational speed increases to conserve angular momentum - similar to a figure skater pulling in their arms. This inertial effect contributes to the intense wind speeds observed in tor