Open In App
Related Articles

Law of Inertia

Like Article
Save Article
Report issue

Law of Inertia is another name for the First Law of Motion given by Sir Isaac Newton. As Law of Inertia has been studied by various scholars, throughout the centuries, and it helped humanity to understand the various concepts of motion in a wide range of fields from aerospace to automobile design.

The origins of the Law of Inertia can be traced back to the ancient Greek philosophers such as Aristotle and Galileo, who first posited the idea of “natural motion.” But until Sir Isaac Newton redefined the Law of Inertia as the “first law of motion” in the 17th century, it was not widely used by science scholars. Their Law of Inertia is also called Newton’s First Law of Motion. In this article, we will dive deeper into the concept of the Law of Inertia and other related topics in good detail.

Inertia Definition

Inertia is a quality of a body that prevents it from changing its state. The inertia of translational motion is measured by a body’s mass.

A body of more mass has a harder time changing its condition of rest or uniform motion, and vice versa.

  • Mass of a body is a numerical or quantitative measure of its inertia.
  • Greater a body’s inertia, the greater its mass.

Newton’s first law of motion

What is Law of Inertia?

Newton’s first law says that a body at rest or in uniform motion will remain at rest or in uniform motion until and unless it is subjected to a net external force.

The image added below the image proves the Law of Inertia, where football does not move till force is applied to it which proves the Law of Inertia.

Law of Inertia Example


Galileo’s Free Fall Experiment

Galileo initially believed in the Aristotal way of motion and Aristotle believed that heavy objects fall faster than light ones because it is in their nature to seek their natural place more quickly. However, Galileo, a math teacher at the University of Pisa, became interested in the rates of falls and hypothesized that a body’s speed should be proportional to its density. 

He conducted an experiment, dropping different balls from a tower, likely the Leaning Tower of Pisa, and found that both balls reached nearly the same speed, except for a small difference caused by air resistance. This result surprised him and forced him to abandon Aristotelian ideas about motion. Galileo also reported that the lighter ball always started out a little faster than the heavy ball, which caught up later, which seems unlikely. Although the experiment’s details are not that reliable whereas if he really did the experiment, it was a crucial turning point in the history of science.

Experiment on Inclined Plane

Galileo’s Experiment on Inclined Planes was a significant contribution to the study of motion and mechanics. Galileo set up an inclined plane and rolled a ball down the slope, measuring the time it took to reach the bottom using the water clock. He repeated the experiment with the same ball, changing the angle of inclination, and found that the time taken to roll down the slope was proportional to the square root of the distance traveled.

Galileo’s findings contradicted the prevailing Aristotelian view that an object’s motion was determined by its weight and paved the way for the development of modern physics. The experiment also demonstrated the principle of inertia, which states that an object in motion will continue to move at a constant speed in a straight line unless acted upon by a force.

If an object is released from rest and gains speed at a steady rate (as it would be in free fall or when rolling down on an inclined plane), then the total distance travelled by the object is proportional to the time squared needed for that travel.

Types of Inertia

There are various examples of Newton’s First Law of Motion or Law of Intertia in everyday life, some of those examples are as follows:

Inertia of Rest

An object at rest tends to remain at rest unless an external force acts on it. Some examples of Inertia of Rest are:

  • The rider falls backward when a horse starts suddenly.
  • Mangoes fall from mango tree branches when we shake them.
  • Passengers on board a bus or train tend to fall back when the vehicle begins rapidly.
  • A coin is placed on cardboard, which is then placed over a tumbler such that the coin is above the tumbler’s mouth. The coin now falls into the tumbler if the cardboard is removed with a sharp movement.

Inertia of Motion

An object in motion tends to remain in motion unless an external force acts on it. Some examples of Inertia of Motion are:

  • A bowler runs the ball before throwing it, so the speed of the run is added to the ball’s speed at the moment of the throw.
  • Passengers on a bus or train lean forward when it comes to an abrupt halt.
  • An athlete runs a specific distance before attempting a long jump because the velocity gained while running is added to the athlete’s velocity at the moment of the leap, allowing him to jump further.
  • A ball tossed upward by a passenger onboard a moving train will fall according to the train’s speed.

Inertia of Direction

A body can’t change its motion direction on its own. Some examples of Inertia of Direction are:

  • The sparks created when a knife is rubbed against a grinding stone move in a tangential direction.
  • Mud is spat out by the vehicle’s rotating wheels, but mudguards installed over the wheels prevent the mud from spreading.

Law of Inertia Examples

Various examples that prove the Law of Inertia in real life are,

  • The movement of the body backward when the bus starts.
  • The sudden jerk experienced by us sitting in the car when the brake is applied.
  • When a straight-running automobile makes a quick turn, the occupant feels a force radiating outwards.
  • When a person leaps off a moving train, he or she may fall forward.
  • When a blanket is thrashed with a stick, the dust particles fall off.

Limitations of Law of Inertia

As Law of Inertia is very useful on one hand but there are many limitations of this law, as this law is given 3 and half centuries ago. Some of the limitations of this law are:

  • Only Applicable in Inertial Frames: The Law of Inertia is only applicable in inertial frames of reference, which are frames that are not accelerating. In other words, the Law of Inertia doesn’t hold for Non-Inertial Frames of Reference.
  • Limited to Non-Relativistic Speeds: The Law of Inertia is only valid for objects moving at non-relativistic speeds i.e., speed not closer to the speed of light. At very high speeds, close to the speed of light, the laws of physics change, and the concept of inertia is no longer applicable.
  • Neglects Friction and Other Forces: The Law of Inertia assumes that there are no external forces acting on an object, but in reality, there are always some forces acting on an object. Friction, air resistance, and other forces can significantly affect the motion of an object and are not taken into account in the Law of Inertia.
  • Applicability at Microscopic Level: As the size of the particle goes to the microscopic, atomic, or subatomic scale, then also the law of inertia fails to hold as there are various other forces that come into play that don’t have any effect on real-life objects.

Read More,

Sample Questions on Law of Inertia

Question 1: Why do Objects Slow Down?


The slowing down of items was thought to be an inherent characteristic of the things prior to Newton and Galileo. Friction and gravity were unknown forces at the time. The frictional force resists an object’s motion, causing it to lose energy and hence slow down.

When we watch a toy car traveling on a concrete surface, for example, the speed of the automobile is governed by the friction between the road and the vehicle wheels. The wheel will meet with little resistance when the toy automobile is driven over a smooth surface. This will create a frictional hurdle, allowing the automobile to accelerate across the entire plain tile surface.

This is in stark contrast to when a vehicle toy is driving on a slick, gravel-filled terrain. The idea of change in the condition of rest or motion is defined by Newton’s first law of motion.

Question 2: What are the effects of External Force on the Motion of the Object?


There are various effects of External Force on the Motion such as:

  • It has the potential to alter the direction of motion.
  • It has the potential to alter the body’s speed.
  • It has the ability to modify the direction as well as the speed of motion.
  • It has the ability to start or halt motion in a stationary body.
  • It has the potential to alter the size and/or form of the body.

Question 2: What is Newton’s first law of motion?


Newton’s first law of motion states that a body at rest will remain at rest, and a body in motion will remain in motion unless it is acted upon by an external force.

Question 3: What is motion?


The change in position of a body/object with regard to time is known as motion. Distance, displacement, speed, velocity, and acceleration are the characteristics used to characterize motion.

Question 4: What do you mean by force?


A force is defined as the push or pull that affects or tends to modify the condition of rest or uniform motion (constant velocity) of a body, according to Newton’s first law of motion.

Question 5: What is inertia?


A characteristic of matter that allows it to remain in its current condition of rest or uniform motion in a straight line until it is disrupted by an external force is called an inertia.

FAQs on Law of Inertia

Q1: Define Law of Inertia.


The law of inertia, also known as Newton’s first law of motion, states that an object at rest will remain at rest and an object in motion will continue in motion with a constant velocity, unless acted upon by a net external force.

Q2: Who discovered the Law of Inertia?


The law of inertia was first formulated by Sir Isaac Newton in his book “Philosophiæ Naturalis Principia Mathematica” (Mathematical Principles of Natural Philosophy) in 1687.

Q3: What is an Example of the Law of Inertia?


An example of the law of inertia is a ball resting on the ground. The ball will remain at rest unless a force is applied to it, such as someone picking it up or kicking it. Once the ball is in motion, it will continue moving in a straight line at a constant speed unless acted upon by another force, such as friction or gravity.

Q4: How does the Law of Inertia Apply on Driving a Car?


The law of inertia applies to driving a car in several ways. For example, when a car is stopped at a red light, it will remain at rest unless the driver applies a force to the gas pedal to accelerate. Similarly, when the car is in motion, it will continue to move forward at a constant speed unless the driver applies the brakes to slow down or stop.

Q5: What is the Difference Between Inertia and Mass?


Inertia is the tendency of an object to resist changes in its state of motion. Mass, on the other hand, is a measure of the amount of matter in an object. The more massive an object is, the greater its inertia, meaning it will be more difficult to move or stop.

Q6: What is an External Force?


An external force is one that originates from outside an item rather than one that originates from within it. The force of gravity exerted by Earth on the moon, for example, is an external force on the moon.

Last Updated : 04 Feb, 2024
Like Article
Save Article
Share your thoughts in the comments
Similar Reads