Law of Inertia (original) (raw)

Last Updated : 23 Jul, 2025

Isaac Newton's first law of motion, also called the _Law of Inertia, is one of the most important ideas in physics. But before we talk about the law, let’s first understand _inertia. Inertia is just a fancy word for the idea that things don’t like to change their state. If something is sitting still, it wants to stay still. If it’s already moving, it wants to keep moving at the same speed and in the same direction. It only stops or changes if something else pushes or pulls on it.

So, the Law of Inertia says that objects will stay at rest or keep moving in a straight line unless an outside force, like a push or a pull, acts on them. This is why when you are in a car that suddenly stops, you feel like you’re being pushed forward. Your body wants to keep moving even though the car has stopped!

This law of Inertia is also called **Newton's First Law of Motion. Here, we will dive deeper into the concept of the law of Inertia and other related topics in good detail. Before discussing the law of inertia, let us know about the definition of inertia.

Inertia definition in physics

Inertia is the property of an object that causes it to resist any changes to its motion. This means that an object will remain stationary if it's not moving or keep moving at the same speed and in the same direction unless something else interferes with it.

A body of greater 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.
• The greater a body's inertia, the greater its mass.

**Read More, Mass and Inertia

Inertia Work On Object

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 football remains stationary until a force is applied, which demonstrates the Law of Inertia, as shown in the image below:

Galileo’s Free Fall Experiment

• Galileo initially believed in the Aristotelian view of motion, which stated that heavy objects fall faster than light ones because it is their nature to seek their natural place more quickly.
• As a math teacher at the University of Pisa, Galileo became interested in the rates of falls and proposed that a body's speed should be proportional to its density.
• He conducted an experiment, likely at the Leaning Tower of Pisa, dropping different balls to test his hypothesis.
• Galileo observed that both balls reached nearly the same speed, with only a small difference due to air resistance, which contradicted Aristotelian ideas.
• He also noted that the lighter ball initially started a little faster than the heavy ball, which caught up later, challenging common beliefs.
• Though details of the experiment are not entirely reliable, if conducted, it marked a significant turning point in the history of science, leading Galileo to reject Aristotelian notions of motion.

Experiment on Inclined Plane

• Galileo's Experiment on Inclined Planes was a significant contribution to the study of motion and mechanics.
• He set up an inclined plane and rolled a ball down the slope, measuring the time with a water clock.
• Galileo repeated the experiment with the same ball, altering the angle of inclination.
• He discovered that the time taken to roll down the slope was proportional to the square root of the distance traveled.
• These findings contradicted the prevailing Aristotelian view on motion and weight.
• Galileo's experiment paved the way for modern physics by challenging established beliefs.
• It also demonstrated the principle of inertia, showing that an object in motion will continue moving at a constant speed 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.

Mathematically, this is expressed as,

**s∝ t 2

Types of Inertia

There is 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.

Inertia with 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.

Practice Questions

**Question 1: Why do Objects Slow Down?

Objects slow down because of forces like friction or air resistance that oppose their motion. These forces gradually reduce the object's speed until it eventually stops, unless another force keeps it moving.

**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 3: What is the Relationship Between Inertia and Mass?

The more mass an object has, the greater its inertia. This means that heavier objects are harder to start or stop moving because they resist changes in their motion more than lighter objects.

**Question 4: How Does Gravity Affect the Motion of an Object?

Gravity is an external force that pulls objects toward the Earth. It affects an object's motion by causing it to fall toward the ground when there is no other force acting on it, such as air resistance or support from a surface.

**Related Reads,

• Forces
• Exploring Forces
• Interaction of forces
• What is frictional force?
• Type of friction.
• Laws of friction.
• Static and Kinetic Friction
• Moment of Inertia
• Newton's laws of motion
• Applications of Newton's first law
• How to calculate moment of inertia?
• Rotational Inertia Formula
• Acceleration
• Motion in Physics