Inertia. Newton’s First Law

21/04/2026

The online inertia simulations on this page help us to better understand the important concept of inertia in physics and its most significant implications.

This Thematic Unit is part of our Physics collection

Equilibrium of Forces

Situation in which the sum of all forces acting on an object is zero, resulting in zero acceleration.

Friction

Force that opposes the relative motion between two surfaces in contact.

Inertia

Property of bodies to maintain their state of rest or uniform linear motion unless an external force acts upon them.

Inertial Mass

Quantitative measure of the resistance offered by a body to changing its state of motion or rest.

Net Force

Vector sum of all individual forces acting on a body at a given moment.

Newton’s First Law

Principle stating that an object will not change its state of motion unless a non-zero net force is applied to it.

State of Rest

Condition of a body whose velocity is zero relative to a given frame of reference.

The concept of inertia in physics

The concept of inertia in physics refers to a fundamental property of matter that consists of the tendency of objects to resist changes in their state of motion or rest. It is a central concept in physics and is related to Newton’s first law of motion, known as the law of inertia.

The concept of inertia. Newton’s first law or law of inertia

The law of inertia states that an object at rest tends to remain at rest, and an object in motion tends to maintain its velocity constant in a straight line unless an external force acts on it. This law is the basis of classical physics.

Inertia in physics. Applications in everyday life

Inertia manifests itself in various ways in everyday life. For example, when in an automobile and the driver brakes sharply, the occupants tend to continue moving forward because of their inertia. Similarly, when accelerating rapidly, the occupants are pushed backward because of their resistance to change from their resting state.

Inertia also explains why heavier objects require more force to move or stop than lighter objects. If a small object and a large object are pushed with the same force, the larger object will have a lower acceleration due to its greater inertia.

Equilibrium of Forces

Situation in which the sum of all forces acting on an object is zero, resulting in zero acceleration.

Friction

Force that opposes the relative motion between two surfaces in contact.

Inertia

Property of bodies to maintain their state of rest or uniform linear motion unless an external force acts upon them.

Inertial Mass

Quantitative measure of the resistance offered by a body to changing its state of motion or rest.

Net Force

Vector sum of all individual forces acting on a body at a given moment.

Newton’s First Law

Principle stating that an object will not change its state of motion unless a non-zero net force is applied to it.

State of Rest

Condition of a body whose velocity is zero relative to a given frame of reference.

Explore the exciting STEM world with our free, online, simulations and accompanying companion courses! With them you’ll be able to experience and learn hands-on. Take this opportunity to immerse yourself in virtual experiences while advancing your education – awaken your scientific curiosity and discover all that the STEM world has to offer!

Inertia simulations

Elevator
Spacecraft

Motion of an elevator

In this simulation we can see what happens to the compression balance readings as the elevator moves up and down. What does this have to do with inertia in physics?

Inertia in a spacecraft

In this animation we can see that in space, where there is no friction, a spacecraft does not come to a screeching halt when it shuts down its engine. Instead, it keeps moving by inertia. What would you do to slow it down?

Horizontal
Vertical
Ball
Water
Elevator

Acceleration and deceleration

In this animation we have two blocks on top of a cart on a frictionless surface. When the cart starts, the block on the left falls to the ground. When the cart stops, it is the block on the right that falls. Explain these results from the concept of inertia in physics.

Uniform motion

In this animation, a cart with a block on it is moving at a constant speed. Suddenly the block is thrown vertically, while the cart continues its motion. Can you explain what happens? What would happen if the mass of the block or the velocity of the cart were changed?

Suspended ball

In this simulation we have a truck carrying a ball suspended by a cable. See what happens when accelerations or decelerations are applied to the truck and when the truck moves with uniform velocity. How does it relate to the concept of inertia in physics?

Water tank

In this simulation we have a truck carrying a tank of water. See what happens to the surface of the water when accelerations or decelerations are applied to the truck and when the truck moves with uniform velocity.

Motion of an elevator

In this simulation we can see what happens to the compression balance readings as the elevator moves up and down. What does this have to do with inertia in physics?

Giants of science

“If I have seen further, it is by standing on the shoulders of giants”

Isaac Newton

Joseph-Louis Lagrange

1736

–

1813

Joseph-Louis Lagrange formulated the Lagrangian equations, key in classical mechanics to describe motion. He also founded variational calculus, applying it to algebra and mathematical physics

“As soon as a mathematical truth is established, it always serves as a starting point for new truth”

William Rowan Hamilton

1805

–

1865

William Rowan Hamilton developed Hamiltonian mechanics and quaternions, unifying geometry and mathematical physics

“Mathematics and physics meet in the harmony of the equations governing motion.”

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Giants of science

“If I have seen further, it is by standing on the shoulders of giants”

Isaac Newton

Joseph-Louis Lagrange

1736

–

1813

Joseph-Louis Lagrange formulated the Lagrangian equations, key in classical mechanics to describe motion. He also founded variational calculus, applying it to algebra and mathematical physics

“As soon as a mathematical truth is established, it always serves as a starting point for new truth”

Gaspard-Gustave de Coriolis

1792

–

1843

Gaspard-Gustave de Coriolis formulated the effect that bears his name, explaining the deflection of bodies on Earth’s rotation and providing key foundations for mechanics.

“Motion is not only trajectory, it is also invisible influence”

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Your path to becoming a giant of knowledge begins with these top free courses

Free mode

Mechanics, Part 2

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Mechanics, Part 1

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Test your knowledge

What is inertia?

Inertia is the fundamental property of objects that explains their tendency to maintain their state of rest or uniform straight-line motion unless acted upon by an external force; this means that an object does not change its speed or direction on its own, and it is precisely this characteristic that helps us understand why objects remain stable when stationary and continue moving steadily once in motion, making it a central concept in classical physics and the basis for the laws of motion, as well as helping to explain everyday phenomena such as why a glass on a table does not move until someone pushes it or why a car on the road does not stop suddenly without a force acting on it.

What does the inertia of an object depend on?

The inertia of an object mainly depends on its mass, so the greater the mass of an object, the more it resists any change in its motion, which means that a larger force is needed to accelerate, slow down, or change the direction of a heavy body; conversely, objects with smaller mass have less inertia and are more easily moved, which can be observed in everyday life when a pencil can be moved effortlessly while a piece of furniture requires considerable effort, clearly showing how mass determines a body’s resistance to changes in motion.

Why does the body move forward when a car brakes suddenly?

When a car brakes abruptly, passengers tend to move forward because of inertia, as their body continues moving at the speed it had before the car slowed down while the vehicle itself decelerates, and this resistance to motion change is why wearing a seatbelt is essential, as it applies the necessary force to safely stop the body and prevent it from continuing to move, providing a clear, everyday example of how inertia affects objects and people even when it is not directly visible in other situations.

Why is it harder to move a heavy object than a light one?

Moving a heavy object requires more effort because it has a larger mass and therefore higher inertia, which means it resists any change to its state of rest or motion; this difference is evident in daily life, for example, when pushing a stationary car versus a bicycle, as both objects require very different amounts of force to start moving, demonstrating how inertia determines the energy needed to alter an object’s movement depending on its mass and how this property is directly linked to the natural resistance objects present against any change.

So… if there are no forces, does an object move forever?

Yes, according to the principle of inertia, an object not subject to any net force will continue moving indefinitely at a constant speed in a straight line, without accelerating or slowing down, forming the basis of Newton’s first law; although it is difficult to observe this in practice because external forces such as friction or air resistance always act on objects, this principle helps us understand that persistent motion does not require continuous energy, but rather the absence of interaction maintains the object’s state, explaining everything from the motion of planets in space to the resistance we feel when pushing a heavy object on the ground.