Collisions in physics. Elastic and inelastic collisions

09/04/2026

The online collision simulations on this page show you in a practical way what happens when two or more objects collide with each other. We will discover what elastic and inelastic collisions are, what physical quantities are involved, and the result of each one depending on the type of collision.

What are collisions in physics

Collisions in physics are events in which two or more objects collide with each other, changing their speed and direction. Collisions are governed by the laws of conservation of linear momentum (quantity of motion) and energy. There are two fundamental types of collisions: elastic and inelastic.

Elastic collisions

An elastic collision is a collision in which there is no loss of kinetic energy in the system. Both the linear momentum and the kinetic energy remain constant. Elastic collisions occur when objects collide and bounce off each other without any change in their shapes. Collisions of billiard balls or collisions between subatomic particles are good examples of elastic collisions.

Inelastic collisions

An inelastic collision is a type of collision in which the kinetic energy is not conserved. In an inelastic collision the internal forces do work, so the kinetic energy of the system no longer remains constant. The main characteristic of this type of collision is that there is a dissipation of energy. Although the kinetic energy is not conserved, the total linear momentum of the system is conserved.

The importance of collisions in physics

The study of collisions has applications in various areas of physics and engineering, such as in automotive mechanics, particle dynamics in nuclear physics, the collision of subatomic particles in particle physics, and in the design and analysis of safety systems in automobiles and other devices.

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!

Collision simulations

Lab
Curling
Two balls

Collision Lab

Investigate a simple 1D collision and more complex 2D collisions. Experiment with the number of balls, their masses and their initial conditions. Vary the elasticity and observe how the total momentum and kinetic energy change during collisions.

Curling stones

Mass ratio

Observe in this simulation what happens when two balls collide in three different situations. Change the mass ratio and check the result.

Inelastic
Elastic
Explosion

Inelastic collision

In this simulation, the two balls remain together after the collision. You can change the mass and velocity of the balls. See what happens to the energy and the amount of motion (momentum) before and after the collision.

Elastic collision

In this simulation, the two balls have an elastic collision. You can change the mass and velocity of the balls. See what happens to the energy and the amount of motion (momentum) before and after the collision.

Explosion

In this simulation, the two balls separate simulating an explosion. You can change the mass of the balls and the amount of energy supplied. See what happens to the energy and the amount of motion (momentum) before and after the explosion.

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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“Nothing takes place in the universe without mathematics expressing it”

Gaspard-Gustave de Coriolis

1792

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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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Test your knowledge

What is a collision in physics?

A collision in physics is an interaction between two or more bodies in which at least one of them is moving and, during a very short interval of time, there is an exchange of energy and momentum between them. This type of event is characterized by sudden changes in the velocities of the objects involved, because the forces acting during the contact are very large even if they act for only a brief moment. Collisions are fundamental for understanding key physical principles such as the conservation of linear momentum, which holds in isolated systems regardless of the type of collision, and they provide a framework for analyzing everything from microscopic interactions between particles to large-scale impacts such as those involving vehicles or moving objects.

What types of collisions exist and how are they different?

Collisions are mainly classified as elastic or inelastic depending on how energy behaves during the interaction; in an elastic collision, both kinetic energy and momentum are conserved, meaning that the objects do not undergo permanent deformation and continue moving after the impact, while in an inelastic collision, kinetic energy is not conserved because part of it is transformed into other forms such as heat, sound, or deformation. There is also the limiting case of a perfectly inelastic collision, in which the objects stick together after the impact and move as a single body, conserving only momentum, which clearly illustrates how energy can be redistributed in different ways depending on the nature of the collision.

Why do objects sometimes bounce after a collision and other times stick together?

This depends on how energy is transformed during the collision, because if most of the kinetic energy is preserved, the objects tend to separate after the impact and appear to bounce, while if a significant portion of that energy is converted into deformation, heat, or sound, the objects may stick together or continue moving as a single system. In general, the more “elastic” a collision is, the more likely it is that the objects will rebound, whereas the more “inelastic” it is, the more likely it is that they will remain together or lose part of their original motion, which explains why different materials and situations produce very different outcomes.

Why do problems always use momentum conservation but not always energy conservation?

This happens because linear momentum is conserved in all collisions as long as the system is isolated and external forces are negligible, making it a reliable principle for solving a wide range of problems. In contrast, kinetic energy is only conserved in elastic collisions, and in many real situations part of that energy is transformed into other forms such as heat, sound, or deformation, so it cannot always be used directly. For this reason, momentum conservation becomes the main tool for analyzing collisions, while energy considerations must be applied carefully depending on the specific type of interaction being studied.

Are real-life collisions more elastic or more inelastic?

In real life, most collisions are inelastic to some extent, because energy losses in the form of sound, heat, or deformation almost always occur, even if they are small and not easily noticeable. For example, collisions between billiard balls are often close to elastic, while a car crash is clearly inelastic due to the large deformations involved, so in practice most collisions lie somewhere between these two extremes, and understanding how much energy is conserved or lost is essential for accurately describing and predicting what happens in each case.