Inelastic Collision - Definition, Formula, Examples, FAQs

Inelastic Collision - Definition, Formula, Examples, FAQs

Vishal kumarUpdated on 02 Jul 2025, 04:49 PM IST

An inelastic collision refers to an event whereby two rigid bodies collide, and as a result, both of them lose a part of their kinetic energy – usually in the form of heat, sound or even changes to the deformation of the two colliding bodies. While these collisions cannot be referred to as elastic because kinetic energy before the collision and after the collision is not the same, momentum is still conserved. Inelastic collision examples can be observed in most daily activities: for instance, in a car crash, the vehicles deform inwards when they collide or when a ball of clay is thrown on the pavement and sticks. This is extremely important when discussing energy loss and how materials behave in practice.

Inelastic Collision -  Definition, Formula, Examples, FAQs
inelastic collision

What is Collision?

A collision is a unique event where two or more bodies interact by getting in contact with each other for a short period of time, whereby they apply force on each other causing a change in their motion (velocity) or shape. In physics, the latter is an important subject since colliding bodies allow students to study the transfer and conservation of momentum and energy.

Collisions are classified based on how energy and momentum are conserved. They are,

  • Elastic collision: Kinetic energy is conserved
  • Inelastic collision: Kinetic energy is not conserved
  • Completely inelastic collision: Kinetic energy is not conserved and the objects are attached after colliding.

Also, read

NEET Highest Scoring Chapters & Topics
This ebook serves as a valuable study guide for NEET exams, specifically designed to assist students in light of recent changes and the removal of certain topics from the NEET exam.
Download E-book

What is Inelastic Collision?

An inelastic collision is a collision in which there is a loss of kinetic energy. While the momentum of the system is conserved in an inelastic collision, kinetic energy is not. This is because some kinetic energy had been transferred to something else.

Inelastic collision examples:

  • Two vehicles crashed.
  • Clay ball when thrown against the wall.
  • A vehicle hit the compound.

What is a Perfectly Inelastic Collision?

A perfectly inelastic collision is a special case of inelastic collision. In this scenario, the two objects stick together after the collision and move as a single unit. For instance, consider a wet ball of mud thrown against a wall - the mud ball sticks to the wall, exemplifying a perfectly inelastic collision.

The Formula for Inelastic Collision

When two objects collide inelastically, they move with a final velocity that can be calculated using the following formula:

$
V=\frac{\left(M_1 v_1+M_2 v_2\right)}{\left(M_1+M_2\right)}
$
Where,
$\mathrm{V}=$ Final velocity
$\mathrm{M}_1=$ Mass of the first object in kgs
$\mathrm{M}_2=$ Mass of the second object in kgs
$\mathrm{V}_1=$ Initial velocity of the first object in $\mathrm{m} / \mathrm{s}$
$\mathrm{V}_2=$ Initial velocity of the second object in $\mathrm{m} / \mathrm{s}$

Inelastic Collision in Two Dimensions

In a two-dimensional inelastic collision, the conservation of momentum is applied separately along each axis. This is because momentum is a vector equation, and there is a separate conservation of momentum equation for each dimension. However, there is only one conservation of energy equation.

Kinetic Energy in Inelastic Collision

In inelastic collisions, kinetic energy isn't conserved. The lost energy is often due to internal friction, which can cause the atoms to vibrate (creating heat) and deform the bodies involved.

Loss of kinetic energy in an inelastic collision

Consider $m_1$ and $m_2$ to be the masses of the two colliding objects. The object with mass $m_1$ moves with velocity $v_1$ and the object with mass $m_2$ is at rest position.

After the collision, the momentum of the objects is conserved. But, the two objects stick to each other. Hence, the kinetic energy is not conserved.

Kinetic energy, $\mathrm{E}=\mathrm{mv}^2 / 2$
The kinetic energy of the object $m_1$ before the collision, $\mathrm{E}_{\mathrm{i}}=\mathrm{m}_1 \mathrm{v}_1{ }^{2 / 2}$
Kinetic energy after the collision is, $E_{\mathrm{f}}=\left(m_1+m_2\right) v_{\mathrm{f}}^2 / 2$
Therefore, the loss of kinetic energy is given by,

$
(E i-E f) / E i=1-\frac{m_1+m_2}{m_1}\left(\frac{v_f^2}{v_i^2}\right)
$

Elastic Collision

Any collision in which the collided objects get separated after the collision is known as an elastic collision. In the case of elastic collision, kinetic energy gets conserved. One must use both conservation of momentum and conservation of energy to find the motions of the objects later.

Difference Between Elastic Collision and Inelastic Collision

AspectElastic Collision

Inelastic Collision

DefinitionA collision in which both momentum and kinetic energy are conserved.

A collision in which momentum is conserved, but kinetic energy is not conserved.

Energy ConservationTotal kinetic energy remains constant before and after the collision.

Some kinetic energy is converted into other forms of energy (e.g., heat, sound).

DeformationNo permanent deformation occurs; objects return to their original shape.

Objects may deform permanently during the collision.

After CollisionObjects separate after the collision and move independently.

Objects may stick together or move separately depending on the situation.

ExampleCollision between two billiard balls.

A car crash or clay ball collision.

Mathematical RelationCoefficient of restitution is equal to 1.

Coefficient of restitution is less than

Frequently Asked Questions (FAQs)

Q: What is the relationship between inelastic collisions and the concept of plasticity in materials?
A:
Plasticity in materials is closely related to inelastic collisions at the atomic and molecular level. When a material undergoes plastic deformation, it experiences numerous inelastic collisions between its constituent particles.
Q: How do inelastic collisions affect the behavior of polymers and other complex materials?
A:
Inelastic collisions in polymers and complex materials lead to energy dissipation through molecular rearrangements, bond breaking, and heat generation. This behavior influences properties like viscoelasticity, impact resistance, and fatigue strength. Understanding these collisions is crucial for developing materials with specific mechanical and thermal properties.
Q: What is the importance of inelastic collisions in the design of crash test dummies?
A:
Inelastic collisions are crucial in the design of crash test dummies. These dummies are engineered to simulate how the human body responds to the inelastic collisions experienced during a vehicle crash. By accurately modeling these collisions, engineers can design and test safety features to minimize injury to vehicle occupants.
Q: How do inelastic collisions contribute to the heating of celestial bodies?
A:
Inelastic collisions contribute to the heating of celestial bodies in several ways. During planet formation, the kinetic energy of colliding bodies is partially converted to heat. In stars, inelastic collisions between particles in the core contribute to maintaining high temperatures. In planetary atmospheres, inelastic collisions between molecules can lead to heating through friction and energy dissipation.
Q: What is the role of inelastic collisions in the process of nuclear fission?
A:
In nuclear fission, inelastic collisions play a crucial role. When a neutron collides inelastically with a fissile nucleus, it can excite the nucleus to a higher energy state. This excitation can lead to the nucleus splitting, releasing more neutrons and energy, and potentially triggering a chain reaction.
Q: How do inelastic collisions affect the behavior of gases at the molecular level?
A:
At the molecular level, inelastic collisions between gas molecules can lead to energy transfer between translational, rotational, and vibrational modes. This energy redistribution affects the gas's thermodynamic properties, such as specific heat capacity and thermal conductivity, and plays a role in phenomena like sound propagation in gases.
Q: What is the significance of inelastic collisions in the field of nanotechnology?
A:
In nanotechnology, understanding inelastic collisions at the nanoscale is crucial for developing and manipulating nanomaterials. These collisions affect how nanoparticles interact, aggregate, and respond to external stimuli. This knowledge is essential for applications in drug delivery, nanoelectronics, and nanoscale energy transfer.
Q: How do inelastic collisions relate to the concept of mechanical energy?
A:
Inelastic collisions demonstrate that mechanical energy (the sum of kinetic and potential energy) is not always conserved in a system. While total energy is conserved, some mechanical energy is converted to non-mechanical forms like heat or sound. This conversion results in a decrease in the system's total mechanical energy.
Q: What is the importance of understanding inelastic collisions in forensic science?
A:
In forensic science, understanding inelastic collisions is crucial for accident reconstruction and crime scene analysis. It helps in determining the forces involved in impacts, the sequence of events in collisions, and the behavior of materials under sudden stresses. This knowledge aids in interpreting evidence and reconstructing incidents accurately.
Q: How do inelastic collisions affect the design of sports equipment?
A:
Inelastic collisions are considered in sports equipment design to control energy transfer and absorption. For example, in helmet design, the goal is to make the collision between the head and an obstacle as inelastic as possible to absorb impact energy. In contrast, in the design of golf club faces, a more elastic collision is desired to maximize energy transfer to the ball.