Neha Kishor
Updated on

A collision is said to be done when the path of one body is changed by the influence of another body or when either two bodies physically collide against each other. Collisions involve forces which means there is  a change in velocity. Closing speed is the magnitude of the velocity difference just before impact. Momentum is conserved in all collisions whereas kinetic energy is not conserved in all collisions. What distinguishes different types of collisions is whether they also conserve kinetic energy. The line that is collinear to the common normal of the surfaces that are closest or in contact during impact is the line of impact. Newton’s coefficient of restitution as well as the  internal force of collision acts during impact along this line.

 Collisions can be classified into three types:

  1. perfectly elastic collision
  2. inelastic collision
  3. perfectly inelastic collision.

Collisions in which both momentum and kinetic energy are conserved are called elastic collisions whereas collision in which momentum is only conserved is called inelastic collision.Here kinetic energy is not conserved.

complete elastic

An inelastic collision is sometimes  called a plastic collision. The limiting case of inelastic collision in which the two bodies coalesce after impact is called a perfectly inelastic collision.

The degree to which a collision is inelastic or elastic is quantified by the coefficient of restitution, a value that generally ranges between zero and one. A perfectly inelastic collision has a coefficient of restitution of zero whereas a perfectly elastic collision has a coefficient of restitution of one.

There are two types of collisions between two bodies

 1)One-dimensional collisions or Head-on collisions  – Here the velocity of each body just before impact is along the line of impact

 2) Two-dimensional collisions  or  oblique collisions – Here the velocity of each body just before impact is not along the line of impact.

According to the coefficient of restitution, there are two special cases of collision.

  1. A collision in which there is no loss of kinetic energy in the collision is defined as a perfectly elastic collision. Any macroscopic collision between objects will convert some kinetic energy to internal energy and other forms of energy, hence no large-scale impacts are perfectly elastic. 
  2. Collision is one in which part of the kinetic energy is changed to some other form of energy in the collision is known as inelastic collision. Momentum is conserved in inelastic collisions, but one cannot track the kinetic energy through the collision since some of it is converted to other forms of energy. Here the coefficient of restitution is not one.

There is a phase where colliding bodies have the same velocity along the line of impact in any type of collision. Here the kinetic energy of bodies reduces to its minimum during this phase and may be called a maximum deformation phase for which momentarily the coefficient of restitution becomes one.

Collisions in ideal gases approach perfectly elastic collisions.Similarly  some large-scale interactions like the slingshot type gravitational interactions between satellites and planets are almost perfectly elastic.

Collisions between hard spheres may be nearly elastic hence it is useful to calculate the limiting case of an elastic collision. The assumption of conservation of kinetic energy  as well as the conservation of momentum makes possible the calculation of the final velocities in two-body collisions.


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Check your knowledge

There are two general types of collisions in physics: elastic and inelastic. An
inelastic collision occurs when two objects collide and do not bounce away from
each other. Momentum is conserved because the total momentum of both objects
before and after the collision is the same.

It’s because when an electric field operates on a Faraday cage (box made of copper), the charges with a cage rearrange themselves to directly counterbalance the field, and hence, the shield (protects) the internal of the cage from the external electric field.

Head-on collision- A collision is said to be a head-on collision when the colliding
objects move along a straight line joining their centres. Oblique Collision- When
the colliding objects do not move along the straight line joining their centres, the
collision is said to be an oblique collision.

An elastic collision is a collision in which there is no net loss in kinetic energy in the system as a result of the collision. Both momentum and kinetic energy are conserved quantities in elastic collisions.

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