Types of mechanical collisions

Mechanical collisions can be classified into three different types depending on what happens to the kinetic energy before and after the collision.

Collisions are classified as perfectly elastic, partially elastic and inelastic.
In the study of collisions between two bodies, the concern is related to what happens to the kinetic energy and the momentum (linear momentum) immediately before and after the collision. The possible variations of these quantities classify the types of collisions.

system definition

A system is the set of bodies that are objects of study , so that any other body that is not being studied is considered an agent external to the system. The forces exerted between the bodies that make up the system are called internal forces, and those exerted on the bodies of the system by an external agent are called external forces.

Momentum and collisions

External forces are capable of generating a change in the momentum of the system as a whole. On the other hand, internal forces can only generate changes in the individual momentum of the bodies that make up the system . A collision takes into account only the internal forces existing between the objects that make up the system, therefore, the momentum will always be the same for any type of collision.

Kinetic energy and collisions

During a collision, the kinetic energy of each participating body can be fully conserved , partially conserved, or fully dissipated. Collisions are classified based on what happens to the kinetic energy of each body. The characteristics of the materials and the conditions of occurrence determine the type of collision that will occur.

Refund Coefficient

The coefficient of restitution (e) is defined as the ratio of the speeds just before and after the collision. They are called relative speeds of approach and departure of bodies.

types of collision

  • Perfectly elastic collision

In this type of collision, the kinetic energy of the participating bodies is totally conserved. Therefore, the relative speed of approach and departure of the bodies will be the same, which will make the coefficient of restitution equal to 1 , indicating that all energy was conserved . The perfectly elastic collision is an idealized situation, and its occurrence in everyday life is impossible, as there will always be a loss of energy.

  • partially elastic collision

When there is a partial loss of kinetic energy from the system, the collision is classified as partially elastic. In this way, the relative speed of departure will be slightly smaller than the relative speed of approach, causing the coefficient of restitution to assume values ​​between 0 and 1.

  • inelastic collision

When there is maximum loss of the kinetic energy of the system, the collision is classified as inelastic. After the occurrence of this type of collision, the participating objects remain stuck together and execute the movement as a single body . As after the collision there will be no separation between the objects, the relative speed of separation will be null, causing the coefficient of restitution to be zero.

The following table can help in memorizing the relationships between the different types of collisions :

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