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