# Elastic or inelastic collision?

Yes, the typical assumption with "stick together" is a head-on collision, ignoring the possibility of a resulting rotation. But a literal reading of the problem statement in #1 above allows the possibility.

If the initial kinetic energy is equal to the final kinetic energy where two objects that collide stick together, this collision is elastic or inelastic?

If my grandmother has wheels she still isn't a school bus.
What collision scenario do you have in mind ?

• russ_watters and davenn
Summary: elastic or inelastic collision?

If the initial kinetic energy is equal to the final kinetic energy where two objects that collide stick together, this collision is elastic or inelastic?
Clearly you do not understand the basic definitions of elastic and inelastic collision. You could at least look those up before proceeding.

• davenn and BvU
An "elastic collision" is, by definition, one in which kinetic energy is conserved. Since, in an inelastic collision, we don't have the "conservation of kinetic energy" equation, we need another condition to solve for the speeds after the collision. Often that is given by requiring that the two objects stick together, but that is not necessary. We can have an inelastic collision in which the two objects do not stick together and we can have an elastic collision in which the two objects do stick together. Here, we are given that kinetic energy is conserved so this has to be an elastic collision.

HallsofIvy said:
An "elastic collision" is, by definition, one in which kinetic energy is conserved. Since, in an inelastic collision, we don't have the "conservation of kinetic energy" equation, we need another condition to solve for the speeds after the collision. Often that is given by requiring that the two objects stick together, but that is not necessary. We can have an inelastic collision in which the two objects do not stick together and we can have an elastic collision in which the two objects do stick together. Here, we are given that kinetic energy is conserved so this has to be an elastic collision.
I thought we weren't supposed to spoon-feed the answers. I had already asked him to look up the defintions so that he could figure that out for himself.

• davenn
Using conservation of momentum (which holds regardless if the collision is elastic or inelastic,unless there is net external force which is comparable in magnitude with the magnitude of the big forces that arise due to collision) we can prove that IF the two bodies stick together then the kinetic energy isn't conserved.

So your scenario is contradictory. Either the bodies don't stick together, or the kinetic energy isn't conserved, you can't have both. OR there are external forces in play.

• Delta2 said:
So your scenario is contradictory. Either the bodies don't stick together, or the kinetic energy isn't conserved, you can't have both. OR there are external forces in play.
Or the objects were spinning just right so that they could latch onto one another without dissipating any energy. They can then spin about one another, each retaining its original (and rotational) kinetic energy in the combined center-of-momentum frame.

jbriggs444 said:
Or the objects were spinning just right so that they could latch onto one another without dissipating any energy. They can then spin about one another, each retaining its original (and rotational) kinetic energy in the combined center-of-momentum frame.
The way I interpreted "stick together" is that they have exactly the same velocity after the collision. In the scenario you describe they don't have the same velocity after the collision (spinning about one another means they have opposite and equal velocities (in the best scenario) if I understand it properly)

Delta2 said:
The way I interpreted "stick together" is that they have exactly the same velocity after the collision. In the scenario you describe they don't have the same velocity after the collision (spinning about one another means they have opposite and equal velocities (in the best scenario) if I understand it properly)
Yes, the typical assumption with "stick together" is a head-on collision, ignoring the possibility of a resulting rotation. But a literal reading of the problem statement in #1 above allows the possibility.

## 1. What is an elastic collision?

An elastic collision is a type of collision between two objects where there is no loss of kinetic energy. This means that the total kinetic energy of the system before and after the collision remains the same.

## 2. What is an inelastic collision?

An inelastic collision is a type of collision between two objects where there is a loss of kinetic energy. This means that the total kinetic energy of the system before the collision is greater than the total kinetic energy after the collision.

## 3. How do you determine if a collision is elastic or inelastic?

To determine if a collision is elastic or inelastic, you can calculate the coefficient of restitution (e) using the formula e = (relative velocity of separation)/(relative velocity of approach). If the value of e is equal to 1, the collision is elastic. If the value is less than 1, the collision is inelastic.

## 4. What are some real-life examples of elastic and inelastic collisions?

A real-life example of an elastic collision is when two billiard balls collide on a pool table. A real-life example of an inelastic collision is when a car collides with a wall, as some of the kinetic energy of the car is lost to deformation and heat.

## 5. Why is the concept of elastic and inelastic collisions important in science?

The concept of elastic and inelastic collisions is important in science because it helps us understand and predict the behavior of objects in motion. It is also crucial in fields such as engineering and physics, where the conservation of energy is a fundamental principle. Additionally, this concept is used in many practical applications, such as in designing safer car bumpers or creating more efficient pool balls.