# 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?

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

• russ_watters and davenn
phinds
Gold Member
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
HallsofIvy
Homework Helper
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.

phinds
Gold Member
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
Delta2
Homework Helper
Gold Member
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 cant have both. OR there are external forces in play.

• jbriggs444
Homework Helper
So your scenario is contradictory. Either the bodies don't stick together, or the kinetic energy isn't conserved, you cant 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.

Delta2
Homework Helper
Gold Member
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)

jbriggs444