# What is the difference between elastic and inelastic collisions? - R

What is the difference between elastic and inelastic collisions?
Christian S.
2012
Introduction
What is the difference between elastic and inelastic collisions? Well, before we can answer that we need to know what elastic and inelastic collisions are. First of all, a collision is where two bodies make contact with another. This can only happen when at least one of the bodies is in motion to hit the other. Collisions are divided into two different kinds, Elastic and Inelastic. An inelastic collision is a collision that when the two bodies collide, the kinetic energy (The energy being used to move to the body) is lost. This means that since the energy that is causing the movement is lost, the movement is lost. An inelastic collision usually results with the bodies stopping, seeing as they no longer have the energy to move it. We will talk more about this later. An elastic collision is a collision where the kinetic energy from the colliding bodies is conserved through the collision. This means that the kinetic energy isn't lost, as if it were an inelastic collision. An elastic collision transfers the kinetic energy to the other body that is hit. If you have ever seen a Billiards ball get hit by the cue ball, you will understand. Unlike the inelastic collisions, elastic collision normally ends in the other body flying off, with the kinetic energy (And momentum) of the first object.
There are also other factors in the collisions as where the collision is perfect or imperfect, but that will be explained later. So from this first paragraph, hopefully you will get a certain understanding of elastic and inelastic collisions. So the difference between inelastic and elastic is what happens with the kinetic energy (or momentum) of the objects. In an elastic collision the energy is conserved, in an inelastic it is not, that is the basic difference, without it getting too confusing.

Things that contribute to Elastic / Inelastic collisions.

Both elastic and inelastic collisions owe their existence to kinetic energy. Kinetic energy is the reason that the objects are moving so they can collide. It’s also responsible for these collisions because in elastic collisions there needs to be kinetic energy that is conserved and in inelastic collisions the kinetic energy must be lost, and if there isn’t any in the first place there is none to be lost. Another thing that contributes is the size and mass of the objects that are colliding. There could be a different way that bodies collide due to the difference in mass from a collision that the bodies have the same mass. The same goes for size, if a small car crashed into a large truck, it will probably be an inelastic collision and the car gets crumpled. The speed and velocity of two colliding objects could affect the outcome as well. A collision with bodies that are moving faster or even one body that is moving faster and one not, could be very different from a collision with slower objects. The speed could also vary in means of friction, the same way a slower collision will be different from a faster one, a collision with less friction ( per say the objects were on ice ) could be different from one with much friction ( Maybe the objects were on something like sandpaper ). As you can see, there are many different variables that contribute to collisions, and although they will always end in an inelastic or elastic collision (Even if it was a collision with different sized objects or something else ) this all shows us how many things are between us and making a perfect collision. ( Head-count : Friction, size & mass, Momentum/speed.. etc. )

Examples of Collisions
Here are some examples that may help you understand elastic and inelastic collisions if you are confused.
Elastic and Inelastic collisions are explained through Isaac Newton's laws of motion, specifically pertaining to his First Law of Motion and his Second Law of Motion
Elastic
• o One ball is dropped and uses gravity to come down and hit the next ball in an elastic collision; this means the energy is conserved. In fact, it’s conserved through all the balls up to the last one which uses the conserved energy to fly up, then come down and do it again. But it will eventually stop because it’s not a perfect elastic collision and not all of the energy is conserved so eventually all the energy will be used up. A perfect elastic collision is only obtained at atom level.
- Billiards
• o The cue ball hits another Billiards ball when they collide, there is an elastic collision. The kinetic energy of the cue ball is transferred to the new billiards ball and the new ball rolls away with the same momentum of the cue ball. Since the energy is transferred, the cue ball now stops in place because it no longer has the energy to move.
- Space Ship
• o If a spaceship flies nearby a planet and the gravitation pull of the planet affects it by making it gain speed (and therefore kinetic energy) that is a collision, but the kinetic energy of the spaceship did not become lost by the collision, as it would if it were a head-on car crash.This is an elastic collision
Inelastic
- Car crash
• o When two cars of equivalent mass and speed collide head on, they crumple and are coupled. The kinetic energy is lost; we know that because the cars are not in motion any more. But we know that in inelastic collisions, when kinetic energy is “lost”, it means mostly all of the kinetic energy was turned into heat and used to crumple the metal of the cars. This would be a perfect inelastic collision.
- Freight Cars
• o Think of two freight cars. One freight car rams into the back of the other, Instead of stopping like a perfect inelastic collision, and instead of transferring the momentum of the first freight car to the second (like an elastic collision), the first car pushes the second. This would be an imperfect inelastic because not all the Kinetic energy is conserved, but not all is lost. The freight car still has kinetic energy to move after it has collided with the other freight car.

Perfect Elastic / Inelastic VS Imperfect.

Now before we get too complicated, understand that there can be perfect collisions. This is where the collision is perfect; it does exactly what it’s supposed to do perfectly. It
 Figure 1, Perfect elastic and Inelastic collisions

would mean it loses EXACTLY all of its kinetic energy, or conserves EXACTLY all of its kinetic energy. Now I will explain the difference of the two, imperfect and perfect. A perfect elastic collision is a collision (Only obtained at atom level) that conserves exactly the amount of energy of the colliding object, without losing any. An imperfect collision is a collision that conserves almost all of the energy of the colliding object. Imagine a newton’s cradle. The collisions are imperfect because every time the balls collide they lose energy, and eventually will stop. If a newton’s cradle was capable of a perfect elastic collision, it wouldn’t lose any kinetic energy and therefore keep going back and forth, but there are certain factors, like air resistance, that would stop it. But that is irrelevant to the example.
A perfect Inelastic collision is collision where all the energy of the two colliding objects is lost from the bodies. And of course by lost we mean turned into heat and sound and some used to deform the bodies. An Imperfect inelastic collision is where not all of the energy is lost; maybe some is used to push the other body after the collision like in the freight car example.

The “Loss” of energy in an inelastic collision
When there is an inelastic collision between two bodies, we saw the energy that caused the movement of the bodies is lost. We say lost because law of conservation of energy saws that energy cannot be destroyed. In reality, most of the energy isn’t lost. What happens in an inelastic collision is the energy turns into heat and sound. The energy also is used in the deformation of the metal. This means that when two objects collide in an inelastic manner, the energy that was taken away from the object is used to deform the objects, create heat, create sound, and THEN the left over energy goes away from the vehicle without a use.
Conclusion

Now we now that the difference between Elastic and Inelastic collisions is what happens with the energy (Conserved or lost?), that there are many factors that can change the manner of collisions but they still are all Inelastic or Elastic regardless of the variable that is changing the collision, that energy cannot be destroyed and when it is “Lost “it is really turned into other forms of energy to do something (Depending on the situation, in Inelastic collisions this is true), many examples of Inelastic collisions and Elastic collisions ( Both perfect and Imperfect ) and the difference between perfect and imperfect collisions (both through Elastic and Inelastic)

 Figure 2 Elastic collision

GLOSSARY

-Conserved- To hold or keep something untouched or unchanged
-Coupled- Morphed together.
-Deformation- To have a major change, mostly in a bad way. (Degeneration is another word that is similar)
-Initial- Something at the beginning of what you are doing.
-Imperfect- Not complete, not perfect
-Kinetic- Active or being used.
-Potential- Capable of becoming something or doing something

CITATIONS
Rosen, Joe. Encyclopedia of Physics. New York: Facts on File, 2004. Print.

Hewitt, Paul G. Conceptual Physics. San Francisco: Pearson Addison Wesley, 2006. Print.

"collision." Encyclopædia Britannica. Encyclopædia Britannica Online School Edition.

Encyclopædia Britannica, Inc., 2012. Web. 24 Sept. 2012.

Wikapedia Contributor. “Collision.”Wikapedia.Wikapedia, the free encyclopedia, 6 September2012.Web.September 23, 2012.