- Good morning.
Let's define elastic and inelastic collisions.
♫ Flipping Physics ♫
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- Gentlemen, those are all worthy causes.
Thank you very much.
Alright, back to the physics.
Let's start with something called an elastic collision.
In an elastic collision, the objects
bounce off of one another.
Billy, is momentum conserved in an elastic collision,
where the two objects bounce off of one another?
- Is momentum conserved in an elastic collision?
I'm sorry, but how should I know?
- [mr.p] You should know because the answer
is actually built into the question.
- Uhh...
- An elastic collision is a collision.
- And momentum is conserved during all
collisions and explosions.
- Right, therefore momentum is conserved
during an elastic collision. Thanks.
- In addition to momentum being conserved,
kinetic energy is also conserved during elastic collisions.
Some examples of elastic collisions are:
Collisions between billiard balls.
Collisions between air-hockey pucks.
And collisions between atomic particles,
are all examples of elastic collisions.
Now, let's talk about inelastic collisions.
Class, is momentum conserved in inelastic collisions?
- Yes, it's a collision.
- What sets an inelastic collision
apart from an elastic collision,
is that kinetic energy is NOT conserved
during an inelastic collision.
Let's make a table with this information.
Realize, we now have the words elastic and inelastic,
which can sound very similar to one another.
Especially when you say something like,
"In an elastic collision, kinetic energy IS conserved.
"However, in an inelastic collision,
"kinetic energy is NOT conserved."
So, please be very careful to carefully
enunciate the words elastic and inelastic.
- I will. - Yeah.
- Okay.
- The reason kinetic energy is not conserved
during inelastic collisions, is because
during inelastic collisions, the objects deform.
And that change in shape of the object, heats up the object.
In other words, the deformation of the object
causes the kinetic energy to be
converted into thermal energy.
I will point out that there is also a specific type
of inelastic collision which is called
a "Perfectly Inelastic" collision.
A perfectly inelastic collision is where the objects
run into, and stick to one another.
Sometimes, you'll also hear a perfectly inelastic collision
referred to as a Totally or Completely inelastic collision.
However, I prefer the term "Perfectly Inelastic" collision.
So, some examples of perfectly inelastic collisions are:
Two football players colliding, and holding onto
one another, or a tackle.
Or, two railroad cars running into one another and coupling.
Okay, so now let's take a look at a
demonstration of some collisions.
When I drop these two balls from the same height,
the one on your right
bounces much higher than the one on your left.
Bo, is the collision on your right
an elastic, inelastic, or perfectly inelastic collision?
- [Bo] Well, I know it's not a perfectly inelastic
collision, because the ball does not stick to the table.
So, is it elastic or inelastic?
That's the question.
In other words, is kinetic energy conserved, or not?
- [Bobby] I think I get it, do you mind, Bo?
- [Bo] No, go ahead, Bobby.
- [Bobby] Kinetic energy is not conserved,
so it must be an inelastic collision.
- [Bo] But, how do you know kinetic energy is not conserved?
- [Billy] I see, if kinetic energy were conserved,
then the ball would have the same kinetic energy
leaving the table as it does when it strikes the table.
Therefore, it would go up to the same height
from which it was dropped.
So, because this ball does not go all the way
up to its original height, kinetic energy is not conserved,
and it is an inelastic collision.
- [mr.p] Very nice.
And Billy, what about the ball on your left?
- [Billy] Well, again it does not stick to the table,
so it is NOT a perfectly inelastic collision.
And also, it does not go up to the original height.
So, I guess this must also be an inelastic collision.
Only, more of the kinetic energy is converted to
internal energy, or Thermal Energy.
- [mr.p] Yes, both of these collisions
are examples of inelastic collisions.
All real-world bounce collisions are inelastic.
On an atomic level, collisions are often elastic.
However, in this macroscopic world that we live in,
an elastic collision is an ideal case,
which is never fully achieved.
Even billiard balls, when they collide,
and we consider them to be elastic.
There is some deformation of the objects,
and therefore, some kinetic energy
converted to thermal energy.
But physicists, and we will for the sake of this class,
consider billiard balls to have elastic collisions.
Thank you very much for learning with me, today.
I enjoyed learning with you.