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What is the physics behind why an egg
breaks against a hard surface,
like a wall, but not a soft surface, like a sheet?
Does it have anything to do
with how fast it comes to a stop?
What about the force it experiences
as it slows down?
We're going to learn all about it
in this segment of "Physics in Motion"
on impulse and momentum.
We'll start with momentum first.
Let's break it down in this demonstration.
When I throw this egg,
the force of my hand causes it to gain velocity.
The egg now has momentum.
Momentum is the quantity of motion an object has,
or put another way, it's mass in motion.
You can calculate it by multiplying the mass
and velocity together.
We use velocity rather than speed
because momentum is a vector quantity.
The formula looks like this.
P, which stands for momentum,
equals mass multiplied by the velocity.
If the egg weighs 0.057 kilograms,
and its velocity is 26 meters per second,
what is its momentum?
We multiply the mass times the velocity
and we find that its momentum is 1.5 kilograms
times meters per second.
Now that we know how to measure momentum,
let's look at the physics of why the egg breaks
against the wall and not the sheet.
Yes, the sheet is softer,
but we're looking for a way to quantify
what's happening.
In order to do that,
we also need to know what impulse is
when we're talking about physics.
Impulse is the product of the force acting on an object
over the time during which the force acts.
The Impulse-Momentum Theorem
is just another way of looking
at Newton's second law of motion.
The acceleration in the second law
becomes a change in velocity over time
in the theorem.
In order for an object's momentum to change
a force must be applied for a period of time.
This is known as an object's impulse.
The theorem says that the impulse applied to an object
is equal to the change in momentum of an object.
The impulse can be a larger force
over a short period of time,
or a smaller force over a longer period of time,
or some other combination of force and time.
You can start to see where we're going with this.
When I throw the egg against the wall
what can we say about the impulse involved?
It's a large force over a short period of time
because it hit the wall,
and its momentum was stopped abruptly.
When I throw the egg against a sheet,
do you think the force is small or large?
It's a weaker force applied over a longer period of time
because the sheet gives.
Even though the impulse is the same as when we throw it
against the wall, the egg will not break
because it experiences a smaller force.
And that's the difference between throwing an egg
against a wall and against a sheet.
The egg experiences the same impulse in both cases,
but the stopping time when it hits the sheet
is much larger than when it hits the wall.
By increasing the stopping time the force required
to stop the egg is decreased.
Let's look at how athletes use momentum and impulse
through an activity called parkour.
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Parkour, or freerunning, was originally used for
firefighter/military training.
Now, people do it for fun and fitness.
One of the most popular maneuvers in parkour
is the vertical wall run,
where a runner tries to reach a vertical height
that a regular jump off the ground couldn't manage.
He has to redirect his horizontal momentum
into vertical momentum to carry him up the wall
as far as he can go,
and he has to max out the force he exerts
over a shorter period of time
as his foot hits the wall,
so he can climb as high as he can.
No way he could've done that without momentum and impulse.
And remember, momentum is a vector quantity,
meaning it has a direction.
You can see how the change in direction is important here
because that transfers the momentum
from horizontal to vertical.
Got it!
Did you see that?
I weakened the force on the egg by moving my hands
to increase the stopping time.
That's a look at momentum and impulse
and how they can be manipulated
to help us jump higher
and even avoid a potential big mess.
That's it for this segment of "Physics in Motion",
and we'll see you next time.
♪♪
(announcer) For more practice problems,
lab activities, and note-taking guides
check out the "Physics in Motion" toolkit.