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- There's nowhere like Atlanta to talk about kinetic energy.
If you've ever been in traffic, you're surrounded by it.
In another segment, we talked about work and energy.
Energy is the ability to do work.
In this segment, we'll look at some specific types of energy,
including kinetic energy.
Kinetic Energy, written as capital KE,
is the energy of motion.
Moving cars, moving trucks and sound waves,
they all have kinetic energy.
A rolling ball and me walking down the street
are examples of things that possess kinetic energy.
Kinetic energy is the energy of movement.
So we would expect that velocity is involved.
Kinetic energy is equal to 1/2 of the moving mass
times its velocity squared.
Kinetic energy is a scalar quantity,
meaning it is not associated with direction.
It can, however, be positive or negative,
depending on whether that energy is being
added to a system or taken away.
This little puppy, Sage, has a lot of kinetic energy.
Let's say our 5.0 kilogram puppy
is moving at 2.2 meters per second.
To figure out his kinetic energy,
you take 1/2 his mass,
2.5 kilograms, times his velocity,
2.2 meters per second squared.
That equals 12.1 kilogram meters squared,
divided by seconds squared.
A kilogram times meters squared divided by seconds squared
equals one joule.
So our answer is 12.1 joules.
That's the kinetic energy expended by the little puppy.
Remember the law of conservation of energy?
Energy is never created or destroyed,
it just gets transformed from one form to another.
Wherever you see objects moving,
there's usually some kind of energy exchange
going on.
And sometimes, that energy can be stored.
That's called potential energy,
written capital PE.
Think of all the ways we store energy.
Batteries store electrical energy.
Springs store elastic energy,
by stretching and compressing,
and a heat pack stores thermal energy.
So a skateboarder standing on top of a ramp
has gravitational potential energy.
The amount depends on
how far he is vertically from a reference point,
usually the bottom of a ramp.
The force of gravity has the ability to do work
on the skateboard, pulling it down the hill.
As the skateboard starts rolling down the ramp,
all that potential energy from gravity
is converted to kinetic energy.
As its height on the ramp decreases,
its speed increases.
The gravitational potential energy,
written PE sub G,
first depends on determining a point of reference.
For example, in this case, it's the bottom of the ramp.
You set that point as the starting point or origin,
that's the point where displacement is equal to 0.
Then, the PE sub G of an object
depends on its mass,
and how high it is above the origin.
Okay, think of other examples.
A skier on top of a slope.
That's a lot of gravitational potential energy.
Or a diver on the high dive platform.
Once they start falling, their potential energies
are converted to kinetic energies as they go down.
Gravitational potential energy
equals the mass of the object, "m",
times gravitational acceleration on Earth, "g",
times the height of the object
above the reference point, "h".
If you're interested in the gravitational potential energy
of a 500.0 kilogram wrecking ball
on a crane 40 feet in the air,
that's 12.2 meters,
set the ground
as your zero reference point,
which is considered ground zero.
Then take the mass,
500.0 kilograms,
and multiply it by the acceleration
due to gravity on Earth,
9.8 meters per second squared,
times the height, 12.2 meters.
So our gravitational potential energy
is 59,780 joules.
With significant figures,
our gravitational potential energy is 5.98,
times ten to the fourth joules.
So think of this skateboarder
poised at the top of a hill.
A skier, a diver,
all of these have gravitational potential energy
and zero kinetic energy.
Once they make their moves,
that gravitational potential energy
begins to change into kinetic energy
until it reaches its reference point.
That's when the kinetic energy is at its maximum
and gravitational potential energy is zero.
That's it for this segment of "Physics in Motion".
We'll see you guys next time.
- For more practice problems,
lab activities and note-taking guides,
check out the "Physics in Motion" toolkit.