- [Instructor] Energy is a
word we hear all the time in seemingly different contexts, almost every single day. We hear about renewable
energy on the news, and particularly in the winter, we hear people talking
about their energy bills, because they're worried about how much it's going
to cost to heat their homes. So this brings about the question of what is energy that we
can talk about it so often and in seemingly such different ways? So in physics, we actually
have a specific definition of what energy is, and you'll see it's
really not that different from how we talk about energy day to day. Energy in physics is defined
as the ability to do work. We can't talk about energy
without talking about work, so we should probably
define that right now, because work is another one of these words that we use an awful lot, but once again, physics has a specific definition for it. In physics, work is performed when you apply force over a distance. We can actually write this as an equation, W equals F times d, where
W is work, F is force, and d is distance or displacement. If you've ever moved a box, a suitcase, or really any object in
your room across the floor just to get it out of your
way, you performed work. You had to apply force
to that box to move it whatever distance it took
to get out of your way. If it was a short distance, you can see from the equation, that that's going to be less work than if you have to move
it across your entire room, down the hall, into another room. And in order to perform
this work to move the box out of your way, you had
to have energy available. That energy is enabling
you to do the work, because you're going
to transfer the energy from yourself to that
box in order to move it. So, another way that we can
think about work and energy is that the change in
energy of the system, in this case, you and the box, is actually equal to the work done. When we define energy this way, it allows us to do a lot
of interesting things. We've set up a way to
measure and calculate energy, so it's actually a quantifiable property. Let's go back to the example of a box, and let's say that instead
of just trying to move a box out of your way in your room, you're actually going to pack
up everything in your room, because you're going to move to a completely different house. And now you have 10 boxes to move. We can actually calculate
the energy required to move all of those boxes. You might be saying to
yourself, "Wait a minute. I'm now moving 10 boxes. That actually sounds kind of tiring. And if I'm getting tired, does that mean I'm
actually losing energy?" It turns out that energy
is coming from somewhere, and in this case, it's
going to come from food. So as you're moving these boxes, you may find yourself getting hungry, so you should probably grab a snack, something like, I don't know, an apple, let's pretend that's
what I've drawn there, so that you can get more energy to move the rest of those 10 boxes. And you might be thinking
now, "Wait a minute. This energy coming from
food to me seems different than energy between me and moving a box." And that's because you can
actually see the box moving, which brings us to our next point, energy comes in various forms and they don't all look the same. We have equations to quantify the energy of these various forms, and we'll talk about
those in another video, but the key here is that energy can transfer between objects and it can also convert
between different forms, such as when you eat and
get energy from that apple, and then you use that
energy to move a box. So to summarize, energy
is the ability to do work. Work is done when you apply
force across a distance, and we can write that as an equation. And because we can calculate the energy of a system using equations, we now know that energy is
a quantifiable property.