 Here we have conservation of energy. So what is energy? Well, some definitions call it a capacity for change or a capacity for action. This is a little bit vague and we really understand energy better when we start looking at specific examples. But focusing in on this concept of change, let's talk a little bit more about what changes are possible. So one is an energy transfer. So energy can be transferred from one object to another. That's a type of change that can happen. Now an example of this could be as simple as a person pushing on a box. The person is using their own energy to push on that box and it results in the box getting a little bit of that energy as it moves, maybe. Another thing that can happen is energy transformation. And by this I mean energy being transferred from one form to another form. Now this semester we've mainly been talking about mechanical energy, including kinetic and potential. But there are other forms of energy as well. Common ones include thermal energy, electrical energy, chemical energy. So all of those are some possible energy types. There's other ones as well, these are just some common ones. One can transform energy from one form into another. Now energy creation, you'll sometimes hear something about energy being created or destroyed. Most commonly when people talk about energy creation, there's really a hidden transfer or transformation coming into place. It might be that the sun was transferring energy to the plant which then grew, which created food, which then we used as energy when we ate it. But it really was a transfer from all of those different objects and different transformations of different forms. Sometimes though you do have a nuclear process, like what's happening inside the sun to create that energy. And in this case there can be a transformation between mass and energy. Einstein was the one who kind of figured out this and came up with his classic equation E equals MC squared where E is our energy, M is our mass, and it was related by C, the speed of light squared. So when people talk about energy conservation, they're really talking about a situation where there was no energy creation. And so that means I could still have transfers or transformations but the total amount of energy didn't change. It's really a matter then of keeping track of everything. What kind of energy did I have? Where did it go? Where did it come from? And all of the little details. Now this semester we focus on mechanical energy conservation. So if we say something as a conservation problem in Physics 1, we're talking about mechanical energy. And because we're only dealing with mechanical energy, when we look at a closed system, we can have transformations between kinetic and potential, or we could have transfers of energy between individual objects in the system. So you might have two objects that are somehow connected or colliding or something like that, and they might transfer energy from one to the other, and they might transform energy between kinetic and potential. But the total amount of mechanical energy on all the objects in the system stays the same. And so that total mechanical energy staying the same is what we mean when we say we have conservation of mechanical energy. So that's your concept. We're going to have to look at some problems to help you understand it a little more.