 So now we're ready to consider non-conservative forces. Well, if we're going to talk about non-conservative forces, we need to compare them to what we've already learned about conservative forces. So as a quick reminder here, we were dealing with mechanical energy, and we were dealing with work that was done that did not depend on the path. And because of this, it acts to transform between kinetic and potential energy. Some of the examples that we looked at were gravity and spring forces. So when gravity and spring forces come into play, kinetic energy can be turned into potential energy, but then that potential energy can be re-released as kinetic energy. So now we have non-conservative forces. So mechanical energy is not conserved. That's what the non-conservative part here is. And oftentimes, its energy is transformed to other forms of energy. It might be that mechanical energy is going to some other form, or some other form is being transformed into mechanical energy. We could also have energy entering or leaving the system. So some examples. The most common example of a non-conservative force you're going to see in all the textbooks is friction. Very closely related to that is air resistance or fluid resistance. So any kind of resistive type force, surface friction, moving it through a fluid, air resistance if something falls. And in both of these cases, what happens is that kinetic energy is transformed into thermal energy. So there's some heating that happens as this resistance happens. You can think about that space shuttle coming in from outer space, and that air resistance causes the front part of the shuttle to heat up so much it's actually glowing from the heat. And this tends to reduce the mechanical energy of the system. Some other examples, motors. So if you've got some sort of motor attached to the system, it's really not a closed system because it uses some sort of outside energy to make that motor work. Similarly, muscles. Well, you could think of muscles as being a type of motor. Again, you use some sort of outside energy. Often in these cases, energy of the system is increased. Your motors or your muscles help get more mechanical energy in. But you could also have a decrease in mechanical energy if you're using those muscles to slow an object down. So let's come back here to one of these main concepts of what's the effect. If I have any form of non-conservative force in the system, if it's there, then I'm not going to have conservation of mechanical energy. To have conservation of mechanical energy, I can have absolutely no non-conservative forces allowed. So that's just a quick introduction. We still need to look at some of the equations and some example problems from this.