 Now, let's look at the physics concept of impulse. As we transition into looking at impulse, we want to change the way we're thinking about our forces. Before, we used to have forces that always acted over the object as it continued to move. Now, we want to ask what happens when our force acts only over a short time. So I give it a little bit of a shove, but then the force goes away as the object continues to move away. Even though the force is only acting over a short period of time, I can still come back to Newton's second law, which told me that the net force was the mass times the acceleration. If I apply that to this case of a short time and an average net force, I can recognize that that average force is going to give me a mass times an average acceleration. And our average acceleration from before was a change in velocity over change in time. I can bring the mass inside this delta V because the mass doesn't change. So as long as I'm multiplying both quantities by the mass, I can bring that up here inside the delta, which means what I really have up here on the top of the equation is the change in momentum. So another way to phrase Newton's second law is that I have a net average force. It's going to result in a change in momentum. Now, specifically, that net average force multiplied by that time span is equal to the change in momentum. So if I have a net average force, which is applied only over a short time span, then I will get a change in momentum. So now we come back to impulse. And we're starting with this equation we just had. And we're now going to define impulse as being equal to both sides of this equation. So that means first that impulse is equal to that average force applied over a time span. And then the impulse is equal to the change in momentum. Now, we want to be careful because both of these quantities are really vector quantities. So the impulse and average force are vectors, and the impulse and change in momentum are vectors. If I take a look at my units for these equations, I can once again start with both of my equations. Now over here on this side, the unit for force is going to be a Newton, and the unit for time is going to be a second, which means I expect impulse to have units of Newtons per second. But if I look over at this equation, impulse also has the units that momentum has, because the change in momentum has the same units as plane momentum. So that's going to be a kilogram meter per second. So impulse can have units of either one of these. Now, if I take my unit for Newton and write it out as a kilogram meter per second squared, which is what a Newton really is defined as, and multiply that by a second, what I see is that this second out here cancels one of my seconds on the bottom, and this whole quantity is really just a kilogram meter per second. So our Newtons per second and our kilogram meter per second are actually equal units to each other, and either one of those can be used for impulse. Now I want to understand impulse a little bit more by giving it back to a related concept of work. Back when I had work, I had force that was applied over a particular displacement, and that creates work when I apply a force over a displacement. And the work changes the mechanical energy of the system. Well, now we've got a force applied over a time span as opposed to a displacement, and that creates an impulse, and the impulse changes the momentum. So there's very similar concepts here, and there's going to be very similar mathematics between what we did for work and what we're doing for impulse. So that's our introduction to impulse.