 Momentum. Here's a new concept we want to introduce. So start with the definition in physics. Momentum is defined as the product of the mass and the velocity, or m times v. But you notice I haven't told you a symbol yet for momentum in there, and we're going to be using a lowercase p. The m's have been used up for other things, so we've got to get a little creative. So that means our equation for momentum is p equals mv. If I take a look at the unit starting with that equation, what I see is the units I expect for mass should be kilograms, while the units I expect for velocity would be meters per second. And that means that my total unit is going to be a kilogram meter per second. Unlike some of our other quantities, we don't have a new unit defined, so you're going to have to just use kilogram meter per second each time you encounter the momentum. Here's some quick examples. I'm going to start with two of them. First would be for a fast moving car. In this case, you have a fairly large mass of probably something over 2,000 kilograms. This might be a typical value for a car. And here are 35 meters per second, and that's somewhere around typical highway interstate speeds. So that gives us a fairly large momentum of 80,500 kilogram meters per second. We can contrast that to the slow moving basketball. So for our slow moving basketball, my mass is much lower, less than a kilogram. And yeah, a velocity of 2 meters per second is kind of quick for a basketball, but nowhere near our highway speeds up there. And so notice again, we get a very small momentum of only 1.2 kilogram meters per second. And of course there's a whole range of values in between these. Now to help clarify this concept of momentum, let's relate it back to some other concepts, particularly let's relate it to inertia. And if you remember, inertia was measured by mass. Well, inertia is actually a word that comes from the Latin for slothfulness. And it was a property of the individual object. Some objects are harder to accelerate. Those would be the very slothful ones. And some of them are much easier to accelerate. So depending on my mass, it affected my acceleration. Now when I get to momentum, that actually comes from Latin as well. It's kind of roughly translated the movement power. And it depends on the object because the mass is in there, but it also depends on the speed. And we can think about this one in terms of easier or harder to stop. An object that's got a lot of momentum going is going to be harder to stop. And that's often the way we use it in our language today. If a football team is moving down the field and they keep progressing, they're said to have momentum. Now I need to clarify a little bit about momentum, which is that there's really two forms of momentum that we study in physics. Linear momentum and angular momentum. We're going to introduce angular momentum a little bit later in more detail. But for now, when we say momentum, it's safe to assume that we're talking about the linear momentum of a particular system. So that introduces the concept of momentum.