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Intro to Momentum

Patrick Ford
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Hey guys, so in this video I'm gonna introduce you to a new physical quantity called momentum. We're gonna be talking a lot about momentum in the next couple of videos. So it's a good idea to get a good conceptual understanding of what it is, how we calculate it, and also how we solve problems with it. Let's check this out, momentum is really just a physical quantity that objects have. It's kind of like energy and it's really just related to an objects mass and it's velocity. So objects that have mass that are moving with some velocity have momentum. The equation for the letter that we use is lower case P. Don't ask me why and it's really just M. Times V. P equals Mv. Very straightforward. So let's talk about the units. The units really just come from these two letters M and V. Right? The units for mass or kilograms and units for velocity are meters per second. So momentum actually doesn't have a fancy unit, like a jewel or watts or newton or anything like that. It's just kilogram meters per second. So if you ever forget, you can always just get back to it by using Mv. So like I said before, we're gonna be talking a lot about momentum's and an objects momentum, how it gets changed or transferred. And so it's a good idea to get a good conceptual understanding of what it is momentum conceptually is really just a measure of how difficult it is to stop moving objects. So let me back up for a second cause we've talked about a similar idea, which is inertia. Remember that inertia was how hard it is to change an object speed. And if you remember inertia was really just a was really just your mass. Mass is a measure of inertia. If you have lots of mass or if you have like 100 kg then you have to push really hard in order to change your speed. Versus if you had like 10 kg or something like that. So momentum is kind of related to inertia except that only just applies to moving objects. So really if you look at the equation for momentum there's actually two ways you can have lots of momentum. So you can have lots of mass, right? Lots of inertia. Therefore it's harder to stop you. But you could also have lots of speed and it's also harder to stop you. Right So let me go ahead and actually work out this problem here. Um Just so I can show you what I mean by this. So here we have a truck and a race car that are moving. So I'm gonna draw this little flat surface like this. So I've got a 4000 kg truck that's the mass. And it's moving to the right with some speed. So this is V. T. I also have a 800 kg race car that's moving to the left with some V. R. What I wanna do is I want to calculate the momentum. Right? This is just the plural word for momentum of both vehicles here. So I'm gonna calculate P. T. And then P. R. Just by using the equation here. So let's talk about the truck first. Well, if you think about this, the truck right really has some momentum because it has mass and it's moving to the rights. Now, what I want to point out here is that momentum is actually a vector. We can see from the equation here that if you have P. M. And V, what happens is that your velocity is an arrow, right? That's a vector. And if your momentum depends on velocity and momentum is also a vector and it just points in the same direction as your velocity. So momentum is always gonna point in the same direction as an object velocity. So if you have knowledge of moving to the right with some velocity, then its momentum is also going to be to the right like this. So when we calculate the momentum for the truck, we're really just gonna use the mass of the truck, times the velocity of the truck, which we know is equal to 10 m per second. So what I'm gonna do is I'm gonna choose the right direction to be positive and therefore my velocity is gonna be positive. So I'm just gonna calculate this is gonna be 4000 times the velocity of 10 And I have the momentum is really just equal to 40,000 kilogram meters per second. So let's talk about the race car. Now, now, the race car is moving to the left, which means that it's actually gonna have a negative velocity. We're told that the race car is 50 m per second. So VR is actually gonna be negative 50 m per second. Signs are gonna be very important in these kinds of problems. So once you pick a direction of positive, you're gonna stick to it. So what happens is if you have an object that's moving in the direction of negative, then the velocity and the momentum are both going to be negative here. So V. R. Is negative 50. And when we calculate the momentum of the race car, we're just gonna use the mass of the race car times the velocity of the race car. So this is going to be 800 times negative 50. So now, when you work this out, the momentum of your race car is gonna be negative 40,000 kg meters per second. So that's how we got the same number. And we also got a negative sign here. So, if you take a look here, we actually have the same number that we calculated for the momentum of the truck versus the momentum of the race car. And really, this just points out, I'm just trying to point out here that it's actually just as difficult to stop both of these moving objects here because they have the same magnitude momentum. So, for the truck, the truck has a high mass, right? It's M. Is very high, but its speed is only 10 m per second. It's not moving that fast, so the race car is actually much lighter. It's only 800 kg. The M is isn't as high, but the speed is actually much greater than the trucks. So, depending, so basically between the two, it's actually just as difficult to stop the race car moving as it is to stop the truck. Alright, so, hopefully, that makes sense. Guys, let me know if you have any questions.