 Today we're going to talk about momentum and impulse. It's a big idea in physics 30 and it comes out of another big idea in physics 20, Newton's Third Law. Newton's Third Law tells us that if one object, like this big guy, applies a force to a second object, like this little guy, the second object applies an equal magnitude, so same size, but opposite direction force, back to the first object. Those two forces are the same size in opposite directions. Now let's take a look at some of the math. The two forces have equal magnitudes, so I can say that the force from the big hockey player is equal to the force from the small hockey player. Then I can use Newton's Second Law to substitute in for force. I'll put in mass times acceleration for each of the forces. And I can go a step further and break the accelerations down into change in velocity divided by change in time. Now this relationship holds even if those two players have different masses, the forces will still be equal, so that means the mass times the velocity divided by the time of each hockey player has to be equal as well. Now during the impact, each force takes place over the same period of time, so that means that the delta t term in each of these two equations is going to be exactly the same. This means that the mv from one side from the big hockey player and the mv from the other hockey player are going to be not necessarily the same numbers exactly, but they'll multiply together to give the same thing. This mass times velocity quantity is called momentum. It gets the symbol p in the equation p equals mv. It's a vector quantity, so we'll get to break it apart into components and add it to other vectors like we have in previous units in Physics 20. It's a fundamental idea for being able to understand motion and how objects interact with one another. If we go back to those equations we looked at a minute ago, we can also see that if we rearrange them, we can get an interesting new formula coming out from momentum. The change in momentum is also equal to force multiplied by the change in time. Closely related to the idea of momentum is the idea of impulse. Impulse is the change in momentum. It's literally just how much your momentum changes by. It doesn't get a fancy symbol or anything like that, just delta p and momentum and impulse mathematically can be represented the same way. The only difference with impulse is we put in the change in velocity and the change in time in the formulas. Impulse can be as simple as just asking what was the initial and the final momentum, subtracting them and seeing what the difference is. Or we can use one of these calculations including the momentum impulse formula from your data sheet to do a calculation. From the equations we can see that momentum has something to do with force and time, but how do force and time work with one another? How are they related? Let's take a look at an example of dropping an egg onto two different types of surfaces. Let's apply some forces. I've got two identical eggs, same mass, and we're going to drop them from a height exactly the same. They're going to land on two different surfaces though. We've got a pillow and we've got a concrete block. So let's see what happens when they hit these two different surfaces. Drop it. When the egg hits the concrete block it breaks. There's no big surprise there. The egg falling downwards has momentum and applies a force to the block and the block applies an upwards force back to the egg, breaking the egg. When the egg hits the pillow it doesn't break, which is weird because the egg experiences the same change in momentum or the same impulse. It has the same mass and the same final velocity. So why is the force smaller in this egg? Our second impulse momentum formula can help us understand this. The change in momentum in object experiences is equal to the force acting on the object multiplied by the change in time. Now if the momentum term has to stay the same, that means we can have different combinations of forces and changes in time that can multiply together to give the same change in momentum or the same impulse. Now this gives us an inverse relationship between the force and the change in time, meaning that the force can be large, but you've got to make your change in time small. Or vice versa, you can have a big change in time and that'll result in a small force. So when the egg hits the concrete, it does so in a very short period of time, making the force acting on the egg really large. When the egg hits the pillow, the force is less because the interaction time between the egg and the pillow is slightly longer and that longer time or larger time is going to make for a smaller force. The idea of an inverse relationship between force and time when objects strike one another, the idea of impulse is a really important one and it comes up all over the place when we talk about things like safety mechanisms, an airbag, or a helmet when you're trying to break a board with your fist. If you're in karate and you want to break a board with your fist, you don't slowly apply a force to the board. You want to do it very quickly. A short period of time will mean a large force. Thanks for watching. For more videos on this topic and others on the topic of momentum and impulse, check out my website, LDStrees.ca. Thanks, Rome. I'm coming over.