>> Hello class, Professor Anderson here. Let's take a look at a problem of conservation of momentum and in this problem we are going to have a wooden block on a table and we are going to fire a bullet through the wooden block. And the bullet emerges out the other side and the wooden block starts to move. And let's calculate how much the velocity of the block changes as this bullet flies right through. So let's give some starting parameters here. We will have the mass of the bullet. We will have the mass of the wooden block. Mb and mw. This is the speed of the bullet initially. This is the speed of the bullet finally. And then this is the speed of the wood. So, this is a conservation of momentum problem. So let's write down what we have for conservation of momentum. What we know is Pi for the system has to be equal to Pf . This is all one dimension so we don't have to worry about two dimensional vectors in this case. If we think about the initial momentum of the system before the collision, all we have is the bullet. The mass of the bullet times the speed -- the initial speed of the bullet. After the collision what are we going to have? Well we have the bullet again. But we also have the block. Okay, so this is before and this stuff is after. And if we have all those numbers we can solve this for the speed of the wooden block. Vw is going to be what? Well I have mbvbi. I need to subtract this thing, mbvbf. And I need to divide by the mass of the wooden block. And now let's try that with some numbers and see how it works out. Okay, so let's say that the block, the mass of the wooden block is 2.6 kilograms. The mass of the bullet is 3 grams, which is 0.00 -- whoops, too many zeroes. 003 kilograms. We've got the speed of the bullet initially, which was 480 meters per second. We've got the speed of the bullet finally, which is 190 meters per second. I believe that is all of our information. So we can plug that in for vw and let's see what we calculate. So we've got the mass of the bullet times the difference in the speeds. So that's 480 minus 190 times .003. And in the numerator we get 0.87. And we got to divide by 2.6 kilograms. And if you do that you should get 0.33 meters per second. Okay, hopefully that one's clear. If not, come see me in my office. Cheers.