 Okay, one more example with the momentum principle and this one's kind of cool. Not necessarily the best way to do it, I mean you're going to see there's lots of ways to approach a problem. Normally if I know something about the time it's very simple. Those problems with the robot, I was firing the rocket for one second, two seconds, that's very appropriate for momentum principle. In the dropping the ball I knew the height. Little more work to find what I needed but I could still do it. So this is one of those cases where later you'll see a better way to do it. Okay, this is actually something interesting and not the best way to do it but in the movie Iron Man, Iron Man, Tony Stark, I hope I'm not spoiling stuff for you but it's from the comic books too so it's official lore so I'm not spoiling anything. So he builds a suit to escape his captors and it's got rockets on it and it kind of works and he shoots up really high in the air and then he falls down and crashes and makes a crater. And so here he is and he's moving down and I'm just going to say v equals negative 50 meters per second y hat and he's going to hit the ground and when he does he makes a crater and so this is where he stops like that. And let's say this crater is a depth of D and so you know you can imagine it probably maybe a meter, I don't know, but that's the distance over which he stops. So what I want to do is calculate the net force on Iron Man while he's stopping. Okay, so if I want I could draw a free body diagram for him. Let me let me do it right here. I have gravity and but since he's moving down I'm going to need a very large force pushing up. I'll call this Fn from the it's from the ground and so but then we'll have a net force going up because his momentum was down and then his momentum was zero so the net force has to be this way. So I don't really need Fn, I'll show you that we'll get it in a way but I'm just going to calculate the net force Fn delta P over delta T. Okay and so he has a mass of something too. Let's say it's, well he's got that suit on. Let me say a hundred kilograms. Okay so I'm just dealing with a one-dimensional problem here so I could say Fnet y is delta Py over delta T equals m times the final velocity in the y direction is zero minus the initial velocity in the y direction which is negative 50. So I get, I get that. That's going to be positive 50 over delta T and again we get the same problem but I just don't know time. Okay, so here again if I assume that it's the constant force I could say v average y, y over delta T and that's also equal to v2y plus v1y over 2. So I can use this to get delta T. Okay so my final velocity is zero so and my delta y it goes from let's call this y equals zero to here a negative D. I'll just write it out. It's going to be final of negative D minus zero over delta T. It has to be negative because he's moving down and that's going to be negative 50 over 2. So delta T is going to be positive. Delta T is going to be negative D over negative 50 times 2 so it's 25. Okay, so now I can just put that in here and I'll get my net force. So let's put in everything we know. I have a hundred kilograms. I have 50 meters per second and then delta T is going to be a D which let me say it's at one meter and then I have multiplied by 25 meters per second. I'm running out of room here. Okay now I want to check the units. I get meters squared per second squared over meters meters per second squared times kilogram does give me units of force. Okay so let's see. I was going to do it in my head but I don't want to make a mistake and I'll just do it in the calculator real quick. Okay so let's see 100, oh that's easy. Okay 125,000 newtons and let's just compare that to the gravitational force on Iron Man which would be 980 newtons right? This times 9.8 so MG is 980 newtons. No, 9,000. 9.8 that's right 980 newtons. So if you wanted to calculate the 4C earth exerts on them during this time you could ignore gravity if you wanted to because other ones so much larger. Now if you want to search for this on my website I have a post about this where I look at this exact situation from a different standpoint. I use something else but the end story is the acceleration of Iron Man is so large it doesn't matter if this would be the suit is he's still going to be accelerating the same amount inside and he'd probably die so that'd be bad. Okay