WEBVTT 00:00:12.405 --> 00:00:14.165 Teacher: Momentum is not the force that you're moving with, 00:00:14.226 --> 00:00:17.626 okay, but that's all right. It very much has to 00:00:17.661 --> 00:00:20.512 do with motion, and a lot of times people will say if 00:00:20.600 --> 00:00:22.864 something has momentum, it wants to keep on moving. 00:00:22.909 --> 00:00:26.224 And they'll make references to Newton's laws of inertia and 00:00:26.384 --> 00:00:28.444 Newton's second law, f=ma, in that way. 00:00:28.596 --> 00:00:31.230 Momentum is certainly related to Newton's second law, as we're 00:00:31.350 --> 00:00:33.018 going to find out. But it's actually something 00:00:33.074 --> 00:00:36.474 much simpler. It's just mass times velocity, and that's 00:00:36.590 --> 00:00:38.085 what we're going to be measuring in the lab today. 00:00:38.585 --> 00:00:42.149 We're going to be taking these cars -- and the cars are fairly 00:00:42.354 --> 00:00:45.574 frictionless on this track, okay -- and we're going 00:00:45.618 --> 00:00:48.564 to be rolling these cars down this track. And the masses of 00:00:48.722 --> 00:00:51.206 the cars are known. They're a half a kilogram 00:00:51.302 --> 00:00:54.000 each, and we're also going to have additional masses we're 00:00:54.141 --> 00:00:55.963 going to weight some of the cars down with. And 00:00:56.291 --> 00:00:59.050 we're going to be using motion sensors to gather the 00:00:59.068 --> 00:01:01.606 velocity data. These are the same motion sensors that 00:01:01.640 --> 00:01:05.385 we used during the lab when you were walking back and forth 00:01:05.549 --> 00:01:07.825 across the classroom and we were trying to 00:01:07.860 --> 00:01:10.246 match that graph -- when we were doing kinematics and velocity 00:01:10.281 --> 00:01:11.658 and acceleration at the time. 00:01:11.768 --> 00:01:14.529 Momentum is very, very practical and very applicable to 00:01:14.722 --> 00:01:16.826 every day life. There are a lot of professions in 00:01:17.012 --> 00:01:20.032 which you have to have a really thorough understanding of 00:01:20.049 --> 00:01:23.498 momentum. Many people who would even be 00:01:23.569 --> 00:01:26.663 something as simple as a police officer who would go and 00:01:26.719 --> 00:01:29.572 investigate an accident scene. They have to use 00:01:29.591 --> 00:01:32.047 the momentum of the cars, by looking at the evidence and 00:01:32.337 --> 00:01:34.542 the skid marks, and they can work backwards 00:01:34.611 --> 00:01:39.344 and find out exactly how fast those cars were going. Also, 00:01:39.466 --> 00:01:41.418 momentum is extremely important in the kinds 00:01:41.607 --> 00:01:44.178 of collisions you have. All of you that have had Driver's 00:01:44.213 --> 00:01:46.384 Ed have probably learned, what is the worst kind of 00:01:46.384 --> 00:01:48.705 collision you could possibly get into? Gracie? 00:01:48.740 --> 00:01:53.071 Student: Intersection. 00:01:53.106 --> 00:01:54.816 Teacher: When two cars are doing this, what would you call 00:01:54.851 --> 00:01:57.220 that? A head on collision. I like that. A head 00:01:57.372 --> 00:02:00.308 on collision is the very worst accident you could ever get 00:02:00.442 --> 00:02:02.635 into, and we're going to see why that is in the 00:02:02.670 --> 00:02:05.942 lab today. It's because the exchange of momentum is the 00:02:05.977 --> 00:02:08.727 highest. Okay? And so you have the greatest risk 00:02:08.762 --> 00:02:12.973 of getting hurt as you come to a stop, sometimes much more 00:02:13.021 --> 00:02:15.604 quickly than you would like. Okay? What is 00:02:15.607 --> 00:02:18.326 another really bad thing to hit if you're in a car wreck? 00:02:18.361 --> 00:02:20.539 Students: A wall. 00:02:20.574 --> 00:02:21.941 Teacher: Okay. Student: A brick wall. 00:02:21.942 --> 00:02:23.959 Teacher: An immovable object. A wall would be a good one, 00:02:24.327 --> 00:02:26.213 but anything that's not moving -- a telephone pole -- 00:02:26.248 --> 00:02:29.164 those are really not good to hit because they're not 00:02:29.199 --> 00:02:31.620 going to give. And we'll talk about that impulse, 00:02:31.709 --> 00:02:34.788 and we'll talk about that exchange between the time of 00:02:35.147 --> 00:02:38.149 impact and the amount of momentum that you 00:02:38.327 --> 00:02:41.161 receive as a driver. So even if you're simply designing 00:02:41.208 --> 00:02:43.203 these cars -- if you want to be an engineer who's 00:02:43.233 --> 00:02:45.544 designing these vehicles -- we're going to find out that 00:02:45.658 --> 00:02:48.415 vehicles are very carefully designed. Not just 00:02:48.450 --> 00:02:51.336 with airbags, but the entire vehicle itself is a structure 00:02:51.371 --> 00:02:54.672 to allow all that momentum not to pass into the 00:02:54.678 --> 00:02:57.824 passengers when they're in a traffic accident. 00:03:02.495 --> 00:03:04.994 Okay, now what do you want to do to get a better look? Yes. 00:03:05.029 --> 00:03:08.567 So we have an initial and final region with 00:03:08.596 --> 00:03:11.273 linear. This one's a little stickier. We definitely have an 00:03:11.431 --> 00:03:13.695 initial, and I would just treat that as final, but before 00:03:13.730 --> 00:03:16.822 it jumps up again, right? And be very careful about your 00:03:16.857 --> 00:03:18.741 negative signs. Okay, just because it's negative on 00:03:18.776 --> 00:03:20.427 the computer screen we're not going to put a negative right? 00:03:21.120 --> 00:03:21.457 Students: Right. 00:03:21.699 --> 00:03:23.550 Teacher: Okay, so think about the direction. Think about 00:03:24.143 --> 00:03:25.659 what happened last time with the data and how 00:03:25.703 --> 00:03:27.347 are you going to keep the negative signs straight this 00:03:27.496 --> 00:03:29.141 time. Okay, I'll come back and ask you that in a minute 00:03:29.193 --> 00:03:31.137 okay. So talk amongst yourselves and figure out what you're 00:03:31.326 --> 00:03:32.648 going to do. Student: So this one was both- 00:03:33.420 --> 00:03:35.828 Teacher: Sometimes when cars collide, Maddy, 00:03:35.829 --> 00:03:38.377 what will happen is the bumpers get interlocked and so 00:03:38.412 --> 00:03:40.483 cars will often skid off together. So in this case, 00:03:40.484 --> 00:03:43.800 this will be a collision that we're going to call perfectly 00:03:43.801 --> 00:03:46.715 inelastic, where two objects stick together. Okay? Student: Okay. 00:03:46.716 --> 00:03:49.547 Teacher: And we want to observe what kind of velocity they 00:03:49.682 --> 00:03:50.301 have afterwards. 00:03:50.302 --> 00:03:51.924 Student: We need to do perfectly -- we need to do inelastic, 00:03:51.992 --> 00:03:53.765 elastic and -- we need to do all of those? 00:03:53.813 --> 00:03:54.785 Teacher: Correct. Student: Okay. 00:03:55.269 --> 00:03:57.425 Student: And we pick one side, we pick like you said. We 00:03:57.474 --> 00:03:59.176 pick one sensor? Teacher: Yes. 00:03:59.305 --> 00:04:01.553 Student: That's negative. And so when we write this down will that 00:04:01.554 --> 00:04:03.928 be positive? Teacher: Very good question. let's talk about it. 00:04:04.692 --> 00:04:07.822 Which -- even though we see two positive slopes beforehand 00:04:07.926 --> 00:04:10.631 and two negative slopes afterwards, obviously you had two 00:04:10.807 --> 00:04:14.270 cars colliding head on here. Okay? So, what are 00:04:14.339 --> 00:04:15.823 you going to choose to call positive and negative as you 00:04:15.824 --> 00:04:18.752 record the data? Student: left is our negative. 00:04:19.728 --> 00:04:20.742 Student: right, which is the red. 00:04:21.642 --> 00:04:24.197 Teacher: Okay. So initial velocities, they're listed as .56 00:04:24.232 --> 00:04:26.971 and .51. Are you going to record one of those as 00:04:27.185 --> 00:04:29.523 negative or both positive, both negative, what are we going to do? 00:04:30.323 --> 00:04:33.021 Student: One would be negative. 00:04:33.056 --> 00:04:34.865 Student: So it would be left. Teacher: Okay. 00:04:34.866 --> 00:04:35.718 Student: left side. 00:04:35.719 --> 00:04:37.787 Teacher: Okay, so one's negative because it's just direction. 00:04:37.788 --> 00:04:39.664 Student: It's just direction. Teacher: Okay 00:04:39.665 --> 00:04:41.281 Student: So just decide left or right negative. 00:04:41.282 --> 00:04:42.730 Teacher: So let's decide that now. 00:04:43.236 --> 00:04:46.026 Students: Left equals negative and right equals positive 00:04:46.030 --> 00:04:49.780 Teacher: Push the car. if that's positive, so is that. 00:04:50.507 --> 00:04:51.979 Student: Okay. Teacher: You got it? Student: Yes. 00:04:52.467 --> 00:04:55.446 Teacher: Make sense, Elizabeth? Everything going toward Ben 00:04:55.566 --> 00:04:57.997 is going to be a positive number. All right? 00:04:58.022 --> 00:05:00.672 And so this one's going to be positive before it hits, if 00:05:00.835 --> 00:05:03.245 it's positive after it hit, and this one's positive after 00:05:03.282 --> 00:05:04.742 it hit because it also went this way. 00:05:04.860 --> 00:05:07.684 What we were looking for today is we were trying to measure 00:05:07.690 --> 00:05:11.015 two different quantities about the cars. We 00:05:11.149 --> 00:05:14.369 wanted to know the mass of the cars and we wanted to know 00:05:14.404 --> 00:05:16.283 the velocity of the cars. We wanted to see 00:05:16.365 --> 00:05:19.045 that before and after a collision. There were different 00:05:19.080 --> 00:05:21.340 collisions that you were simulating today. We were 00:05:21.400 --> 00:05:24.327 simulating perfectly inelastic collisions, where cars stuck 00:05:24.362 --> 00:05:26.828 together; we were simulating elastic collisions, 00:05:26.902 --> 00:05:29.349 where they were bouncing off each other with hopefully 00:05:29.439 --> 00:05:32.698 little to no loss of kinetic energy; and then we 00:05:32.702 --> 00:05:34.822 were simulating just inelastic collisions, where they were 00:05:35.080 --> 00:05:36.290 just kind of hitting and bouncing a little more 00:05:36.291 --> 00:05:39.229 abruptly. And hopefully we'll see within these that 00:05:39.264 --> 00:05:41.692 momentum is conserved in each one of these types of 00:05:42.495 --> 00:05:45.229 collisions, by measuring the velocity before and after and 00:05:45.264 --> 00:05:47.104 taking the mass of the cars with it. 00:05:47.147 --> 00:05:49.588 There was also one or two really good questions asked by 00:05:49.676 --> 00:05:51.893 students and they said, "Oh, Mister Brown, what 00:05:51.894 --> 00:05:53.706 about the velocity? Isn't the velocity supposed to be the 00:05:53.707 --> 00:05:55.800 same afterwards as it is in the beginning?" And the 00:05:56.150 --> 00:05:59.567 answer is emphatically no. Velocity is not conserved in 00:05:59.602 --> 00:06:02.932 these collisions. It's mass times velocity, which is not 00:06:02.967 --> 00:06:04.466 the same thing.