[Script Info] Title: [Events] Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text Dialogue: 0,0:00:00.09,0:00:01.41,Default,,0000,0000,0000,,- [Lecturer] Let's solve\Na couple of problems Dialogue: 0,0:00:01.41,0:00:02.91,Default,,0000,0000,0000,,on Newton's second law. Dialogue: 0,0:00:02.91,0:00:03.78,Default,,0000,0000,0000,,Here's the first one. Dialogue: 0,0:00:03.78,0:00:05.97,Default,,0000,0000,0000,,We have an elevator, which is moving up. Dialogue: 0,0:00:05.97,0:00:07.35,Default,,0000,0000,0000,,And let's say the mass of the elevator, Dialogue: 0,0:00:07.35,0:00:11.88,Default,,0000,0000,0000,,including the passenger\Ninside, is 1,000 kilograms. Dialogue: 0,0:00:11.88,0:00:14.94,Default,,0000,0000,0000,,Now the force, the tension\Nforce of the cable, Dialogue: 0,0:00:14.94,0:00:17.40,Default,,0000,0000,0000,,let's say that's about 7,800 newtons, Dialogue: 0,0:00:17.40,0:00:20.88,Default,,0000,0000,0000,,our goal is to figure\Nout what the acceleration Dialogue: 0,0:00:20.88,0:00:22.71,Default,,0000,0000,0000,,of this elevator is. Dialogue: 0,0:00:22.71,0:00:26.07,Default,,0000,0000,0000,,We're also given that the\Ngravitational force acting Dialogue: 0,0:00:26.07,0:00:29.31,Default,,0000,0000,0000,,on that elevator, including\Nthe passenger over here, Dialogue: 0,0:00:29.31,0:00:32.43,Default,,0000,0000,0000,,is about 9,800 newtons. Dialogue: 0,0:00:32.43,0:00:33.99,Default,,0000,0000,0000,,So, how do we figure this out? Dialogue: 0,0:00:33.99,0:00:35.62,Default,,0000,0000,0000,,Well, the first thing\Nthat comes to my mind is, Dialogue: 0,0:00:35.62,0:00:37.83,Default,,0000,0000,0000,,"Hey, we have some forces Dialogue: 0,0:00:37.83,0:00:40.86,Default,,0000,0000,0000,,and we have some motion\Nvariables like acceleration. Dialogue: 0,0:00:40.86,0:00:43.44,Default,,0000,0000,0000,,What connects forces and motion variables? Dialogue: 0,0:00:43.44,0:00:45.06,Default,,0000,0000,0000,,What connects forces and motion? Dialogue: 0,0:00:45.06,0:00:47.49,Default,,0000,0000,0000,,Newton's second law. Dialogue: 0,0:00:47.49,0:00:49.14,Default,,0000,0000,0000,,So, the first thing I try to do Dialogue: 0,0:00:49.14,0:00:50.88,Default,,0000,0000,0000,,before applying Newton second law is I try Dialogue: 0,0:00:50.88,0:00:53.52,Default,,0000,0000,0000,,to draw a free body diagram. Dialogue: 0,0:00:53.52,0:00:55.08,Default,,0000,0000,0000,,So lemme do that. Dialogue: 0,0:00:55.08,0:00:56.67,Default,,0000,0000,0000,,What's the way to draw\Na free body diagram? Dialogue: 0,0:00:56.67,0:00:58.92,Default,,0000,0000,0000,,We try to get rid of unnecessary details. Dialogue: 0,0:00:58.92,0:01:01.35,Default,,0000,0000,0000,,Use a box to represent\Nyour object of interest. Dialogue: 0,0:01:01.35,0:01:02.91,Default,,0000,0000,0000,,Our object of interest is this elevator Dialogue: 0,0:01:02.91,0:01:03.75,Default,,0000,0000,0000,,and the person over here. Dialogue: 0,0:01:03.75,0:01:05.49,Default,,0000,0000,0000,,So, that's our box. Dialogue: 0,0:01:05.49,0:01:07.02,Default,,0000,0000,0000,,It's mass is 1,000 kilograms Dialogue: 0,0:01:07.02,0:01:08.88,Default,,0000,0000,0000,,and let's draw all the\Nforces acting on it, Dialogue: 0,0:01:08.88,0:01:09.99,Default,,0000,0000,0000,,which are the forces acting on it. Dialogue: 0,0:01:09.99,0:01:12.33,Default,,0000,0000,0000,,Well, we have an upward\Nforce that's tension Dialogue: 0,0:01:12.33,0:01:15.30,Default,,0000,0000,0000,,and we have a downward force\Nthat's the force of gravity Dialogue: 0,0:01:15.30,0:01:18.48,Default,,0000,0000,0000,,and our goal is to calculate\Nwhat the acceleration is. Dialogue: 0,0:01:18.48,0:01:19.83,Default,,0000,0000,0000,,And how do we do that? Dialogue: 0,0:01:19.83,0:01:20.95,Default,,0000,0000,0000,,Well, we use Newton second law, Dialogue: 0,0:01:20.95,0:01:23.19,Default,,0000,0000,0000,,which says the acceleration Dialogue: 0,0:01:23.19,0:01:26.31,Default,,0000,0000,0000,,should equal the net\Nforce acting on an object Dialogue: 0,0:01:26.31,0:01:29.10,Default,,0000,0000,0000,,divided by its mass. Dialogue: 0,0:01:29.10,0:01:31.86,Default,,0000,0000,0000,,And of course, since we're\Ndealing with vectors, Dialogue: 0,0:01:31.86,0:01:33.99,Default,,0000,0000,0000,,we can put arrow marks over here. Dialogue: 0,0:01:33.99,0:01:35.82,Default,,0000,0000,0000,,The direction of the\Nacceleration will be the same Dialogue: 0,0:01:35.82,0:01:37.14,Default,,0000,0000,0000,,as the direction of the net force. Dialogue: 0,0:01:37.14,0:01:40.35,Default,,0000,0000,0000,,Okay, now we can calculate\Nthe net force from this Dialogue: 0,0:01:40.35,0:01:42.15,Default,,0000,0000,0000,,and we can calculate, we know the mass, Dialogue: 0,0:01:42.15,0:01:43.80,Default,,0000,0000,0000,,so from that we can\Ncalculate the acceleration. Dialogue: 0,0:01:43.80,0:01:45.24,Default,,0000,0000,0000,,So why don't you pause the video Dialogue: 0,0:01:45.24,0:01:46.68,Default,,0000,0000,0000,,and see if you can plug in the numbers Dialogue: 0,0:01:46.68,0:01:48.73,Default,,0000,0000,0000,,and find the acceleration yourself first. Dialogue: 0,0:01:49.98,0:01:51.54,Default,,0000,0000,0000,,All right, let's try. Dialogue: 0,0:01:51.54,0:01:54.30,Default,,0000,0000,0000,,So our acceleration\Nwould be the net force. Dialogue: 0,0:01:54.30,0:01:55.89,Default,,0000,0000,0000,,How do I figure the net force out? Dialogue: 0,0:01:55.89,0:01:57.00,Default,,0000,0000,0000,,Well, the total force, Dialogue: 0,0:01:57.00,0:01:59.43,Default,,0000,0000,0000,,well they're since in\Nthe opposite direction, Dialogue: 0,0:01:59.43,0:02:00.54,Default,,0000,0000,0000,,we'll subtract them. Dialogue: 0,0:02:00.54,0:02:02.40,Default,,0000,0000,0000,,So, I'll just take the bigger number. Dialogue: 0,0:02:02.40,0:02:06.00,Default,,0000,0000,0000,,So 9,800 newtons, which\Nis acting downwards. Dialogue: 0,0:02:06.00,0:02:08.01,Default,,0000,0000,0000,,We need to take care of the direction. Dialogue: 0,0:02:08.01,0:02:10.65,Default,,0000,0000,0000,,From that, I'll subtract\Nthe smaller number, Dialogue: 0,0:02:10.65,0:02:15.65,Default,,0000,0000,0000,,that is 7,800 newtons\Nupwards divided by the mass, Dialogue: 0,0:02:16.35,0:02:18.90,Default,,0000,0000,0000,,which is 1,000 kilograms. Dialogue: 0,0:02:20.61,0:02:22.32,Default,,0000,0000,0000,,And if you simplify, Dialogue: 0,0:02:22.32,0:02:26.88,Default,,0000,0000,0000,,we will get 9,800 minus 7,800 is 2,000. Dialogue: 0,0:02:26.88,0:02:29.64,Default,,0000,0000,0000,,Nice numbers, 2000 newtons. Dialogue: 0,0:02:29.64,0:02:31.38,Default,,0000,0000,0000,,But what direction is it? Dialogue: 0,0:02:31.38,0:02:34.66,Default,,0000,0000,0000,,The downward one wins, right? It's bigger. Dialogue: 0,0:02:34.66,0:02:36.75,Default,,0000,0000,0000,,So, the net force will be\Nin the downward direction. Dialogue: 0,0:02:36.75,0:02:41.75,Default,,0000,0000,0000,,It's divided by 1,000 kilograms\Nand that gives us two. Dialogue: 0,0:02:42.84,0:02:45.21,Default,,0000,0000,0000,,So, our acceleration becomes Dialogue: 0,0:02:45.21,0:02:49.62,Default,,0000,0000,0000,,two meters per second square downwards. Dialogue: 0,0:02:49.62,0:02:50.79,Default,,0000,0000,0000,,Okay, we found our answer, Dialogue: 0,0:02:50.79,0:02:54.33,Default,,0000,0000,0000,,but one of the best ways to\Ngain deeper insights is to try Dialogue: 0,0:02:54.33,0:02:55.89,Default,,0000,0000,0000,,and see if this kind of makes sense. Dialogue: 0,0:02:55.89,0:02:58.05,Default,,0000,0000,0000,,Can you get a feeling for\Nwhat's going on over here? Dialogue: 0,0:02:58.05,0:02:59.91,Default,,0000,0000,0000,,Okay, now the first question Dialogue: 0,0:02:59.91,0:03:01.83,Default,,0000,0000,0000,,that we could be having\Nover here is, look, Dialogue: 0,0:03:01.83,0:03:03.72,Default,,0000,0000,0000,,the net force is downwards Dialogue: 0,0:03:03.72,0:03:04.86,Default,,0000,0000,0000,,because gravity is winning, right? Dialogue: 0,0:03:04.86,0:03:07.29,Default,,0000,0000,0000,,So the total force acting on\Nthis elevator is downwards Dialogue: 0,0:03:07.29,0:03:10.08,Default,,0000,0000,0000,,and yet the elevator is moving up. Dialogue: 0,0:03:10.08,0:03:11.88,Default,,0000,0000,0000,,Why is that? (chuckles) Dialogue: 0,0:03:11.88,0:03:15.09,Default,,0000,0000,0000,,Well, remember, force does not dictate Dialogue: 0,0:03:15.09,0:03:17.07,Default,,0000,0000,0000,,the direction of motion. Dialogue: 0,0:03:17.07,0:03:20.94,Default,,0000,0000,0000,,Force tells you the direction\Nof acceleration, okay? Dialogue: 0,0:03:20.94,0:03:22.02,Default,,0000,0000,0000,,The elevator could be moving, Dialogue: 0,0:03:22.02,0:03:24.24,Default,,0000,0000,0000,,objects can be moving\Nwhatever direction it wants. Dialogue: 0,0:03:24.24,0:03:25.77,Default,,0000,0000,0000,,When you put a force on it, it tells you Dialogue: 0,0:03:25.77,0:03:27.21,Default,,0000,0000,0000,,what direction it should accelerate. Dialogue: 0,0:03:27.21,0:03:28.17,Default,,0000,0000,0000,,So that's the key thing. Dialogue: 0,0:03:28.17,0:03:31.32,Default,,0000,0000,0000,,So, there is no problem that\Nthe force is acting downwards, Dialogue: 0,0:03:31.32,0:03:32.91,Default,,0000,0000,0000,,but the elevator is moving upwards. Dialogue: 0,0:03:32.91,0:03:34.53,Default,,0000,0000,0000,,Secondly, what does it mean Dialogue: 0,0:03:34.53,0:03:36.21,Default,,0000,0000,0000,,that the acceleration is downwards? Dialogue: 0,0:03:36.21,0:03:38.01,Default,,0000,0000,0000,,See, if the net force is downwards, Dialogue: 0,0:03:38.01,0:03:39.06,Default,,0000,0000,0000,,the acceleration has to be downward. Dialogue: 0,0:03:39.06,0:03:41.13,Default,,0000,0000,0000,,But what does it mean\Nthe elevator is moving up Dialogue: 0,0:03:41.13,0:03:42.75,Default,,0000,0000,0000,,and the acceleration is downwards? Dialogue: 0,0:03:42.75,0:03:46.56,Default,,0000,0000,0000,,Ooh, this means since the velocity Dialogue: 0,0:03:46.56,0:03:48.72,Default,,0000,0000,0000,,and the acceleration is\Nin the opposite direction, Dialogue: 0,0:03:48.72,0:03:52.74,Default,,0000,0000,0000,,this means the elevator is slowing down. Dialogue: 0,0:03:52.74,0:03:54.09,Default,,0000,0000,0000,,That's what it means for velocity Dialogue: 0,0:03:54.09,0:03:55.56,Default,,0000,0000,0000,,and acceleration to be in\Nthe opposite direction. Dialogue: 0,0:03:55.56,0:03:56.46,Default,,0000,0000,0000,,If they're in the same direction, Dialogue: 0,0:03:56.46,0:03:57.78,Default,,0000,0000,0000,,it means they're speeding up. Dialogue: 0,0:03:57.78,0:04:00.24,Default,,0000,0000,0000,,So, this means our elevator is going up, Dialogue: 0,0:04:00.24,0:04:03.15,Default,,0000,0000,0000,,but it is slowing down, which\Nprobably means that, you know, Dialogue: 0,0:04:03.15,0:04:04.86,Default,,0000,0000,0000,,it's probably about to stop. Dialogue: 0,0:04:04.86,0:04:06.99,Default,,0000,0000,0000,,Maybe this person has probably reached Dialogue: 0,0:04:06.99,0:04:08.67,Default,,0000,0000,0000,,his destination or something. Dialogue: 0,0:04:08.67,0:04:10.56,Default,,0000,0000,0000,,All right, onto the next problem. Dialogue: 0,0:04:10.56,0:04:15.12,Default,,0000,0000,0000,,This time we have a sledge at\Nrest whose is 70 kilograms. Dialogue: 0,0:04:15.12,0:04:16.62,Default,,0000,0000,0000,,Our goal is to push it Dialogue: 0,0:04:16.62,0:04:18.81,Default,,0000,0000,0000,,and accelerate to six meters per second Dialogue: 0,0:04:18.81,0:04:20.43,Default,,0000,0000,0000,,in about two seconds, let's say, Dialogue: 0,0:04:20.43,0:04:22.11,Default,,0000,0000,0000,,so that, you know, it\Ncan nicely slide down Dialogue: 0,0:04:22.11,0:04:23.76,Default,,0000,0000,0000,,and we can enjoy the ride. Dialogue: 0,0:04:23.76,0:04:26.01,Default,,0000,0000,0000,,Now, there is going to\Nbe some frictional force. Dialogue: 0,0:04:26.01,0:04:28.14,Default,,0000,0000,0000,,Even though we're on ice and everything, Dialogue: 0,0:04:28.14,0:04:29.37,Default,,0000,0000,0000,,there'll be some frictional force. Dialogue: 0,0:04:29.37,0:04:30.20,Default,,0000,0000,0000,,Let's say the friction Dialogue: 0,0:04:30.20,0:04:32.85,Default,,0000,0000,0000,,that this ledge will experience\Nis about 200 newtons. Dialogue: 0,0:04:33.69,0:04:36.84,Default,,0000,0000,0000,,The question now is what is\Nthe force with which we have Dialogue: 0,0:04:36.84,0:04:39.99,Default,,0000,0000,0000,,to push on it so as to\Nachieve all of this? Dialogue: 0,0:04:39.99,0:04:42.06,Default,,0000,0000,0000,,So how do we figure this out? Dialogue: 0,0:04:42.06,0:04:43.98,Default,,0000,0000,0000,,Well, again, the first thing\Nthat comes to my mind is Dialogue: 0,0:04:43.98,0:04:47.01,Default,,0000,0000,0000,,that look, we're dealing with\Nforces and we have motion. Dialogue: 0,0:04:47.01,0:04:50.49,Default,,0000,0000,0000,,What connects forces and\Nmotion? Newton's second law. Dialogue: 0,0:04:50.49,0:04:52.32,Default,,0000,0000,0000,,So, the first thing I'll\Ndo is I'm gonna draw Dialogue: 0,0:04:52.32,0:04:53.31,Default,,0000,0000,0000,,a free-body diagram. Dialogue: 0,0:04:53.31,0:04:56.43,Default,,0000,0000,0000,,Again, I encourage you\Nto pause the video now Dialogue: 0,0:04:56.43,0:04:57.60,Default,,0000,0000,0000,,or anytime later on. Dialogue: 0,0:04:57.60,0:04:58.71,Default,,0000,0000,0000,,Whenever you feel more comfortable, Dialogue: 0,0:04:58.71,0:05:01.05,Default,,0000,0000,0000,,pause the video and see if\Nyou can complete it yourself. Dialogue: 0,0:05:01.05,0:05:03.12,Default,,0000,0000,0000,,Okay, so anyways, let's first try Dialogue: 0,0:05:03.12,0:05:04.23,Default,,0000,0000,0000,,to draw a free-body diagram. Dialogue: 0,0:05:04.23,0:05:05.06,Default,,0000,0000,0000,,How do we do that? Dialogue: 0,0:05:05.06,0:05:06.78,Default,,0000,0000,0000,,Well, again, we'll take\Nthe object of our interest. Dialogue: 0,0:05:06.78,0:05:09.51,Default,,0000,0000,0000,,In this case, the object of\Nour interest is this ledge Dialogue: 0,0:05:09.51,0:05:12.93,Default,,0000,0000,0000,,whose mass is 70 kilograms\Nand just make it a square Dialogue: 0,0:05:12.93,0:05:14.37,Default,,0000,0000,0000,,and then draw all the forces acting on it. Dialogue: 0,0:05:14.37,0:05:15.45,Default,,0000,0000,0000,,What are the forces acting on it? Dialogue: 0,0:05:15.45,0:05:18.00,Default,,0000,0000,0000,,I know there's frictional\Nforce acting backwards Dialogue: 0,0:05:18.00,0:05:21.66,Default,,0000,0000,0000,,of 200 newtons, but then I\Nalso have the applied force Dialogue: 0,0:05:21.66,0:05:23.58,Default,,0000,0000,0000,,by this person, which I,\Nwe need to figure out. Dialogue: 0,0:05:23.58,0:05:24.81,Default,,0000,0000,0000,,This is what we need to calculate. Dialogue: 0,0:05:24.81,0:05:27.78,Default,,0000,0000,0000,,That is the applied force.\NWhat are other forces? Dialogue: 0,0:05:27.78,0:05:30.60,Default,,0000,0000,0000,,Well, I know that there's\Nalso gravity acting on it Dialogue: 0,0:05:30.60,0:05:33.15,Default,,0000,0000,0000,,and then there's a normal\Nforce acting on it upwards. Dialogue: 0,0:05:33.15,0:05:35.22,Default,,0000,0000,0000,,But we know that these forces are balanced Dialogue: 0,0:05:35.22,0:05:37.59,Default,,0000,0000,0000,,and so they're not going to be useful. Dialogue: 0,0:05:37.59,0:05:41.19,Default,,0000,0000,0000,,They're not going to affect\Nour situation over here. Dialogue: 0,0:05:41.19,0:05:42.84,Default,,0000,0000,0000,,So, I'm just gonna draw them over here. Dialogue: 0,0:05:42.84,0:05:44.95,Default,,0000,0000,0000,,They're there, of course, but\Nthey're completely balanced. Dialogue: 0,0:05:44.95,0:05:46.74,Default,,0000,0000,0000,,They're in the verticals,\Nthey're not gonna affect it, Dialogue: 0,0:05:46.74,0:05:48.15,Default,,0000,0000,0000,,so we'll not draw it. Dialogue: 0,0:05:48.15,0:05:50.16,Default,,0000,0000,0000,,Okay, now that I have\Nmy free-body diagram, Dialogue: 0,0:05:50.16,0:05:52.47,Default,,0000,0000,0000,,let's go ahead and write\Ndown on Newton second law. Dialogue: 0,0:05:52.47,0:05:57.47,Default,,0000,0000,0000,,It says acceleration should\Nalways equal the net force, Dialogue: 0,0:05:57.93,0:06:00.42,Default,,0000,0000,0000,,which let me draw using pink now. Dialogue: 0,0:06:00.42,0:06:04.92,Default,,0000,0000,0000,,Net force divided by the mass. Dialogue: 0,0:06:04.92,0:06:06.51,Default,,0000,0000,0000,,Divided by the mass. Dialogue: 0,0:06:06.51,0:06:09.87,Default,,0000,0000,0000,,Okay, so what do we do next? Dialogue: 0,0:06:09.87,0:06:12.63,Default,,0000,0000,0000,,I need to find this applied force. Dialogue: 0,0:06:12.63,0:06:16.62,Default,,0000,0000,0000,,So this means if I can\Ncalculate the net force, Dialogue: 0,0:06:16.62,0:06:19.65,Default,,0000,0000,0000,,then I can figure out what\Nthe applied force is, right? Dialogue: 0,0:06:19.65,0:06:21.15,Default,,0000,0000,0000,,So, from Newton's second law, Dialogue: 0,0:06:21.15,0:06:22.77,Default,,0000,0000,0000,,I just need to figure out\Nwhat the net force is. Dialogue: 0,0:06:22.77,0:06:25.47,Default,,0000,0000,0000,,For that, I ask myself, do I know m? Dialogue: 0,0:06:25.47,0:06:27.96,Default,,0000,0000,0000,,I do. I know that m is 70 kilograms. Dialogue: 0,0:06:27.96,0:06:29.94,Default,,0000,0000,0000,,Do I know the acceleration? Dialogue: 0,0:06:29.94,0:06:31.14,Default,,0000,0000,0000,,Hmm, it's not given directly, Dialogue: 0,0:06:31.14,0:06:34.74,Default,,0000,0000,0000,,but wait a second, I know\Nthe initial velocity, Dialogue: 0,0:06:34.74,0:06:36.18,Default,,0000,0000,0000,,I know the final velocity and I know Dialogue: 0,0:06:36.18,0:06:38.73,Default,,0000,0000,0000,,that the change in velocity\Nshould happen in two seconds. Dialogue: 0,0:06:38.73,0:06:39.94,Default,,0000,0000,0000,,Whoo-hoo! Dialogue: 0,0:06:39.94,0:06:43.86,Default,,0000,0000,0000,,That means I can calculate the\Nacceleration from this data. Dialogue: 0,0:06:43.86,0:06:46.14,Default,,0000,0000,0000,,I can plug in and from\Nthat I can figure out Dialogue: 0,0:06:46.14,0:06:48.63,Default,,0000,0000,0000,,what the total force is going to be, Dialogue: 0,0:06:48.63,0:06:50.07,Default,,0000,0000,0000,,the net force is going to be. Dialogue: 0,0:06:50.07,0:06:52.59,Default,,0000,0000,0000,,And from that we can figure out Dialogue: 0,0:06:52.59,0:06:54.33,Default,,0000,0000,0000,,what the applied force should be. Dialogue: 0,0:06:54.33,0:06:56.28,Default,,0000,0000,0000,,Okay, so if you haven't done this before, Dialogue: 0,0:06:56.28,0:06:57.69,Default,,0000,0000,0000,,why don't you now pause the video Dialogue: 0,0:06:57.69,0:06:59.43,Default,,0000,0000,0000,,and see if you can put it all together Dialogue: 0,0:06:59.43,0:07:01.56,Default,,0000,0000,0000,,and solve the problem? Dialogue: 0,0:07:01.56,0:07:03.18,Default,,0000,0000,0000,,Okay, let's do this. Dialogue: 0,0:07:03.18,0:07:05.13,Default,,0000,0000,0000,,So, let me first calculate\Nthe acceleration. Dialogue: 0,0:07:05.13,0:07:06.24,Default,,0000,0000,0000,,So our acceleration is going Dialogue: 0,0:07:06.24,0:07:07.62,Default,,0000,0000,0000,,to be, well how do we figure this out? Dialogue: 0,0:07:07.62,0:07:11.67,Default,,0000,0000,0000,,This is the final velocity\Nminus the initial velocity Dialogue: 0,0:07:11.67,0:07:13.95,Default,,0000,0000,0000,,divided by the time taken. Dialogue: 0,0:07:13.95,0:07:15.30,Default,,0000,0000,0000,,So, that's going to be, in our case, Dialogue: 0,0:07:15.30,0:07:17.50,Default,,0000,0000,0000,,the final velocity is\Nsix meters per second Dialogue: 0,0:07:19.26,0:07:21.81,Default,,0000,0000,0000,,to the right minus... Dialogue: 0,0:07:21.81,0:07:23.04,Default,,0000,0000,0000,,What's the initial velocity? Dialogue: 0,0:07:23.04,0:07:25.65,Default,,0000,0000,0000,,Well, it's zero, it's at rest. Dialogue: 0,0:07:25.65,0:07:27.69,Default,,0000,0000,0000,,So there is no initial velocity divided Dialogue: 0,0:07:27.69,0:07:30.36,Default,,0000,0000,0000,,by time is two, two seconds. Dialogue: 0,0:07:30.36,0:07:31.59,Default,,0000,0000,0000,,That gives us what? Dialogue: 0,0:07:31.59,0:07:36.59,Default,,0000,0000,0000,,That gives us six by two is\Nthree meters per second squared Dialogue: 0,0:07:37.56,0:07:39.87,Default,,0000,0000,0000,,to the right. Dialogue: 0,0:07:39.87,0:07:42.39,Default,,0000,0000,0000,,So I know my acceleration has to be Dialogue: 0,0:07:42.39,0:07:44.61,Default,,0000,0000,0000,,to the right, which is\Ngood news. (chuckles) Dialogue: 0,0:07:44.61,0:07:45.66,Default,,0000,0000,0000,,It has to be to the right. Dialogue: 0,0:07:45.66,0:07:46.92,Default,,0000,0000,0000,,We want this ledge to move to the right. Dialogue: 0,0:07:46.92,0:07:47.75,Default,,0000,0000,0000,,So, that makes sense. Dialogue: 0,0:07:47.75,0:07:49.44,Default,,0000,0000,0000,,So, we are on the right track over here. Dialogue: 0,0:07:49.44,0:07:50.73,Default,,0000,0000,0000,,Okay, now, I can plug in Dialogue: 0,0:07:50.73,0:07:52.56,Default,,0000,0000,0000,,and figure out what the\Nnet force is going to be. Dialogue: 0,0:07:52.56,0:07:56.10,Default,,0000,0000,0000,,So, if I just simplify that,\Nso net force is going to be, Dialogue: 0,0:07:56.10,0:07:58.77,Default,,0000,0000,0000,,if I rearrange this equation\Nmultiply by m on both sides. Dialogue: 0,0:07:58.77,0:08:02.22,Default,,0000,0000,0000,,So I get net force to be\Nmass times the acceleration. Dialogue: 0,0:08:03.24,0:08:06.06,Default,,0000,0000,0000,,And so now I can plug in\Nfor mass and acceleration. Dialogue: 0,0:08:06.06,0:08:09.15,Default,,0000,0000,0000,,What do I get? Well I get 70 kilograms. Dialogue: 0,0:08:09.15,0:08:10.53,Default,,0000,0000,0000,,That's the mass, Dialogue: 0,0:08:10.53,0:08:14.16,Default,,0000,0000,0000,,times the acceleration is three\Nmeters per second squared, Dialogue: 0,0:08:14.16,0:08:15.62,Default,,0000,0000,0000,,70 times three, seven times these 21. Dialogue: 0,0:08:15.62,0:08:18.09,Default,,0000,0000,0000,,So, this is 210. Dialogue: 0,0:08:18.09,0:08:21.91,Default,,0000,0000,0000,,So, I get that my net force is 210 newtons Dialogue: 0,0:08:23.61,0:08:25.23,Default,,0000,0000,0000,,to the right of this, Dialogue: 0,0:08:25.23,0:08:27.21,Default,,0000,0000,0000,,this acceleration was to the right. Dialogue: 0,0:08:27.21,0:08:30.66,Default,,0000,0000,0000,,So, this will also be to the right. Dialogue: 0,0:08:30.66,0:08:34.74,Default,,0000,0000,0000,,So, now that I've found my\Nnet force, so my net force, Dialogue: 0,0:08:34.74,0:08:37.86,Default,,0000,0000,0000,,the total force will be to the right Dialogue: 0,0:08:37.86,0:08:40.02,Default,,0000,0000,0000,,and that is 210 newtons. Dialogue: 0,0:08:40.02,0:08:44.07,Default,,0000,0000,0000,,From that, can I calculate\Nthe applied force? Yes. Dialogue: 0,0:08:44.07,0:08:45.03,Default,,0000,0000,0000,,So now, first of all, Dialogue: 0,0:08:45.03,0:08:47.55,Default,,0000,0000,0000,,I know my applied force should be bigger. Dialogue: 0,0:08:47.55,0:08:48.66,Default,,0000,0000,0000,,It has to be bigger Dialogue: 0,0:08:48.66,0:08:51.51,Default,,0000,0000,0000,,because my total force\Nshould be towards the right, Dialogue: 0,0:08:51.51,0:08:54.03,Default,,0000,0000,0000,,which means if I subtract\Nthe two from this, Dialogue: 0,0:08:54.03,0:08:57.03,Default,,0000,0000,0000,,if I subtract this number,\NI should get this number. Dialogue: 0,0:08:57.03,0:08:58.53,Default,,0000,0000,0000,,That's the net force, all right? Dialogue: 0,0:08:58.53,0:09:00.39,Default,,0000,0000,0000,,So now, now that I know all the directions Dialogue: 0,0:09:00.39,0:09:02.07,Default,,0000,0000,0000,,and everything, I can\Njust subtract the numbers. Dialogue: 0,0:09:02.07,0:09:04.53,Default,,0000,0000,0000,,So I can now say, "Hey, my net force Dialogue: 0,0:09:04.53,0:09:08.64,Default,,0000,0000,0000,,that is 210 newtons should\Nequal this bigger number, Dialogue: 0,0:09:08.64,0:09:13.44,Default,,0000,0000,0000,,the bigger force minus 200 newtons. Dialogue: 0,0:09:14.61,0:09:17.85,Default,,0000,0000,0000,,Now, to get applied force,\NI just add 200 on both sides Dialogue: 0,0:09:17.85,0:09:20.28,Default,,0000,0000,0000,,so that I can get rid of this. Dialogue: 0,0:09:22.05,0:09:27.05,Default,,0000,0000,0000,,And look, the applied\Nforce becomes 210 plus 200. Dialogue: 0,0:09:27.15,0:09:31.17,Default,,0000,0000,0000,,That is 410 newtons. Dialogue: 0,0:09:31.17,0:09:32.88,Default,,0000,0000,0000,,And I already know it's to the right, Dialogue: 0,0:09:32.88,0:09:34.44,Default,,0000,0000,0000,,so I know it's direction. Dialogue: 0,0:09:34.44,0:09:37.11,Default,,0000,0000,0000,,So if I were to write down its direction, Dialogue: 0,0:09:37.11,0:09:39.12,Default,,0000,0000,0000,,it's going to be, oops. (chuckles) Dialogue: 0,0:09:39.12,0:09:39.99,Default,,0000,0000,0000,,Okay. Okay. Dialogue: 0,0:09:39.99,0:09:42.99,Default,,0000,0000,0000,,I mean, okay, not the most\Norganized board over here, Dialogue: 0,0:09:42.99,0:09:45.42,Default,,0000,0000,0000,,but let me just write it down\Na little bit more neatly. Dialogue: 0,0:09:45.42,0:09:49.41,Default,,0000,0000,0000,,So 410 newtons to the right. Dialogue: 0,0:09:49.41,0:09:51.39,Default,,0000,0000,0000,,That's how much force we need to apply Dialogue: 0,0:09:51.39,0:09:53.01,Default,,0000,0000,0000,,for all of this to happen.