1 00:00:00,601 --> 00:00:01,434 - [Voiceover] We should talk a little more 2 00:00:01,434 --> 00:00:02,572 about Newtons's Third Law, 3 00:00:02,572 --> 00:00:04,482 because there are some deep misconceptions 4 00:00:04,482 --> 00:00:06,493 that many people have about this law. 5 00:00:06,493 --> 00:00:08,148 It seems simple, but it's not nearly 6 00:00:08,148 --> 00:00:09,732 as simple as you might think. 7 00:00:09,732 --> 00:00:11,358 So people often phrase it as, 8 00:00:11,358 --> 00:00:14,193 for every action there's an equal and opposite reaction. 9 00:00:14,193 --> 00:00:16,510 But that's just way too vague to be useful. 10 00:00:16,510 --> 00:00:18,115 So a version that's a little better, 11 00:00:18,115 --> 00:00:21,821 says that for every force, there's an equal 12 00:00:21,821 --> 00:00:23,612 and opposite force. 13 00:00:23,612 --> 00:00:24,849 So this is a little better. 14 00:00:24,849 --> 00:00:26,933 The equal sign means that these forces 15 00:00:26,933 --> 00:00:28,567 are equal in magnitude. 16 00:00:28,567 --> 00:00:31,391 And this negative sign means they're just different 17 00:00:31,391 --> 00:00:33,272 by the direction of the vector. 18 00:00:33,272 --> 00:00:35,771 So these are vectors, so this says that this pink 19 00:00:35,771 --> 00:00:38,087 vector F, has the opposite direction, 20 00:00:38,087 --> 00:00:41,947 but equal in magnitude to this green vector F. 21 00:00:41,947 --> 00:00:44,594 But to show you why this is still a little bit too vague, 22 00:00:44,594 --> 00:00:46,254 consider this, if this is all you knew about 23 00:00:46,254 --> 00:00:48,347 Newtons's Third Law, that for every force, 24 00:00:48,347 --> 00:00:50,167 there's an equal and opposite force, 25 00:00:50,167 --> 00:00:52,117 you might wonder, if you were clever, 26 00:00:52,117 --> 00:00:54,230 you might be like, wait a minute, 27 00:00:54,230 --> 00:00:58,063 if for every force F, right, there's got to be 28 00:00:59,319 --> 00:01:02,152 a force that's equal and opposite. 29 00:01:03,249 --> 00:01:04,418 Well why doesn't that just mean 30 00:01:04,418 --> 00:01:07,149 that every force in the universe cancels? 31 00:01:07,149 --> 00:01:09,782 Shouldn't every force just cancel then, at that point? 32 00:01:09,782 --> 00:01:12,240 Doesn't that just mean that there's 33 00:01:12,240 --> 00:01:15,024 no acceleration that's even possible? 34 00:01:15,024 --> 00:01:17,918 Because if I go and exert a force F on something, 35 00:01:17,918 --> 00:01:19,836 if there's gonna be a force negative F, 36 00:01:19,836 --> 00:01:22,803 doesn't that mean that no matter what force I put forward, 37 00:01:22,803 --> 00:01:24,501 it's just gonna get cancelled? 38 00:01:24,501 --> 00:01:26,765 And the answer no, and the reason it's no 39 00:01:26,765 --> 00:01:28,512 is because these two forces 40 00:01:28,512 --> 00:01:30,474 are exerted on different objects. 41 00:01:30,474 --> 00:01:31,844 So you have to be careful. 42 00:01:31,844 --> 00:01:33,225 So the reason I say that this statement 43 00:01:33,225 --> 00:01:35,846 of Newtons's Third Law is still a little bit too vague, 44 00:01:35,846 --> 00:01:38,244 is because this is really on different objects. 45 00:01:38,244 --> 00:01:41,190 So if this is the force on object A, 46 00:01:41,190 --> 00:01:45,233 exerted by object B, then this force over here 47 00:01:45,233 --> 00:01:49,400 has to be the force on object B, exerted by object A. 48 00:01:53,238 --> 00:01:55,554 In other words, these forces down here are exerted 49 00:01:55,554 --> 00:01:56,712 on different objects. 50 00:01:56,712 --> 00:01:58,988 I'm gonna move this over to this side. 51 00:01:58,988 --> 00:02:00,930 I'm gonna move this over to here. 52 00:02:00,930 --> 00:02:02,230 Let's draw two different objects 53 00:02:02,230 --> 00:02:04,912 to show explicitly what I mean. 54 00:02:04,912 --> 00:02:07,388 So if there was some object A, 55 00:02:07,388 --> 00:02:09,919 so I put some object A in here. 56 00:02:09,919 --> 00:02:11,451 Just wanna make sure there's an object A. 57 00:02:11,451 --> 00:02:13,413 Let's say this is object A, and it had 58 00:02:13,413 --> 00:02:16,937 this green force exerted on it, F. 59 00:02:16,937 --> 00:02:19,651 So this object right here is A. 60 00:02:19,651 --> 00:02:23,166 Well, there's gonna be another object, object B. 61 00:02:23,166 --> 00:02:26,689 We'll just make it another circle. 62 00:02:26,689 --> 00:02:29,424 So we'll make it look like this. 63 00:02:29,424 --> 00:02:31,007 So here's object B. 64 00:02:31,992 --> 00:02:34,015 And it's gonna have this pink force, 65 00:02:34,015 --> 00:02:35,824 F, negative F exerted on it. 66 00:02:35,824 --> 00:02:37,681 So I'm gonna call this object B. 67 00:02:37,681 --> 00:02:41,867 Now we're okay, now we know these forces can't cancel, 68 00:02:41,867 --> 00:02:43,625 and the reason these forces can't cancel, 69 00:02:43,625 --> 00:02:45,666 is cause they're on two different objects. 70 00:02:45,666 --> 00:02:46,925 But when you just say that Newtons's Third Law, 71 00:02:46,925 --> 00:02:48,710 is that every force has an equal and opposite force, 72 00:02:48,710 --> 00:02:51,311 it's not clear that it has to be on different objects. 73 00:02:51,311 --> 00:02:53,180 But it does have to be on different objects. 74 00:02:53,180 --> 00:02:55,630 So these Newtons force law pairs, 75 00:02:55,630 --> 00:02:57,711 often times is called force pairs, 76 00:02:57,711 --> 00:03:00,210 or Newton's third law partner forces, 77 00:03:00,210 --> 00:03:02,587 are always on different objects. 78 00:03:02,587 --> 00:03:05,371 So the convention I'm using is that the first letter 79 00:03:05,371 --> 00:03:09,548 represents the object that the force is on. 80 00:03:09,548 --> 00:03:11,862 So this A represents that this green force F 81 00:03:11,862 --> 00:03:16,029 this green force F, is on A and it's exerted by B. 82 00:03:18,183 --> 00:03:20,946 And this shows that it's exerted on B, 83 00:03:20,946 --> 00:03:23,172 because the first letter's on the first one, 84 00:03:23,172 --> 00:03:26,589 and it's exerted by the second object, A. 85 00:03:27,723 --> 00:03:30,486 So this pink force is exerted on B. 86 00:03:30,486 --> 00:03:32,425 This green force is exerted on A. 87 00:03:32,425 --> 00:03:34,741 They're equal and opposite, they do not cancel, 88 00:03:34,741 --> 00:03:38,492 they cannot cancel because they're not on the same object. 89 00:03:38,492 --> 00:03:40,036 So that's why these don't cancel. 90 00:03:40,036 --> 00:03:41,702 And they are the same magnitude, 91 00:03:41,702 --> 00:03:43,429 even if the two objects are not the same size. 92 00:03:43,429 --> 00:03:45,144 This is another misconception, 93 00:03:45,144 --> 00:03:48,090 if object A is a planet, a big planet. 94 00:03:48,090 --> 00:03:50,885 Or maybe a star, this is yellow, it looks like a star. 95 00:03:50,885 --> 00:03:53,018 Let's say this is some big star, 96 00:03:53,018 --> 00:03:55,486 and this is some smaller planet orbiting that star. 97 00:03:55,486 --> 00:03:59,424 This is not to scale, unless this planet was enormous. 98 00:03:59,424 --> 00:04:02,167 So this is some planet, but this planet could be 99 00:04:02,167 --> 00:04:05,540 hundreds, thousands of times, millions of times 100 00:04:05,540 --> 00:04:08,892 less massive than this star but it would still 101 00:04:08,892 --> 00:04:10,010 exert the same force. 102 00:04:10,010 --> 00:04:11,951 So if this star is pulling on the planet 103 00:04:11,951 --> 00:04:15,040 with this pink force negative F, 104 00:04:15,040 --> 00:04:17,324 then this planet has to be pulling on the star 105 00:04:17,324 --> 00:04:19,144 with this green force F and they have to have 106 00:04:19,144 --> 00:04:23,318 the same magnitude, even if they are different sizes. 107 00:04:23,318 --> 00:04:24,711 So people quote Newtons's Third Law, 108 00:04:24,711 --> 00:04:27,047 but sometimes they don't really believe it. 109 00:04:27,047 --> 00:04:28,571 If I told you this planet was a million times 110 00:04:28,571 --> 00:04:31,709 less massive than this star, people would want to say 111 00:04:31,709 --> 00:04:36,048 that well, then the star obviously pulls more on the planet, 112 00:04:36,048 --> 00:04:37,743 than the planet pulls on the star. 113 00:04:37,743 --> 00:04:40,172 But that's not true according to Newtons's Third Law. 114 00:04:40,172 --> 00:04:43,330 And Newtons's Third Law says that they have to be the same, 115 00:04:43,330 --> 00:04:44,570 even if they're different sizes. 116 00:04:44,570 --> 00:04:46,561 So if this was the earth and this was the moon, 117 00:04:46,561 --> 00:04:48,267 the earth pulls on the moon, just as much 118 00:04:48,267 --> 00:04:50,270 as the moon pulls on the earth. 119 00:04:50,270 --> 00:04:51,945 And you might still object, you might say, 120 00:04:51,945 --> 00:04:54,577 wait that makes no sense, I know the star just basically 121 00:04:54,577 --> 00:04:58,600 sits there and the planet gets whipped around in a circle. 122 00:04:58,600 --> 00:05:00,409 How come this planet's getting whipped around 123 00:05:00,409 --> 00:05:02,054 and the star's just staying put? 124 00:05:02,054 --> 00:05:04,867 That's because, just because the forces are equal, 125 00:05:04,867 --> 00:05:06,492 that doesn't mean that the result is equal. 126 00:05:06,492 --> 00:05:08,138 In other words, the forces could be equal, 127 00:05:08,138 --> 00:05:11,226 but the accelerations don't have to be equal. 128 00:05:11,226 --> 00:05:15,940 Acceleration is gonna be the net force divided by the mass. 129 00:05:15,940 --> 00:05:18,024 So even if the force is the same, 130 00:05:18,024 --> 00:05:19,426 you divide by that mass, you'll get 131 00:05:19,426 --> 00:05:22,697 a different acceleration and that's why the result of 132 00:05:22,697 --> 00:05:24,688 the force does not have to be the same, 133 00:05:24,688 --> 00:05:27,399 even though the forces do have to be the same, 134 00:05:27,399 --> 00:05:29,410 because of Newtons's Third Law. 135 00:05:29,410 --> 00:05:31,320 Another misconception people sometimes make, 136 00:05:31,320 --> 00:05:33,860 is they think there might be a delay in the creation 137 00:05:33,860 --> 00:05:36,391 of this Newtons's Third Law partner force. 138 00:05:36,391 --> 00:05:39,020 And people think, maybe if I exert this first force 139 00:05:39,020 --> 00:05:42,048 fast enough, I can catch the universe sleeping, 140 00:05:42,048 --> 00:05:43,888 and there might be some sort of delay 141 00:05:43,888 --> 00:05:45,603 in the creation of this other force. 142 00:05:45,603 --> 00:05:48,581 But that's not true, Newtons's Third Law is universal. 143 00:05:48,581 --> 00:05:49,719 No matter what the situation, 144 00:05:49,719 --> 00:05:52,532 no matter what the acceleration or non acceleration, 145 00:05:52,532 --> 00:05:54,900 or motion or no motion, whether one object 146 00:05:54,900 --> 00:05:57,562 is bigger or smaller, if their Newtons's Third Law 147 00:05:57,562 --> 00:06:00,447 partner forces, they are equal they are opposite 148 00:06:00,447 --> 00:06:02,060 and they are always equal and opposite, 149 00:06:02,060 --> 00:06:03,372 at every given moment in time. 150 00:06:03,372 --> 00:06:06,368 So even if I came in all guns a blazing, 151 00:06:06,368 --> 00:06:09,143 Chuck Norris style, trying to dropkick some wall. 152 00:06:09,143 --> 00:06:12,869 That does not look like the correct form for a drop kick. 153 00:06:12,869 --> 00:06:15,319 But even if I came in, flying at this wall, 154 00:06:15,319 --> 00:06:18,854 as soon as I start to make contact with the wall, 155 00:06:18,854 --> 00:06:20,143 I'm gonna exert a force on the wall, 156 00:06:20,143 --> 00:06:22,215 and the wall has to exert a force back. 157 00:06:22,215 --> 00:06:26,441 So I'd exert a force on the wall to the right. 158 00:06:26,441 --> 00:06:30,383 And this would be the force on the wall, by my foot. 159 00:06:30,383 --> 00:06:32,069 There'd have to be an equal and opposite force 160 00:06:32,069 --> 00:06:36,275 instantly transmitted backwards, on my foot. 161 00:06:36,275 --> 00:06:39,221 So this would be the force on my foot, by the wall. 162 00:06:39,221 --> 00:06:42,240 This happens instantaneously, there is no delay. 163 00:06:42,240 --> 00:06:43,743 You can't kick this wall fast enough, 164 00:06:43,743 --> 00:06:46,962 for this other force to not be generated instantaneously. 165 00:06:46,962 --> 00:06:49,664 As soon as your foot starts to exert any force on the wall 166 00:06:49,664 --> 00:06:51,798 what so ever, the wall is gonna start exerting 167 00:06:51,798 --> 00:06:54,317 that same force back on your foot. 168 00:06:54,317 --> 00:06:56,085 So Newtons's Third Law is universal, 169 00:06:56,085 --> 00:06:58,392 but people still have trouble identifying 170 00:06:58,392 --> 00:07:00,566 these third law partner forces. 171 00:07:00,566 --> 00:07:01,774 So one of the best ways to do it, 172 00:07:01,774 --> 00:07:03,259 is by listing both objects, 173 00:07:03,259 --> 00:07:05,413 as soon as you list both objects, 174 00:07:05,413 --> 00:07:06,856 well to figure out where the partner force is, 175 00:07:06,856 --> 00:07:09,079 you can just reverse these labels. 176 00:07:09,079 --> 00:07:11,711 So I know over here, if one of my forces 177 00:07:11,711 --> 00:07:14,211 is the force on the wall by my foot, 178 00:07:14,211 --> 00:07:16,292 to find the partner force to this force, 179 00:07:16,292 --> 00:07:18,852 I can just reverse the labels and say it's gotta be 180 00:07:18,852 --> 00:07:22,915 the force on my foot, by the wall, which I drew over here. 181 00:07:22,915 --> 00:07:24,329 So this is a great way to identify 182 00:07:24,329 --> 00:07:25,913 the third law partner forces, 183 00:07:25,913 --> 00:07:27,446 cause it's not always obvious 184 00:07:27,446 --> 00:07:29,417 what force is the partner force. 185 00:07:29,417 --> 00:07:30,920 So to show you how this can be tricky, 186 00:07:30,920 --> 00:07:32,557 consider this example. 187 00:07:32,557 --> 00:07:34,508 Say we got the ground and a table. 188 00:07:34,508 --> 00:07:38,031 So this example drives people crazy for some reason. 189 00:07:38,031 --> 00:07:40,217 If I've got a box sitting on a table, 190 00:07:40,217 --> 00:07:42,257 we'll call it box A. 191 00:07:42,257 --> 00:07:44,260 Box A is gonna have forces exerted on it. 192 00:07:44,260 --> 00:07:47,673 One of those forces is gonna be the gravitational force. 193 00:07:47,673 --> 00:07:49,360 So the force of gravity is gonna pull straight down 194 00:07:49,360 --> 00:07:51,595 on box A, and if I were to ask you, 195 00:07:51,595 --> 00:07:54,764 what force is the third law partner force to this force 196 00:07:54,764 --> 00:07:58,076 of gravity, I'm willing to bet a lot of people might say, 197 00:07:58,076 --> 00:08:00,941 well there's an upwards force on box A, 198 00:08:00,941 --> 00:08:02,578 exerted by the table. 199 00:08:02,578 --> 00:08:04,000 And that's true. 200 00:08:04,000 --> 00:08:06,081 And if this box A is just sitting here, 201 00:08:06,081 --> 00:08:08,478 not accelerating, these two forces are going 202 00:08:08,478 --> 00:08:09,953 to be equal and opposite. 203 00:08:09,953 --> 00:08:12,806 So it's even more tempting to say that these two forces 204 00:08:12,806 --> 00:08:15,183 are equal and opposite because of the third law, 205 00:08:15,183 --> 00:08:16,484 but that's not true. 206 00:08:16,484 --> 00:08:18,068 These two forces are equal and opposite 207 00:08:18,068 --> 00:08:19,592 because of the second law. 208 00:08:19,592 --> 00:08:21,929 The second law says if there's no acceleration, 209 00:08:21,929 --> 00:08:23,688 then the net force has to be zero, 210 00:08:23,688 --> 00:08:25,301 the forces have to cancel. 211 00:08:25,301 --> 00:08:26,308 And that's what's happening here. 212 00:08:26,308 --> 00:08:27,516 These forces are equal and opposite, 213 00:08:27,516 --> 00:08:29,762 they're canceling on box A. 214 00:08:29,762 --> 00:08:31,945 Which is a way to know that they are not third law 215 00:08:31,945 --> 00:08:34,293 partner forces, cause third law partner forces 216 00:08:34,293 --> 00:08:36,894 are always exerted on different objects. 217 00:08:36,894 --> 00:08:40,164 They can never cancel if they're third law partner forces. 218 00:08:40,164 --> 00:08:41,696 So what's going on over here? 219 00:08:41,697 --> 00:08:43,780 We've got two forces that are canceling, 220 00:08:43,780 --> 00:08:45,874 that are equal and opposite, but they're not third law 221 00:08:45,874 --> 00:08:49,044 partner forces, they're partner forces are somewhere else. 222 00:08:49,044 --> 00:08:50,832 I haven't drawn their partner forces yet. 223 00:08:50,832 --> 00:08:53,331 So let's try to figure out what they're partner forces are. 224 00:08:53,331 --> 00:08:55,220 So let's get rid of this, let's come back to here, 225 00:08:55,220 --> 00:08:58,126 let's slow it down to figure out what the partner force is, 226 00:08:58,126 --> 00:09:00,207 name the two objects interacting. 227 00:09:00,207 --> 00:09:02,868 So this force of gravity, I shouldn't be vague, 228 00:09:02,868 --> 00:09:05,571 I should call it the force on object A, 229 00:09:05,571 --> 00:09:09,625 our box A exerted by, well you can't just say gravity. 230 00:09:09,625 --> 00:09:11,300 Gravity is not an object. 231 00:09:11,300 --> 00:09:12,804 So the object that is exerting 232 00:09:12,804 --> 00:09:15,668 this gravitational force on A, is the earth. 233 00:09:15,668 --> 00:09:17,578 So this force really, this gravitational force, 234 00:09:17,578 --> 00:09:19,865 if I wanna be careful, is the force on object A 235 00:09:19,865 --> 00:09:21,174 exerted by the earth. 236 00:09:21,174 --> 00:09:23,827 Now it's easy to figure out where the partner force is. 237 00:09:23,827 --> 00:09:25,035 The partner force can be found just 238 00:09:25,035 --> 00:09:26,712 by reversing these labels. 239 00:09:26,712 --> 00:09:28,683 So instead of the force on A by the earth, 240 00:09:28,683 --> 00:09:30,390 there's gotta be an equal and opposite force, 241 00:09:30,390 --> 00:09:33,269 which is the force on the earth, by box A. 242 00:09:33,269 --> 00:09:35,768 So opposite means it has to point up. 243 00:09:35,768 --> 00:09:37,698 So it has to be an upward force. 244 00:09:37,698 --> 00:09:40,328 And that upward force has to be exerted on the earth, 245 00:09:40,328 --> 00:09:42,734 by box A, and this is kind of weird, 246 00:09:42,734 --> 00:09:44,984 because you may not have realized it, 247 00:09:44,984 --> 00:09:48,751 but if the earth is pulling down on a box, or you, 248 00:09:48,751 --> 00:09:50,905 that means you are pulling up on the earth. 249 00:09:50,905 --> 00:09:52,367 And this might seem ridiculous, 250 00:09:52,367 --> 00:09:55,639 I mean if you jump up, you jump up, you fall back down, 251 00:09:55,639 --> 00:09:57,577 you move around, but the earth just sits there. 252 00:09:57,577 --> 00:09:59,540 If your forces are equal, how come the earth doesn't 253 00:09:59,540 --> 00:10:01,765 move around like you do. 254 00:10:01,765 --> 00:10:03,452 And again, it's because just because 255 00:10:03,452 --> 00:10:05,594 the forces are the same, the acceleration 256 00:10:05,594 --> 00:10:06,865 doesn't have to be the same. 257 00:10:06,865 --> 00:10:08,897 The mass of the earth is so big, 258 00:10:08,897 --> 00:10:11,892 compared to your mass, there's basically no acceleration. 259 00:10:11,892 --> 00:10:13,985 Even though the forces on you 260 00:10:13,985 --> 00:10:16,655 and the forces on the earth are the same. 261 00:10:16,655 --> 00:10:19,532 So these two are third law partner forces. 262 00:10:19,532 --> 00:10:22,222 These two are joined together forever. 263 00:10:22,222 --> 00:10:25,758 They have to be equal, no matter what happens, 264 00:10:25,758 --> 00:10:27,891 these two forces will always be equal. 265 00:10:27,891 --> 00:10:29,812 I don't care if this box is accelerating 266 00:10:29,812 --> 00:10:33,255 or not accelerating, or that there's motion or no motion. 267 00:10:33,255 --> 00:10:35,376 Whether it's hitting a wall, sitting on a table, 268 00:10:35,376 --> 00:10:38,436 falling through space, these two forces must 269 00:10:38,436 --> 00:10:41,512 always be equal and opposite, because of the third law. 270 00:10:41,512 --> 00:10:42,833 So how about this other force, 271 00:10:42,833 --> 00:10:45,727 this force that the table was exerting. 272 00:10:45,727 --> 00:10:48,652 So this is, the force on A by the table. 273 00:10:48,652 --> 00:10:50,147 So if I wanna label it correctly, 274 00:10:50,147 --> 00:10:54,150 I'd call it the force on box A, exerted by the table. 275 00:10:54,150 --> 00:10:56,425 Now finding the third law partner force is easy, 276 00:10:56,425 --> 00:10:57,978 I can just reverse these labels, 277 00:10:57,978 --> 00:10:59,664 and I'd get that there must be, 278 00:10:59,664 --> 00:11:01,168 instead of an upwards force, a downwards 279 00:11:01,168 --> 00:11:03,008 force on the table, by A. 280 00:11:03,008 --> 00:11:05,527 So I'm gonna have another force here on the table. 281 00:11:05,527 --> 00:11:07,742 It's gonna be a downward force. 282 00:11:07,742 --> 00:11:09,693 Downward force on the table by A, 283 00:11:09,693 --> 00:11:12,871 that's the third law partner force to this upward 284 00:11:12,871 --> 00:11:14,923 force that the table is exerting. 285 00:11:14,923 --> 00:11:18,734 These two forces are also third law partner forces. 286 00:11:18,734 --> 00:11:20,632 these forces are going to be equal 287 00:11:20,632 --> 00:11:22,736 and opposite no matter what happens. 288 00:11:22,736 --> 00:11:25,000 This force on box A by the table. 289 00:11:25,000 --> 00:11:27,235 And this force on the table by box A 290 00:11:27,235 --> 00:11:30,425 must be equal no matter what happens, 291 00:11:30,425 --> 00:11:32,680 but the force on box A by the table, 292 00:11:32,680 --> 00:11:34,439 does not have to be equal and opposite 293 00:11:34,439 --> 00:11:36,541 to the force on A by the earth. 294 00:11:36,541 --> 00:11:38,096 It happens to be equal and opposite, 295 00:11:38,096 --> 00:11:40,128 in a case where there's no acceleration. 296 00:11:40,128 --> 00:11:43,489 If we stuck this whole situation into an elevator, 297 00:11:43,489 --> 00:11:47,939 or a rocket that had some huge acceleration upwards, 298 00:11:47,939 --> 00:11:50,052 even if there's acceleration upwards, 299 00:11:50,052 --> 00:11:52,470 these partner forces have to be equal. 300 00:11:52,470 --> 00:11:53,912 So the force on A by the table, 301 00:11:53,912 --> 00:11:56,472 and the force on the table by A will have to be equal. 302 00:11:56,472 --> 00:11:58,910 Similarly the force on the earth by A, 303 00:11:58,910 --> 00:12:00,861 and the force on A by the earth have to be equal. 304 00:12:00,861 --> 00:12:03,746 But no longer will these two forces have to be equal, 305 00:12:03,746 --> 00:12:05,473 cause they're not partner forces. 306 00:12:05,473 --> 00:12:08,155 They might be equal and opposite in some circumstances, 307 00:12:08,155 --> 00:12:10,105 but they don't always have to be equal and opposite. 308 00:12:10,105 --> 00:12:11,730 If we're accelerating upwards, 309 00:12:11,730 --> 00:12:13,417 this upward force on the box, 310 00:12:13,417 --> 00:12:15,916 must be bigger than the downwards force on the box. 311 00:12:15,916 --> 00:12:17,533 So these won't be equal. 312 00:12:17,533 --> 00:12:19,950 Recapping quickly, Newtons's Third Law is a statement 313 00:12:19,950 --> 00:12:22,124 about the forces on two different objects. 314 00:12:22,124 --> 00:12:23,819 And because it's about two different objects, 315 00:12:23,819 --> 00:12:25,993 those forces can never cancel. 316 00:12:25,993 --> 00:12:28,220 To find the Newtons's Third Law partner force, 317 00:12:28,220 --> 00:12:30,362 just reverse the label after you've identified 318 00:12:30,362 --> 00:12:32,405 the two objects that are interacting. 319 00:12:32,405 --> 00:12:36,001 The third law partner forces have to be equal in magnitude, 320 00:12:36,001 --> 00:12:38,448 even if one object is larger than the other, 321 00:12:38,448 --> 00:12:41,150 or has more charge or any property that might 322 00:12:41,150 --> 00:12:44,677 seem like it would convey more force, than another object. 323 00:12:44,677 --> 00:12:46,363 If those are the two objects interacting, 324 00:12:46,363 --> 00:12:47,977 their forces must be of equal magnitude 325 00:12:47,977 --> 00:12:51,738 and opposite directions, the forces instantaneously 326 00:12:51,738 --> 00:12:53,921 generated this partner forces. 327 00:12:53,921 --> 00:12:58,127 And be careful, some forces might seem like partner forces, 328 00:12:58,127 --> 00:12:59,630 and might be equal and opposite, 329 00:12:59,630 --> 00:13:02,425 but they're not necessarily third law partner forces. 330 00:13:02,425 --> 00:00:00,000 They made just be equal and opposite for other reasons.