WEBVTT 00:00:02.570 --> 00:00:05.110 - There are a lot of misconceptions out there, 00:00:05.110 --> 00:00:09.270 and this is a video about one of the most common ones. 00:00:09.270 --> 00:00:11.307 So I went around asking people 00:00:11.307 --> 00:00:13.800 "What makes the Moon go around the Earth?" 00:00:13.800 --> 00:00:15.530 And they told me... 00:00:15.530 --> 00:00:18.560 - The Earth puts a gravitational force on the Moon. 00:00:18.560 --> 00:00:23.103 - But does the Moon put a gravitational pull on the Earth? 00:00:24.788 --> 00:00:26.790 - Pull on the Earth? Yes it does, 00:00:26.790 --> 00:00:28.420 hence we have tides, et cetera. 00:00:28.420 --> 00:00:29.700 - The Moon pulls on the Earth too. 00:00:29.700 --> 00:00:34.171 It affects like the tides and women. 00:00:34.171 --> 00:00:36.247 (both laughing) 00:00:36.247 --> 00:00:38.350 It does, it's very powerful. 00:00:38.350 --> 00:00:40.030 - Does the Moon pull on the Earth? 00:00:40.030 --> 00:00:41.810 - Probably that as well, yeah. 00:00:41.810 --> 00:00:44.150 - So what I want to know is how does that force, 00:00:44.150 --> 00:00:45.710 that the Moon exerts on the Earth, 00:00:45.710 --> 00:00:47.780 how does that compare in terms of size 00:00:47.780 --> 00:00:50.140 to the force the Earth exerts on the Moon? 00:00:50.140 --> 00:00:52.520 - Well, I'm no scientist, 00:00:52.520 --> 00:00:55.240 but I think that one would be more powerful than that one. 00:00:55.240 --> 00:00:57.260 - It's got a greater force coming from the Earth, 00:00:57.260 --> 00:00:58.340 'cause it's greater mass. 00:00:58.340 --> 00:01:00.000 - 'Cause the Earth probably has more mass. 00:01:00.000 --> 00:01:01.610 - 'Cause it's a big mass. 00:01:01.610 --> 00:01:04.230 I thought greater mass is more like, is more force. 00:01:04.230 --> 00:01:05.710 - Does the Moon pull on the Earth? 00:01:05.710 --> 00:01:07.030 - Yes. 00:01:07.030 --> 00:01:08.560 But a lot less. 00:01:08.560 --> 00:01:10.540 - Yeah, Not as much as the Earth pulls on the Moon. 00:01:10.540 --> 00:01:12.177 - Yeah, but a little bit. 00:01:12.177 --> 00:01:13.780 - But not very strongly. 00:01:13.780 --> 00:01:16.918 - Yes, but much, much, much, much smaller. 00:01:16.918 --> 00:01:17.960 - Because of its mass, size 00:01:17.960 --> 00:01:18.960 - It's got less Mass 00:01:19.957 --> 00:01:22.800 - Because it's smaller, it's much smaller than the Earth. 00:01:22.800 --> 00:01:23.633 - Because it's smaller. 00:01:23.633 --> 00:01:25.760 - Because it's smaller. 00:01:25.760 --> 00:01:28.600 - Allow me to let you in on a little secret. 00:01:28.600 --> 00:01:30.883 Everyone got it wrong. 00:01:31.720 --> 00:01:34.220 The force that attracts the Moon to the Earth 00:01:34.220 --> 00:01:35.960 is exactly the same size 00:01:35.960 --> 00:01:38.410 as the force that attracts the Earth to the Moon. 00:01:39.560 --> 00:01:40.820 So what's going on here? 00:01:40.820 --> 00:01:42.860 Why did everyone get it wrong? 00:01:42.860 --> 00:01:45.860 Well I think it comes down to cause and effect. 00:01:45.860 --> 00:01:49.840 The effect of the force on the Moon is quite clear. 00:01:49.840 --> 00:01:53.010 The Moon goes in circles around the Earth, 00:01:53.010 --> 00:01:55.600 but the effect of that force on the Earth 00:01:55.600 --> 00:01:57.100 is basically negligible. 00:01:57.100 --> 00:01:59.480 The Earth barely wobbles at all. 00:01:59.480 --> 00:02:03.177 So people interpret this negligible effect 00:02:03.177 --> 00:02:07.640 as indicating there's very little force affecting the Earth, 00:02:07.640 --> 00:02:10.660 but that is forgetting the third key piece of the puzzle 00:02:10.660 --> 00:02:12.690 which is inertia. 00:02:12.690 --> 00:02:14.970 Inertia is the tendency of mass 00:02:14.970 --> 00:02:17.690 to maintain its state of motion. 00:02:17.690 --> 00:02:20.720 Since the Earth has a greater mass, 00:02:20.720 --> 00:02:22.470 it has a greater inertia. 00:02:22.470 --> 00:02:25.320 And so even with the same amount of force on it, 00:02:25.320 --> 00:02:28.100 it doesn't accelerate that much. 00:02:28.100 --> 00:02:31.620 Now the funny thing is many of the people I interviewed 00:02:31.620 --> 00:02:34.820 could state Newton's third law, which is... 00:02:34.820 --> 00:02:36.420 Every force has... 00:02:36.420 --> 00:02:38.190 - an equal and opposite reaction. 00:02:38.190 --> 00:02:39.340 - Something about Newton's law 00:02:39.340 --> 00:02:41.626 doesn't seem to fit into that. 00:02:41.626 --> 00:02:42.880 (laughing) 00:02:42.880 --> 00:02:44.088 - You're good at this. 00:02:44.088 --> 00:02:44.921 (laughing) 00:02:44.921 --> 00:02:46.260 Which Newton's law are we talking about? 00:02:46.260 --> 00:02:48.590 - The whole equal and opposite force thing? 00:02:48.590 --> 00:02:49.770 Yeah, that one. 00:02:49.770 --> 00:02:51.000 - So tell me what you're thinking now. 00:02:51.000 --> 00:02:53.770 - Uh well, one would think that 00:02:53.770 --> 00:02:55.140 if the Earth putting a force on me, 00:02:55.140 --> 00:02:57.800 I would be putting an equal force upon it. 00:02:57.800 --> 00:03:00.100 - So why didn't they apply it to this problem? 00:03:01.210 --> 00:03:03.870 Well, I think they may have memorized the words, 00:03:03.870 --> 00:03:07.990 but not really believe Newton's third law in their core. 00:03:07.990 --> 00:03:10.493 Do they really feel it in their spleen? 00:03:11.490 --> 00:03:12.620 I don't think they did. 00:03:12.620 --> 00:03:16.860 So, allow me to try to convince you, all of you, 00:03:16.860 --> 00:03:18.650 spleen included 00:03:18.650 --> 00:03:21.470 that Newton's third law really is true. 00:03:21.470 --> 00:03:24.230 Let's consider two objects. 00:03:24.230 --> 00:03:28.410 Initially they have the same mass, one kilogram each. 00:03:28.410 --> 00:03:31.460 So obviously the gravitational force of attraction 00:03:31.460 --> 00:03:34.283 must be the same on both of the objects. 00:03:35.600 --> 00:03:40.070 Now let's add a second kilogram to the first object. 00:03:40.070 --> 00:03:43.940 The force on the second object will now be twice as great 00:03:43.940 --> 00:03:47.310 because that one kilogram is attracted equally 00:03:47.310 --> 00:03:50.580 to each of the kilograms in the first object. 00:03:50.580 --> 00:03:53.810 But what is often forgotten, is that new kilogram 00:03:53.810 --> 00:03:56.440 is also attracted to the second object. 00:03:56.440 --> 00:03:59.350 Meaning that the total force on each object 00:03:59.350 --> 00:04:00.700 is still the same. 00:04:00.700 --> 00:04:02.080 They're attracted to each other 00:04:02.080 --> 00:04:04.373 with an equal and opposite force. 00:04:05.560 --> 00:04:07.840 We could add a third kilogram 00:04:07.840 --> 00:04:09.290 and we would find the same thing, 00:04:09.290 --> 00:04:11.770 the force on both objects is still the same, 00:04:11.770 --> 00:04:14.860 even though the object on the left has three times the mass 00:04:14.860 --> 00:04:16.650 of the object on the right. 00:04:16.650 --> 00:04:19.910 So we can see that no matter what the mass, 00:04:19.910 --> 00:04:23.120 any two objects will have the same gravitational force 00:04:23.120 --> 00:04:24.193 towards each other. 00:04:25.410 --> 00:04:29.610 Can you feel Newton's third law in your spleen now? 00:04:29.610 --> 00:04:33.023 It should settle inside you and become a part of you.