WEBVTT 00:00:00.000 --> 00:00:03.240 This is College Physics Answers with Shaun Dychko. 00:00:03.280 --> 00:00:06.160 We are going to calculate the force exerted by the pivot 00:00:06.200 --> 00:00:08.960 using the second condition of equilibrium which is to say that 00:00:08.960 --> 00:00:13.240 the total clockwise torques have to equal the total counter-clockwise torques. 00:00:13.980 --> 00:00:18.440 So the pivot point exerts a torque that is counter-clockwise 00:00:18.440 --> 00:00:20.780 about a pivot located here 00:00:20.840 --> 00:00:22.940 and you can choose your pivot wherever you want it to be, 00:00:22.940 --> 00:00:24.300 it doesn't have to be... 00:00:24.300 --> 00:00:25.800 I mean the fulcrum seems to be 00:00:25.800 --> 00:00:27.880 the most obvious place to choose the pivot 00:00:27.920 --> 00:00:32.020 but we can't choose it there and figure out the force exerted by the pivot 00:00:32.020 --> 00:00:35.800 since it would have zero torque about a pivot that is 00:00:35.800 --> 00:00:37.420 zero meters away. 00:00:37.660 --> 00:00:39.880 So we have to choose a pivot in a different position 00:00:39.920 --> 00:00:43.440 in order to use a second condition of equilibrium here for this force. 00:00:43.780 --> 00:00:48.680 So this pivot point is exerting a counter-clockwise torque 00:00:48.760 --> 00:00:51.860 a distance r 1 from this pivot 00:00:51.920 --> 00:00:57.760 and that's going to equal the clockwise torque exerted by child number 2 00:00:57.820 --> 00:01:03.160 which is a weight distance r 1 plus r 2 from this pivot. 00:01:04.160 --> 00:01:07.260 So we'll divide both sides by r 1 00:01:07.960 --> 00:01:10.560 and solve for the force exerted by the pivot 00:01:10.620 --> 00:01:16.120 and then substitute in mass 2 times g in place of the weight of child 2. 00:01:16.420 --> 00:01:19.700 And that's 32.0 kilograms times 9.80 newtons per kilogram 00:01:19.740 --> 00:01:22.200 times 1.60 meters plus 1.30 meters 00:01:22.200 --> 00:01:25.780 and this 1.30 meters—the distance of child 2 from the pivot— 00:01:25.780 --> 00:01:27.900 was solved for in the example 00:01:28.020 --> 00:01:32.260 and so we'll use that result here and we divide by 1.60 and we get 00:01:32.260 --> 00:01:34.080 568 newtons.