WEBVTT 00:00:00.000 --> 00:00:03.240 This is College Physics Answers with Shaun Dychko. 00:00:03.820 --> 00:00:06.800 A child sits on the end of a playground see-saw 00:00:06.800 --> 00:00:08.020 and we have to guess 00:00:08.060 --> 00:00:12.000 which of these estimates of the torque are correct? 00:00:12.320 --> 00:00:15.480 So we have to make just assumptions about what the mass of the child is 00:00:15.520 --> 00:00:17.940 and what their distance is from the pivot point? 00:00:17.940 --> 00:00:20.560 Suppose the child is 30 kilograms, which is about 00:00:20.560 --> 00:00:24.340 64 lbs or something 00:00:24.440 --> 00:00:27.900 and this maybe is about 2 meters 00:00:28.200 --> 00:00:31.720 and so the torque would be this distance of 2 meters 00:00:31.760 --> 00:00:35.980 times 30 kilograms times 10 newtons per kilogram; 00:00:36.040 --> 00:00:39.260 we can substitute 10 for g because 00:00:39.260 --> 00:00:40.760 you know, we are just dealing with estimates here 00:00:40.760 --> 00:00:42.140 so 10 is close enough to 9.8 00:00:42.140 --> 00:00:44.140 and that makes our calculations a little easier 00:00:44.540 --> 00:00:48.720 and then times by sin of 90 because the force is straight down— 00:00:48.820 --> 00:00:50.080 this is the force of gravity 00:00:50.120 --> 00:00:52.500 and it's perpendicular to the lever-arm. 00:00:53.180 --> 00:00:54.800 So 600 newton meters 00:00:54.800 --> 00:00:56.600 looks like a reasonable estimate for the torque 00:00:56.600 --> 00:00:58.740 and that is answer (c).