WEBVTT 1 00:00:01.877 --> 00:00:06.761 If common usage confuses the ideas of the weight and the mass of an object 2 00:00:06.762 --> 00:00:09.869 these two quantities are actually quite different. 3 00:00:11.542 --> 00:00:16.217 The mass of an object is a quantity that depends solely on the nature of the object 4 00:00:16.218 --> 00:00:18.150 and not on its location. 5 00:00:18.151 --> 00:00:20.893 It is a number that is expressed in kilograms. 6 00:00:22.513 --> 00:00:25.752 Thus, an object with a mass of 4 kg on Earth 7 00:00:25.753 --> 00:00:29.278 has the same mass of 4 kg on the Moon 8 00:00:29.279 --> 00:00:31.760 or anywhere else in the Universe. 9 00:00:32.806 --> 00:00:34.267 Weight, on the other hand 10 00:00:34.268 --> 00:00:38.343 is a force whose intensity depends on the location of the object. 11 00:00:40.904 --> 00:00:46.571 Like all other forces, weight is a vector, having a direction, pointing somewhere 12 00:00:46.572 --> 00:00:49.836 and an intensity which is expressed in Newtons. 13 00:00:51.300 --> 00:00:54.878 The intensity of this force is measured using a dynamometer 14 00:00:54.879 --> 00:00:58.117 which balances the weight against the tension in a spring 15 00:00:58.118 --> 00:01:00.338 with a known stiffness constant. 16 00:01:01.514 --> 00:01:06.086 The vector Weight and the number mass should thus not be confused. 17 00:01:06.087 --> 00:01:12.955 But it is true that these two quantities are related by the expression: W =mg 18 00:01:12.956 --> 00:01:19.538 where W is the weight and g is the vector representing the gravitational field. 19 00:01:19.539 --> 00:01:23.900 This vector varies according to the place where it is measured. 20 00:01:23.901 --> 00:01:32.076 On Earth, its intensity varies from 9.78 at the Equator to 9.83 at the poles. 21 00:01:32.077 --> 00:01:37.379 It equals 9.77 at the top of Mount Everest. 22 00:01:37.380 --> 00:01:41.272 It is only 1.6 at the surface of the Moon. 23 00:01:41.273 --> 00:01:43.231 It is therefore correct to say 24 00:01:43.232 --> 00:01:47.672 that our weight on the Moon is not the same as our weight on Earth 25 00:01:47.673 --> 00:01:53.627 but it is incorrect to express the intensity of the force producing weight in kilograms. 26 00:01:53.628 --> 00:01:56.628 As for our mass, it is the same everywhere.