28 JUNIOR GRADE SCIENCE 



which a body is attracted by the earth is its weight. But it must be 

 remembered that this force is just the same whether things actually 

 fall to the ground or not. A spring balance shows the weight of a 

 thing, and masses are compared by means of the ordinary balance or 

 pair of scales. 



The attraction of gravity. Bearing in mind that weight is really 

 a measure of the attraction between an object and the earth, it will 

 be clear from Newton's law of gravitation that since a thing up in a 

 balloon is farther away from the earth (which acts exactly as if its 

 whole mass were collected at its centre) than when at the sea-level, 

 the weight of this thing ought to be less than it is at the sea-level. This 

 is found to be the case, but, actually to demonstrate the difference 

 in weight, the weight must be measured by a spring balance as in 

 Experiment 12(c). 



Similarly, because the earth is not a perfect sphere, but is flattened 

 at the poles, points at the surface of the earth in the region of the 

 tropics are at a greater distance from the centre than points similarly 

 situated in the neighbourhood of the poles. Consequently, the weight 

 of a mass situated on the earth in the tropics should be less than the 

 weight it would have if it were moved into the polar regions. This 

 has been found to be the case. 



The rotation of the earth is another disturbing influence. While 

 places on the equator are carried round with a velocity of over a thousand 

 miles an hour, those near the poles have but a small velocity of rotation, 

 while the pole itself is at rest. It is clear that if we consider a mass at 

 the equator its tendency is to obey the first law of motion (p. 23), and 

 to fly off at a tangent, and part of the force of gravitation is expended 

 in preventing this flight the remainder of the force of gravity is 

 operative as the weight of the mass under consideration. At the pole 

 there is no tendency to move off tangentially, and the whole of the 

 force of gravitation is felt as the weight of the body. For this reason 

 alone the mass would weigh less at the equator. At places intermediate 

 between the poles and the equator the diminution in the weight oi the 

 body, or the diminution in the acceleration due to gravity, is less ; it 

 diminishes as the nearness to the pole is increased. 



