Weight on and offthe Earth We are so used to.our life on the surface of the earth that it can bequite an effort for our mind to break free of all the ideas that we take for

granted.Because wecan feel that things are heavy,we think of "weight"as being a fixedquality in an object,but it is not really fixed at all.If you could take aone-pound packet

of butter 4,000miles out from the earth,it would weigh only a quarter of a pound. Why wouldthings weigh only a quarter as much as they do at the surface of the earth ifwe

took them 4,000miles out into space?The reason is this:all objects have a natural attractionfor all other objects;this is called gravitational attraction.But this power of

attraction betweentwo obects gets weaker as they get farther apart.When the butter was at thesurface of the earth,it was 4,000 miles from the center.When we took the butter

4,000 miles out,itwas 8,000 miles from the center,which is twice the distance.If you double thedistance between two objects,their gravitational attraction decreases two times

two.If you treblethe distance, it gets nine times weaker(three times three)and so on. So this isone of the first things we need to remember that the weight of an object inspace

is not the same asits weight on the surface of the earth. What about the weight of our pound ofbutter on the surface of the moon?At the distance the puii of the earth isabout

4,000 times smallerthan it is here on the surface,so we can forget all about the earth-pull on ourbutter. On the other hand,on the moon there will be an attraction between the

butter and themoon,but the butter will weigh only about one-sixth as much as it does on theearth.This is because the moon is so much smaller than the earth.The amount of

gravitational pullthat a body produces depends on the amount of material in it.A packet of butterhas a gravitational pull of its own;but this is very small in relation to thepull

of something aslarge as the moon,or the earth,or the sun.The main idea ofthis passage is

A.different weightsystems in different places

B.freedom fromtraditional ideas

C.traveling from theearth to the moon

D.the effect ofgravitation on weight