[What is Gravity?]

everything that goes up must come down that's a common thing and no wonder everyday people work hard just lifting things up that would otherwise stay down whenever we lift anything up we work against a pull this downward pull is called gravity gravity is the force that pulls things to earth when you climb a ladder you work to lift your body up against the force of gravity peddling your bike up a hill is hard work you are working against the force of gravity but gravity can also work for you gravity pulled the bike down the hill while it's gravity that pulls you down into the pool gravity may pull water down to your house from an elevated storage tank it's the downward pull of gravity that forces water through dams turning machines that make electricity but gravity is of still more important than that gravity holds everything on the earth in place without the pull of gravity everything would fly off the earth our atmosphere would fly off too and throw what our oceans and lakes without water and air nothing could live soon the earth itself with no gravity to hold it together would fall apart gravity as you can see is very important to us we don't know what causes gravity but we do know a lot about how it behaves Galileo Galilei an Italian mathematician astronomer and physicist was one of the first men to scientifically study the effects of gravity you can repeat one of Galileo's experiments yourself a break weighs a great deal more than a sponge but they both hit the ground at the same time let's watch again in slow motion the heavy and light objects fall to earth at the same speed but this one experiment may not be enough to convince you that the speed of a falling object does not depend upon its weight it's obvious for example that a piece of paper will fall more slowly than a coin but this has nothing to do with the lightness of the paper it's because the paper takes up a lot of space for its weight and must push aside a large amount of air as it falls if the paper is crumpled into a small ball so that it doesn't take up much space it drops just as quickly as the coin Galileo's work was most important in the history of science because unlike most scholars before him he tested his ideas by actually trying them out but it remained for Sir Isaac Newton the great English physicist and mathematician to discover the basic laws of the behavior of gravity the story goes that Newton became interested in gravity when he noticed an Apple falling from a tree ouch this may be so but some of Newton's first studies of gravity had to do with the movement of the moon around the earth for it is the force of gravity which keeps the moon near the earth just as the force the athlete puts on the handle keeps the hammer near his body but he lets go the handle the force disappears in the same way if the force caused by gravity were to disappear the moon too would go sailing off into space Nutan carefully studied among other things the path of the moon around the earth by 1686 he had worked out his law of gravity the first part of Newton's law states that every object in the universe pulls on every other object with a force that depends on the mass or the amount of material in the object the bigger the object the stronger the pull now let's see what that means these marbles are pulling on each other but they don't roll together because their pull is very weak the pull is weak because there isn't much material in a marble and its mass is very small a bowling ball is much larger than a marble and has much more mass the pull of a bowling ball should be much larger than a marbles pull but the bowling balls don't go together either the pull is so weak that it can be measured with only the most delicate instruments here is an instrument that can measure the pull of small objects these spheres are made of a non-magnetic material these small spheres are also non-magnetic they can swing freely suspended on an extremely thin strand of material which is protected from air currents by a metal shield a tiny mirror is attached to this supporting strand it reflects a spot of light through this prism and onto this scale a small movement of the spheres causes a large movement of the spot of light across the scale now let's test Newton's law with this instrument the large spheres are placed close to the small ones if the small sphere is attracted by the large one the small sphere should move towards the large sphere the movement is too small to notice here but the spot on the scale shows us that the small spheres are moving this experiment indicates that even small objects have some attraction for each other but with objects of this size the attraction is extremely weak the earth however is so huge and has so much mass that its pull on objects is very apparent and can be easily measured this boy is measuring the pull of the earth on his body the scale shows that the earth is pulling on his body with a force of 100 pounds now let's take our boy on an imaginary trip through outer space to see if he still weighs a hundred pounds on places other than the earth the first stop will be the moon [Music] remember on earth the boy weighed 100 pounds but on the moon he weighs only 16 there's Newton's law tell you why the boy weighs less on the moon because the moon is smaller than the earth and has less mass this means that the gravity of the Moon is weaker and pulls on the boy with less force now we'll take the boy to Jupiter you astronomers tell us that Jupiter is not a solid planet it does have a small solid Center but this is surrounded by a thick heavy layer of gas no one could land there but after all this is an imaginary trip [Music] if we could put the boy down on the gaseous surface of Jupiter we would find that he now weighs 264 pounds why because Jupiter has far more mass than the earth and that means that its gravity is much stronger remember Newton's law tells us that the attraction between two objects depends on the masses of the objects Newton's law also states that the attraction depends upon one more thing distance the greater the distance between two objects the less the force of attraction what does this mean to the boy as he leaves Jupiter and heads back toward Earth as the distance between Jupiter and the ship becomes greater the force of attraction between them becomes smaller finally the pull of Jupiter on the ship becomes very small and the rocket motors can be shut off and the ship coasts towards Earth now the boy has no weight at all he has escaped the gravity of Jupiter and has not yet entered into the gravitational pull of the earth without gravity pulling on him he is weightless [Music] as the ship moves back into the gravitational pull of the earth it will be pulled faster and faster towards the earth now the rocket motors must be turned on again to slow it down [Music] back on earth the boy weighs a hundred pounds again Newton's law states that all objects attract each other and that the force of the attraction depends on the mass or amount of material in the object only objects as huge as the earth are large enough to have a sizable attraction the force of attraction between objects also depends on the distance between the objects it grows less as the distance increases objects which are beyond the gravitational pull of any planet or star have no weight at all do you understand why [Music]