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ounting out from the Sun, Earth is the rst planet with a satellite, or moon. Mercury, closest planet to the Sun, doesn’t have a moon, nor does Venus. Mars, the fourth planet out, has two moons, but they are probably just a couple of big old rocks that ended up in Mars’ orbit after they were fully formed. It is suspected that Earth didn’t have a moon at rst, but acquired one early in its history as a result of a gigantic planetary collision. Visualize the event as it may have happened about 4.5 billion years ago. MAKING THE MOON Earth was pretty much formed as a planet. Most of the dust and gas in the region had been pulled in, and the proto-Earth was revolving around the Sun more or less in the orbit it travels today. However, these were the early days of the Solar System. There were a lot of large chunks of matter ying around in unstable orbits. Some of the chunks were huge—the size of small planets. Planetary scientists now think that one of these large planetesimals, perhaps the size of Mars, was traveling around the Sun in an exaggerated elliptical orbit. It’s not known why it had such a peculiar orbit—perhaps it was pulled by the gravitational inuence of a large planet, or perhaps there were lots of such strange objects early in the Solar System’s history. Anyway, it ended up heading for Earth. Had you been on Earth to witness the event, the incoming object would have rst appeared as a dot in the heavens. Over a period of days and weeks, it grew bigger and bigger until it completely lled the eld of view above Earth. Then it struck. Because the colliding object was so large, the impact itself seemed to happen in slow motion, lasting several minutes, even though the planetesimal was traveling at perhaps 40,000 km/h. What chaos must have followed the crash! The incoming object was destroyed on impact, reduced to vapor, dust, and chunks. Large surviving parts were driven deep into the interior of Earth. A signicant portion of Earth was destroyed as well. The energy that resulted from the crash produced an explosion of unimaginable magnitude. The force of the impact threw a tremendous quantity of matter into motion—at least 20 billion cubic kilometers of matter. One portion of the matter, the pieces traveling at 67 542-1460_Force_RB_Pgs_1-76.indd 67 5/7/08 11:46:59 AM the highest speeds, ew out into space, never to be seen again. Another portion of the matter ew up into the air and then was pulled back to Earth by gravity. Some of the falling matter fell back almost immediately as huge rocks, some a little later as granules of various sizes, and some months or even years later in the form of dust and chemicals held aloft in the atmosphere. A third and signicant portion of the debris didn’t y off into space, and it didn’t return to Earth. It began orbiting Earth in a disk, like the rings of Saturn. The ring was probably about two Earth diameters from the surface of Earth. Over the next millions of years, the force due to gravitation started bringing the pieces of matter together. Tiny grains formed larger and larger chunks, which eventually all pulled together to form the Moon. the mass, the stronger the force due to gravitation. The gravitational force plays two major roles in planetary system formation. Gravitational force pulls bits of matter together to form massive objects. If a little bit of matter is pulled together, the shape of the object might be irregular because the gravity will not be strong. Asteroids are funny shapes because of their low mass. If a lot of matter accumulates, however, the mass will be pulled into a sphere. The other role of gravitation is to hold planets and satellites in orbit. The straight-line paths of planets are deected into circular paths by the constant application of a force. That force is the universal force of gravitation. GRAVITY VERSUS VELOCITY GRAVITY AND ITS EFFECTS When the planetesimal hit Earth, matter ew everywhere. Pieces of matter launched with high velocity escaped Earth’s gravitational attraction and began a phase of existence as loose space debris. Matter that didn’t achieve escape velocity continued to be inuenced by Earth’s gravity. It’s just that simple. Gravity is one of the four known forces in the universe. Gravity, along with electromagnetism and two forces at work in the nucleus of atoms, makes everything in the world behave in ways we understand. It is the force due to gravitation that causes two masses to attract each other. Gravitational force exerted by a small mass, like a marble or an apple, is so small that we can’t detect it. But the force exerted by a large mass, like a planet or a star, is tremendous. The larger Before we think more about the matter that didn’t y into space, consider another piece of information about the behavior of mater. Isaac Newton gured out that an object in motion will travel in a straight line forever unless it is acted on by a force that changes its direction. In other words, things don’t travel in curves, circles, spirals, zigzags, or any other nonstraight paths unless something acts on them to change their motion. We had a moon where previously there was none, and it must have been a sight hanging up there maybe 30,000 km above Earth, rather than the 385,000-km distance we see today. 68 542-1460_Force_RB_Pgs_1-76.indd 68 5/7/08 11:26:35 AM F gra Imagine taking a yo-yo by the end of its string and swinging it around over your head in a nice circle. If you let go of the string, what happens? The yo-yo stops going in a circle and ies off in a straight line. As long as you keep applying a force (pulling on the string) to change the direction of the yo-yo, it continues to orbit your st. y vit F gra Earth “Natural” path of Moon vit y Back to the impact debris. Some of the matter fell back to Earth in the usual way. But some of the matter that ew out in a straight line had its path altered by the force due to Earth’s gravity. Take a rock the size of a trash can as an example. It ew off in a straight line like that shown on the illustration labeled “Natural” path of Moon. If there were no gravity, the rock would keep going off into space. However, the force of gravity pulls the rock toward Earth. The pull didn’t bring the rock to Earth’s surface, but it did change the direction of the rock’s travel. Remember, an object travels in a straight line until acted on by a force. The force that changed the rock’s direction of travel in this case was due to Earth’s gravity. The continuously altered path of the rock brought it into orbit. Fgravity Force due to gravitation is the “string” pulling on the Moon to keep it in a circular path. Similarly, gravitation is the force keeping Earth in a circular orbit around the Sun. In fact, everything that is going around something else in the Solar System is doing so because of gravitational force. Gravity rules! 69 542-1460_Force_RB_Pgs_1-76.indd 69 5/21/08 3:35:26 PM ! ! ! ! 6$7!-$!8&/!(+)!6$#)!9+/$!:!;$$+!<=$>5&!(+)!;$/*$+?8>()&!@A! 4/3)&+/!23&%/*$+%! $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ 0L U4+-$&5$+(3-4',$P3,J$I3,$5)(',,:('$+5$'D*H+&('J$&($-4'$I&,5-$5'(-'(%'V$U4+-$J&J$-4'$+8-43,$85'$ -3$5&K(&IT$-4&5$J'I&(&-&3(V$"&%N$+(3-4',$*H+%'$&($-4'$I&,5-$5'(-'(%'$P4','$-4'$+8-43,$J3'5$-4'$ 5+.'$-4&(KL$$$$ 1L W($-4'$-4&,J$*+,+K,+*4$-4'$+8-43,$5+T5$X*',4+*59$3('$3I$-4'5'$H+,K'$*H+('-'5&.+H59$*',4+*5$-4'$ 5&Y'$3I$:+,59$P+5$-,+7'H&(K$+,38(J$-4'$58($&($+($'D+KK',+-'J$'HH&*-&%+H$3,Q&-LZ$U4+-$&5$-4'$+8-43,$ ,'I','(%&(KV$ @L )'5%,&Q'$P4+-$4+**'('J$-3$-4'$*H+('-'5&.+H$+I-',$&-$%,+54'J$&(-3$-4'$[+,-4L$ \L U4+-$P+5$-4'$,'58H-$3I$-4'$X-4&,J$+(J$5&K(&I&%+(-$*3,-&3($3I$-4'$J'Q,&5Z$I,3.$-4'$%,+549$ J'5%,&Q'J$&($*+,+K,+*4$>V$W($T38,$3P($P3,J5$J'5%,&Q'$43P$-4&5$4+**'('JL$$)'5%,&Q'$P4+-$ 4+**'('J$-3$-4'$3-4',$-P3$*3,-&3(5$3I$-4'$J'Q,&5L$ !"#$#%&'(%'$)'*+,-.'(-$/$0102$)3,%4'5-',$67'(8'$/$)3,%4'5-',9$:6$;101<$ "43('$=20>?$2@<AB><;$/$C+D$=20>?$2@<AEB;0$ F$1;0@$!"#$#%&'(%'$)'*+,-.'(-$$ ! ! ! $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ ! <L U4+-$%+($T38$%3(%H8J'$+Q38-$-4'$.33($-3J+T$P&-4$-4'$5-+-'.'(-9$XW-$.85-$4+7'$Q''($+$5&K4-$ 4+(K&(K$8*$-4','$.+TQ'$@;9;;;$N.$+Q37'$[+,-49$,+-4',$-4+($-4'$@B<9;;;$N.$J&5-+(%'$P'$5''$ -3J+TVZ$$$ 2L U4+-$I3,%'$%+85'5$-P3$.+55'5$-3$Q'$+--,+%-'J$-3$'+%4$3-4',V$U4T$%+(]-$P'$I''H$-4'$ K,+7&-+-&3(+H$*8HH$3I$+($+**H'V$ >L [D*H+&($43P$K,+7&-T$+II'%-'J$-4'$J'Q,&5$I,3.$-4'$&.*+%-$'7'(-$53$-4+-$-4'$J'Q,&5$Q'K+($-3$3,Q&-$ -4'$[+,-4L$ !"#$#%&'(%'$)'*+,-.'(-$/$0102$)3,%4'5-',$67'(8'$/$)3,%4'5-',9$:6$;101<$ "43('$=20>?$2@<AB><;$/$C+D$=20>?$2@<AEB;0$ F$1;0@$!"#$#%&'(%'$)'*+,-.'(-$$ ! ! ! 6$7!-$!8&/!(+)!6$#)!9+/$!:!;$$+!<=$>5&!(+)!;$/*$+?8>()&!@A! 4(B1#&!:+%7&>%! $ $ ! ! ! ! ! CD EF(/!*%!(+$/F&>!7$>)!G$>!H!"#$%%&#$'!(%!&.1#(*+&)!*+!/F&!G*>%/!%&+/&+5&I!EF(/!)*)!/F&!(3/F$>! 3%&!/$!%*,+*GJ!/F*%!)&G*+*/*$+I!K*5L!$3/!$+&!B$>&!1#(5&!*+!/F*%!G*>%/!%&+/&+5&!/F(/!/F&!(3/F$>! )$&%!/F&!%(B&!/F*+,()$$ .*"(;'6$#"6+$<"6$5)(',,:('$:5$=""*>$%;'$)3(;"6$),5"$+"'5$:($("$*)='$?"36$@,)*'(0$A)6(;>B$ C%')7;'6$*"('D$E($:5$6'),,F$:=@"6()*($(;)($5(3+'*(5$3*+'65()*+$;"#$(;'$7"==)$:5$35'+$:*$(;:5$ <:65($5'*('*7'>$%;:5$:5$"<('*$5"='(;:*G$(;)($5(3+'*(5$+"$*"($3*+'65()*+>$$.$7"==)$:5$),5"$35'+$)$ <'#$"(;'6$(:='5$:*$(;'$<:65($@)6)G6)@;$("$7,)6:<F$)$#"6+>$AH>$?/'6736F0$(;'$7,"5'5($@,)*'($("$(;'$ 23*0IJ$ MD N+!/F&!/F*>)!1(>(,>(1F!/F&!(3/F$>!%(J%!H1&>F(1%!$+&!$G!/F&%&!#(>,&!1#(+&/&%*B(#%O!1&>F(1%!/F&! %*P&!$G!;(>%O!7(%!/>(Q&#*+,!(>$3+)!/F&!%3+!*+!(+!&.(,,&>(/&)!&##*1/*5(#!$>R*/DS!EF(/!*%!/F&! (3/F$>!>&G&>&+5*+,I$ %;'$)3(;"6$:5$6'<'66:*G$("$"*'$"<$(;'$,)6G'$7;3*95$"<$=)('6:),$<,F:*G$)6"3*+$+36:*G$(;'$')6,F$+)F5$ "<$(;'$2",)6$2F5('=>$ TD 0&%5>*R&!7F(/!F(11&+&)!/$!/F&!1#(+&/&%*B(#!(G/&>!*/!5>(%F&)!*+/$!/F&!U(>/FD! E($#)5$+'5(6"F'+0$K':*G$6'+37'+$("$L)@"60$+35(0$)*+$7;3*95>$ VD EF(/!7(%!/F&!>&%3#/!$G!/F&!H/F*>)!(+)!%*,+*G*5(+/!1$>/*$+!$G!/F&!)&R>*%S!G>$B!/F&!5>(%FO! )&%5>*R&)!*+!1(>(,>(1F!@I$N+!J$3>!$7+!7$>)%!)&%5>*R&!F$7!/F*%!F(11&+&)D!!0&%5>*R&!7F(/! F(11&+&)!/$!/F&!$/F&>!/7$!1$>/*$+%!$G!/F&!)&R>*%D!$ %;'$(;:6+$@"6(:"*$6'53,('+$:*$)$6:*G$)6"3*+$(;'$A)6(;0$#;:7;$G6)+3),,F$7",,'7('+$:*("$7;3*95$)*+$ (;'*$<"6='+$(;'$/""*>$%;'$6'5($"<$(;'$+'K6:5$':(;'6$<,'#$"3($:*("$5@)7'$"6$6'(36*'+$("$A)6(;>$$ WD EF(/!5(+!J$3!5$+5#3)&!(R$3/!/F&!;$$+!/$)(J!7*/F!/F&!%/(/&B&+/O!HN/!B3%/!F(Q&!R&&+!(!%*,F/! F(+,*+,!31!/F&>&!B(JR&!TXOXXX!LB!(R$Q&!U(>/FO!>(/F&>!/F(+!/F&!TYWO!XXXLB!)*%/(+5&!7&!%&&! /$)(JIS!!! %;'$/""*$:5$<36(;'6$)#)F$*"#>$$ ZD EF(/!5(3%&%!/7$!B(%%&%!/$!(//>(5/!/$!&(5F!$/F&>I!EFJ!5(+[/!7&!G&&#!/F&!,>(Q*/(/*$+(#!13##!$G! (+!(11#&I! &6)L:(F$:5$(;'$<"67'$(;)($7)35'5$(#"$=)55'5$("$)((6)7($"*'$)*"(;'6>$.*$)@@,'$;)5$(""$5=),,$"<$)$ =)55$<"6$35$("$+'('7($:(>$$ @D U.1#(*+!F$7!,>(Q*/J!(GG&5/&)!/F&!)&R>*%!G>$B!/F&!*B1(5/!&Q&+/!%$!/F(/!/F&!)&R>*%!R&,(+!/$! $>R*/!/F&!U(>/FD! &6)L:(F$@3,,'+$(;'$+'K6:5$K)79$("$A)6(;$)*+$;',+$5"='$"<$(;'$+'K6:5$:*$"6K:($)6"3*+$(;'$A)6(;>$ 2"='$"<$(;'$+'K6:5$#)5$@3,,'+$+"#*$("$A)6(;M5$536<)7'0$#;:,'$5"='$6'=):*'+$:*$"6K:($)*+$ 'L'*(3),,F$<"6='+$(;'$/""*>$ !"#$#%&'(%'$)'*+,-.'(-$/$0102$)3,%4'5-',$67'(8'$/$)3,%4'5-',9$:6$;101<$ "43('$=20>?$2@<AB><;$/$C+D$=20>?$2@<AEB;0$ F$1;0@$!"#$#%&'(%'$)'*+,-.'(-$$