
1)Whymustyoubendforwardwhencarryingaheavyloadonyourback?(hint:considerstability)
Youbendforwardwhencarryingaheavyloadonyourbacktoshiftthecenterofgravityofyouandyour
loadabovetheareaboundedbyyourfeet—otherwiseyoutopplebackward.Theconditionforstability
isthataverticallinedroppeddownfromthecenterofgravitymeetsapointinthebaseoftheobject.
2)IfthepolaricecapsoftheEarthweretomelt,theoceanswouldbedeeperbyabout30m.What
effectwouldthishaveontheEarth’srotation?
Themasscontainedintheicewouldspreadmoreevenlyaroundtheworldinsteadofbeinglocalized
nearthepoles.Inaccordwiththeconservationofangularmomentum,sincemassismovinfartherfrom
theaxisofrotation(thisgoesthroughthepoles),therotationalspeeddecreases.SoEarthwouldslowin
itsdailyrotationandthedaywouldbecomelonger.
3)AfriendsaysthatsinceEarth’sgravityissomuchstrongerthanmoon’sgravity,rocksonthemoon
couldbedroppedtotheEarth.Whatgoeswrongwithhisconclusion?
TheforceofgravityonMoonrocksattheMoon’ssurfaceisconsiderablystrongerthantheforceof
gravityofthedistantEarth.Theforceofgravityofaplanetormoonontherockdependsnotonlyonthe
massesoftheplanetormoonandrockbutalsoonhowfartherockisfromtheplanet.Rocksdropped
ontheMoonfallontotheMoonsurfacesincethegravitationalforcefromtheMoonissomuchstronger
thanfromtheEarthwhentherockisnearthemoon’ssurfacebecausethemoonissomuchcloserto
therock.(TheforceoftheMoon’sgravityisabout1⁄6oftheweighttherockwouldhaveonEarth;the
forceoftheEarth’sgravityatthatdistanceishoweveronlyabout1⁄3600oftherock’sEarth‐weight.)
4)Mostpeopletodayknowthattheoceantidesarecausedprincipallybythegravitationalpullofthe
moon.AndmostpeoplethereforethinkthatthegravitationalpullofthemoonontheEarthisgreater
thanthatofthesunontheEarth.Whatdoyouthink?
ThegravitationalpulloftheSunontheEarthisgreaterthanthegravitationalpulloftheMoon,about
180timesaslarge.Thetides,however,arecausedbythedifferencesingravitationalforcesbytheMoon
onoppositesidesoftheEarth.ThedifferenceingravitationalforcesbytheMoononoppositesidesof
theEarthisgreaterthanthecorrespondingdifferenceinforcesbythestrongerpullingbutmuchmore
distantSun.