高中物理题
1.Howisthegravitationalforcebetweentwomassiveobjectssimilartoanddifferentfromtheelectricforcebetweentwochargedobjects?
2.Placeafridgemagnet(F)onthedoorofarefrigerator(R).HowdoweknowthatF,butnotR,isthesourceofapermanentmagneticfield?Youmaywanttoexperimentwithsomepins,staples,orpaperclipstoproveyourpoint.
3.Supposeatoneinstantthatamediumsuchasastringappearsflat.Doesthismeanthattherearenowavespropagatinginthemedium?
麻烦各位用英文来解释答案
贺知明回答：
1.问的是两个物体问的引力,与两个带电电荷间的electricforce的相同与不同.其实就是想要你去比较两条公式.
solution:
accordingtoNewton'slawofuniversalgravitation,gravitationalforcebewtwwntwomassiveobjects
F=G(M1M2/r^2)
accordingtoCoulomb'slaw,whichdescribestheinteractionofelectriccharges
F=1/(4Лε）(q1q2/r^2)
Similaritiesbetweenelectrostaticandgravitationalforces:
1.Bothactinavacuum.
2.Botharecentralandconservative.
3.Bothobeyaninverse-squarelaw(bothareinverselyproportionaltosquareofr).
4.Bothpropagatewithfinitespeedc.
Differencesbetweenelectrostaticandgravitationalforces:
1.Electrostaticforcesaremuchgreaterthangravitationalforces(byabout1036times).
2.Gravitationalforcesareattractiveforlikecharges,whereaslectrostaticforcesarerepulsiveforlikecharges.
3.Therearenonegativegravitationalcharges(nonegativemass)whiletherearebothpositiveandnegativeelectriccharges.Thisdifferencecombinedwithpreviousimpliesthatgravitationalforcesarealwaysattractive,whileelectrostaticforcesmaybeeitherattractiveorrepulsive.
4.Electricchargeisinvariantwhilerelativisticmassisn't
2.这条问,把磁铁靠近冰箱的门,磁铁会粘在门上,为什么说磁铁是永久的磁体而冰箱不是.其实是要求你知道sourceofapermanentmagneticfield的定义.
答案是：Permanentmagnetsareobjectsthatproducetheirownpersistentmagneticfields.Allpermanentmagnetshavebothanorthandasouthpole.Regularpinsarenotattractedbythedooroftherefrigeratorbutareattractedbyfridgemagnet.
3.在某一瞬间,一个媒介,例如,一条绳子,是平的.是不是代表有这个媒介上没有波?
波可分为横波纵波,这条绳子上的波可以是纵波,所以看上去上平的.有些波的波长是肉眼看不出来的（例如声波）.不能因为看上去是平的就认为没有波的传递.
