普通物理英语课件11_Gravitation.pdfVIP

Gravitation

Physics1425Lecture11

MichaelFowler,UVa

TheInverseSquareLaw

•Newton’sidea:thecentripetalforcekeepingthe

MooncirclingtheEarthisthesamegravitational

forcethatpullsustotheground.

•BUT:theMoon’scentripetalaccelerationisonly

1/3600ofg!

•TheMoonis60timesfurtherfromtheEarth’s

centerthanweare.

•SO:thissuggeststhegravitationalpullfromthe

EarthdecreaseswithdistancefromtheEarth’s

2

centeras1/r.

GravitationalAttractionisProportionaltoMass

•Thegravitationalforcecauseseverythingto

fallwiththesameacceleration:

•FordownwardmotionFgrav=ma=mg

•Thatis,F/mhasthesamevalueforall

grav

masses.

•Conclusion:F∝m:thegravitationalforceon

amassisproportionaltothemass.

ClickerQuestion

DoestheMoonattracttheEarthgravitationally?

A.No

B.Yes,butwithamuchweakerforcethanthe

EarthattractstheMoon.

C.Yes,withthesameforcetheEarthattracts

theMoon.

ClickerAnswer

DoestheMoonattracttheEarthgravitationally?

•Yes,withthesameforcetheEarthattracts

theMoon.

•Newton’sThirdLawappliesheretoo!

•Theattractionsareequalandopposite.

Sowhydoesn’ttheEarthgoroundthe

Moon?

•Themoon’smassisabout1%oftheEarth’s

mass.

•Infact,theMoon’sattractiondoescausethe

Earthtogoinacircle—bothEarthandMoon

circletheircommoncenterofmass(whichis

insidetheEarth!)We’lldiscussthismore

later.

GravitationalAttractionisProportional

toBothMasses

•We’vealreadyseen,fromGalile