普通物理英语课件03_Falling.pdfVIP

FreelyFallingObjects

Physics1425Lecture3

MichaelFowler,UVa.

Today’sTopics

•Inthepreviouslecture,weanalyzedone-

dimensionalmotion,definingdisplacement,

velocity,andaccelerationandfindingformulas

formotionatconstantacceleration.

•Todaywe’llapplythoseformulastoobjects

falling,butfirstwe’llreviewhowweknow

thatfallingmotionisatconstantacceleration.

Galileo’sIdea

•BeforeGalileo,itwasbelievedthatfalling

objectsquicklyreachedanaturalspeed,

proportionaltoweight,thenfellatthatspeed.

•Galileoarguedthatinfactfallingobjects

continuetopickupspeed(unlessair

resistancedominates)andthatthis

accelerationisthesameforallobjects.

•Buthowtoconvincepeople?Watchinga

fallingobject,it’salloversoquickly.

DroppingaBrick

•Galileoclaimedpeoplealreadyknewthiswithout

realizingit:

•Imaginedrivinganailintoaboardbydroppinga

weightonitfromvariousheights.Everyone

alreadyknowsthatthefurtheritfalls,themore

impact—whichmustmeanit’smovingfaster.

•Buthowmuchfaster?Notsoeasytotell!

Istheresomewaytoslowdownthemotion?

Slowingdownthemotion…

•Afeatherfallsslowly—butGalileoarguedthat

thatmotion(fairlysteadyspeed)was

dominatedbyairresistance,sowasunlike

ordinaryfallingofaweightyobject.

•Hefoundanotherwaytoslowthingsdown…

here’shisexperiment—intwoparts,the

pendulumandtheramp.

ATwo-TimingPendulum

•Pendulumwithpeg•Firsthetooka

pendulumswinging

freelybackandforth,

thenheintroduced