Kinetics • Newton's Third Law of Motion

For every action, there is an equal and opposite reaction. By action and reaction it is meant action and reaction forces. We know that forces arise in ... Obsah Následujícíkapitola Předchozíkapitola 26 Kinetics Newton'sThirdLawofMotion–Lawofactionandreaction Foreveryaction,thereisanequalandoppositereaction. Byactionandreactionitismeantactionandreactionforces.Weknowthatforcesariseininteractionsoftwobodies41.Whenonebodyexertsa forceonanotherbody,thesecondbodyexertsa forceonthefirstbodythatisequalinmagnitudebutoppositeindirection. Whena journalistinterviewedthehockeyplayerJaromírJágr,hepaidhima compliment:„beingsobiggivesyouanadvantagewhensomebodytriestoboardyou“.However,JaromírJágrsmartlyreplied:„butbeingbigalsomeansthatIhavetocopewithgreaterweightwhenIamboarded“.Letushavea lookata situationwhena bigicehockeydefensemanramsintoa smallerforwardwhoismovingwiththesamevelocitybutintheoppositedirection.Whichofthemexertsa greaterforce?Theybothexertforcesofthesamemagnitudebutoppositedirection(Newton'sThirdLaw)!What,however,istheeffectoftheseforces?FromNewton'sSecondLawweknowthataccelerationofa bodydependsnotonlyonforcebutalsoonmass.Becausethesmallerforwardhaslessmass,afterthecollisionhewillaccelerateinthedirectionoftheforceexertedbythedefenseman(assumingtheirfootholdisthesame).Insucha collisionwecanseethatrammingintoanopponentwithlessmasshasmorekineticeffect.Thebiggerthemassofa player,thelesskineticeffect(acceleration)wecanseeaftera collisionwitha playeroflessermass.Newton'sThirdLawhelpsustoexplainhowforcesactandwhattheyacton.Butitdoesnotdirectlystatewhatmotionsofbodiesforcescause.Itonlystatesthatforcesariseinpairsandthateachofthemactona differentbody.Thereforewedonotaddvectorsoftheforcesofactionandreaction. 41Withtheexceptionofinertiaforcesinmechanics.Zpět