《大学物理简明教程》 课件 英文 6-1 Torque.pptx

TorquerotationforcebalanceStaticequilibrium

Wehaveconsideredforcesactingonasinglepoint.-point-massmodeltheobjectmovesasapointmass–atranslationalmotion.ForcesandTorquesNowweconsidertheobjecthavingcertainshapeandsize.Whatiftheforcesactondifferentpointsoftheobject?

Atorqueisaphysicalquantitythatis:aproductofaforceandadistance.ATorqueisalsoknownasthe“MomentofForce”.Itoperatesatsomedistancefromthepivotpoint.It’sactionrotatesanobject.Torque/MomentofForceFdpivot?TheSIunitisN·m,butitisNOTjoule(J)!

Fd?dF??????F?Thedirectionsoftorqueare“clockwise”or“anti-clockwise”

ForceCoupleTwooppositeandequalforcesthatactondifferentpointsofanobjectareknownasa“couple”.Acouplewillrotatetheobject-itexertsatorque.*FFNocontributiontothenetforcecontributetothenettorque*FF*l/2l/2?clockwisecenterofrotation

ExampleAmechanicexertsaforceof100Nperpendiculartotheendofawrench300mmlong.Whatisthetorqueappliedtothescrewnut???Clockwise–tofastenthescrewAnti-clockwise–toloosenthescrew

Achildofweight150Nsitsontheendofasee-sawoftotallength3m.Herfatherstands0.5mfromthemiddleofthesee-sawandpushesdown.Whatforcemustheexerttobalancehisdaughter?Ifherolderbrother(weight200N)getsonthesee-saw,whereshouldhesittobalancehissister?150N3mF=?0.5ma)Anti-clockwisetorque=positiveClockwisetorque=negativeTobalance,thesumofthemomentsmustbezero.(150Nx1.5m)+(-Fx0.5m)=0Example:balanceoftorquesF=(150x1.5)/0.5=450N

F1d1+F2d2=0d2=-(F1d1)/F2=-(-1.5mx150N)/200N150N3md2=?200Nd2=1.125mfromthecenter(or1.5–1.125=0.375mfromend)

LeversWecanusetheconceptoftorque/momentstoexplainhowasimpleleverworks:Momentduetoweight=+l1·MgMomentduetoperson=-l2·WMaximummasswhenthesecancel