动力学三大运动定律一level物理课件4 dynamics.pptx

Dynamics

Newton’sthreelawsofmotionNewton’sfirstlawofmotion:

Newton’ssecondlawofmotion:

Newton’sthirdlawofmotion:identical:magnitudetypelineofactiontimeofactiondifferent:oppositedirectionactondifferentbodies

EgAnobjectofmass1.5kgistobeacceleratedat2.2ms-2.Whatforceisrequired?

EgAcarofmass1.5tones(1.5×103kg),travellingat80kmh-1,istobestoppedin11s.Whatforceisrequired?

Momentummomentum=mass×velocityp=m×vmassisascalarquantity,velocityisavectorquantity,somomentumisavectorquantity

Therateofchangeofmomentumofabodyisdirectlyproportionaltotheresultantforceappliedtothebody,andisinthesamedirectionastheforce.F=△p/△t=dp/dt=d(mv)/dtIfmisaconstantF=m(dv/dt)=ma

Aballofmass0.2kghitsafixedverticalwallatrightangleswithspeed3.5m/s.Theballreboundswithspeed2.5m/s.Findthechangeofmomentumoftheball.△p=0.2×2.5–0.2×(-3.5)=1.2Ns

Impulse

TheprincipleofconservationofmomentumThetotalmomentumofaclosedsystemofcollidingobjectsremainsconstantifnoexternalforcesactonthatsystem.m1v1+m2v2=m1u1+m2u2

m1(-v1)+m2v2=m1u1+m2u2-m1v1+m2v2=m1u1+m2u2

Theprincipleofconservationoflinearmomentumstatesthatthetotalmomentumafterimpact:momentum=mass×velocitym1u1+m2u2=m1v1+m2v2Momentumismeasuredinkgms-1.p=p1+p2=constant

Collisionsp=mvEgWhatisthemagnitudeofthemomentumofanα-particleofmass6.6×10-27kgtravellingwithaspeedof2.0×107ms-1?

Impulse:ifaconstantforceFactsonabodyforatime,theimpulseoftheforceisgivenby(Ns).Theimpulseofaforceisequaltothechangeinmomentum.

Theimpulse-momentumprinciplestatesthattheimpulseofaforceisequaltothechangeinmomentumproduced:Impulse=force×timeI=mv-muImpulseismeasuredinNewton-seconds(Ns)

EgSometennisplayerscanservetheballataspeedof55ms-1.Thetennisballha