NEWTON’S LAWS OF MOTIONChapter 2: pages 37-53Review questions 1, 5-10, 14, 17, 21-24, 30click for 9/page to printSir Isaac NewtonSir Isaac Newton Born 1642 1665 began individual studies Proved universal gravitationg Invented the CalculusReflector telescope 1672Reflector telescope 1672Mathematical Principles of Natural PhilosophyNatural Philosophy(“Principia”)First Law of MotionFirst Law of Motion“E bj ttiiit tt f t“Every object continuesin its state of rest, or of uniform motion in a straight line, unless it is compelled to change that stateunless it is compelled to change that state by forces impressed upon it.”“CONTINUES” INERTIA“CONTINUES” = INERTIA• http://www.physicsclassroom.com/mmedia/newtlaws/cci.htmlMassMassHhttHow much matter Measure of inertiaInertiaInertiaBird InertiaBird InertiaCoin InertiaCoin InertiaSecond Law of Motion “The acceleration of an object is directlyThe acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object.”Force Means “acceleration” ~massForce “~” (say “is proportional to”)Second LawFa=ma=AltiiditlAltiii lAcceleration is directly proportional to force•Acceleration is inversely proportional to massIncrease forcesIncrease forcesForce on brick createsForce on brick creates accelerationTwice force on brickTwice force on brick creates more accelerationacceleration Double mass of bricks requires double forcerequires double force to pushAcceleration equationFFa =If there is double the forcemIf there is double the force Also need to double the massTiti ttltiTo maintain constant acceleration Direct proportion—constant ratioAccelerationvΔtvΔCh i l it titChange in velocity over time Definition of accelerationAccelerationChange in velocity over timevΔChange in velocity over timeta=FmFa = Force over mass AND velocity over time? How can this be?AccelerationForcecausesFForce causesaccelerationForce over massmFa=mSolve for the force:maF=maFFreely falling objectsAcceleration of gravity is Fam 9.81Use g, aconstantma=2smUse g, a constant¾981F2m¾9.81¾For lecture calculations dt 10mFg=2sround to 10 m2smWeight is a Force Function of g (acceleration of Fggravity) Proper units:mg=à GravityàMassm2smkàMassàForceFmg=mkg•kgForce Different planet—wFmg=2skg•pdifferent weight for the same massWeight is a ForceWeight is a ForceWihWihtgWeight=mgWeight=mgIncreasing mass(Neglect air resistance in this example)• http://www.physicsclassroom.com/mmedia/newtlaws/efff.htmlGalileo’s investigation ofGalileo s investigation of motionSurface area changes airSurface area changes air resistanceObjects reach terminal velocityObjects reach terminal velocity due to air resistanceIn vacuum this is not a factorIn vacuum, this is not a factorFreely falling objectsFreely falling objectsBoulder vs featherBoulder vs. featherF=amFmFa =m Boulder has more inertia, but not more acceleration—so how come it falls faster?Air Resistance Acceleration is less than g due to air resistancedue to air resistance Friction of air against falling objectobject Air resistance depends onSdàSpeedà Frontal area exposed to airAir ResistanceAir ResistanceFlli bjth tt ttFalling object has constant mass, constant weightTilli hdh iTerminal velocity reached when air resistance matches weightAir resistance function of speed and area Falling object has variable frontal area if you deploy a parachuteAir ResistanceAir ResistanceParachute increases frontal area increases air resistancParachute increases frontal area, increases air resistanc Increased air resistance balanced with slower speed• http://www.physicsclassroom.com/mmedia/newtlaws/sd.htmlAir ResistanceGreater air resistance forGreater air resistance for elephant, because it is largerGreater weight because it isGreater weight because it is more massiveMore speed required to gain airMore speed required to gain air resistance to overcome the greater weightgreater weight• http://www.physicsclassroom.com/mmedia/newtlaws/efar.htmlAir Resistance Force of air resistance balances greater mass at greater speedat greater speed Heavier skydiver has greater terminal velocity than lightweight skydivervelocity than lightweight skydiver• http://www.physicsclassroom.com/mmedia/newtlaws/efar.htmlZero AccelerationZero AccelerationOne case: Motionless objectsOne case: Motionless objects (no change in velocity) Downward force created by gravityyg y Upward force created by surfaceZero AccelerationZero AccelerationAth Ct i tAnother case: Cart crossing room at constant velocity (no change in velocity)Nf iNet force is zero Force applied pushing force = frictional forceZero AccelerationZero Acceleration Push down on springpg Spring pushes up on youEach molecule of table actsEach molecule of table acts like microscopic spring pushinguponobjectpushing up on objectFrictionFrictionWk itfWorks against forces Opposite direction Not dependent on speed Not dependent on area of contactp Only dependent on weightThird Law of MotionThird Law of Motion“Wh bj t t f“Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first”an equal and opposite force on the first. Force is an interaction between objectsAction—reaction pairsForce interactionForce interactionFFFamF==Fma=mam•Forces in action-reaction pair are equalForces in actionreaction pair are equal• Different masses must have different accelerationsAction—reaction pairsActionreaction pairsFbjtitForce on object moves it Force by object acts on other thingsAlways equalAction—reaction pair Hammer exerts force on nail Nail exerts equal force on hammerAction—reaction pairsF Consider firing a cannon=mFFF Force on cannon and on cannonball the =aFsame cannonball has less thmmass than cannon Cannonball has greatergreater accelerationAction—reaction pairsActionreaction pairsRifle has lessRifle has less acceleration than the bulletbullet Because it has greater massgreater mass Forces are the sameAction—reaction pairsActionreaction pairsRocket acceleratesRocket accelerates upward Recoil from exhaust gasThe force that propels a rocket
View Full Document
Unlocking...