Momentum and EnergyChapter 3Click for 9/page to printMomentumMomentum• Momentum is inertia in motion• Mass x velocityy• Has both magnitude and direction• Large mass or high speed can give object pgjgreat amount of momentumMomentum=m•vMomentum mv•Change momentum byChange momentum by – changing mass or velocityusually velocity–or velocity—usually velocity• What causes changes in velocity?•Force• Time is also importantpMomentum and ImpulseMomentum and Impulse•Apply force over time to change velocityApply force over time to change velocity and momentum•Greater time of application greater•Greater time of application, greater change in momentumFtiitliIMPULSE•Force x time interval is IMPULSEImpulse=F•tImpulse Ft•ForcextimeintervalForce x time interval• Impulse changes momentum• Technically:)(mvFtΔ=• Realistically:vmFtΔ=Impulse Increasing MomentumImpulse Increasing MomentumvmFtΔ=• Pushing a child on a swing—the force)(mvFtΔ=g• Increases momentum•Longer pushLonger push increases momentum more than a short one• http://www.bcm.edu/oto/research/cochlea/Volta/16.htmlImpulse Decreasing MomentumImpulse Decreasing Momentum•Stopping the out of control carvmFtΔ=pp g• Change momentum to zero•Less force if time is greaterLess force if time is greaterImpulse Changes MomentumImpulse Changes Momentum•Can change force by changing timeCan change force by changing timeMitidf•Move away increases time, reduces force• Toward decreases time, increases forceBounce increases impulseBounce increases impulse•There is a change invmFtΔ=There is a change in velocity direction, making a greater ΔvvmFtΔ• So greater force is required• Water changesdirection, has greaterilh flimpulse than a flatpaddleConservation of MomentumConservation of Momentum• The system: the cannon and the ball•When it fires momentum is conserved•When it fires, momentum is conserved—they both have momentum: in opposite directionsdirections• Momentum = m•vConservation of MomentumConservation of Momentum•No net force• Momentum = m•vNo net force• No net momentumConservation of MomentumConservation of Momentum•In the absence of external force theIn the absence of external force, the momentum of the system remains unchangedunchanged• Momentum = m•v• Consider individual balls as individual systems: momentum of each does changeConservation of MomentumConservation of Momentum•Net momentum is the same before theNet momentum is the same before the collision•As after the collision•As after the collisionMt•Momentum = m•vConservation of MomentumConservation of MomentumYll bll t t ttia.Yellow ball starts stationaryb. Both balls moving opposite directionsc. Green ball moving faster• Momentum = m•vEnergyEnergy•Property of a system that enables it to doProperty of a system that enables it to do work•Potential Energy•Potential Energy– SpringsCh i l–Chemicals– Gravity• Kinetic EnergyPotential EnergyPotential Energy• http://www.howstuffworks.com/inside-clock.htm• http://www.himalayan.pdx.edu/virtualjourney/slideshow/se_photos_web/pages/Boy%20with%20Slingshot%2C%20J.htmPotential EnergyPotential Energy•Fuel is chemicalFuel is chemical potential energy• http://www.alternativefuels.com.au/Biodiesel/dragster.htm• http://www.lilligren.com/Redneck/redneck_lawnmower.htmWork is Energy•Lift heavy ram of pile driverWork is EnergyLift heavy ram of pile driver• Work transfers energy to lift intolift into gravitational potential energyGravitational potential energyGravitational potential energy•Due to object’s positionDue to object s position• Relative to a surface= weight x height =mgh= weight x height =mgh• Work done for object to gain potential energyGravitational potential energyGravitational potential energyGravitational potential energyGravitational potential energyEP= mass x (acceleration of gravity) x heightEP= mass x (acceleration of gravity) x heightHihti b f l lHeight is above some reference levelPotential energy is always referenced to a zero level defined in the systemGravitational potential energyGravitational potential energy•E=mgh•EP= mgh• mg = weight• h = heightGravitational potential energyGravitational potential energy•EP= mgh •Path to the height is not factor in E•Path to the height is not factor in EP• Horizontal distance is not factor in EPKinetic Energy of MotionKinetic Energy of Motion•EK=mv21EK mv2• Work is a change in kinetic energy•W = ∆EKK• ∆ Delta ‘change’Kinetic Energy of MotionKinetic Energy of Motion•HeatHeat• SoundEl t i it d li ht•Electricity and lightKinetic Energy of MotionKinetic Energy of Motion•W=∆EWork is change in kinetic energy•W = ∆EKWork is change in kinetic energy• Work-energy theorem• Net work = force x distance W = Fd–Due to net force2mv E2mv Fd=2K E=2 FdConservation of EnergyConservation of Energy•Cannot be created or destroyedCannot be created or destroyed• Can be converted from one form to anotherKinetic energy and momentumKinetic energy and momentum•Properties of moving thingsProperties of moving things• Momentum is a vector quantityb ll d ith it t–can be cancelled with opposite momentum• Kinetic Energy is a scalar quantity– Cannot ever be cancelled• http://www.littletheatre.net/Firecracker_Miss/miss_firecracker_contest.htmConservation of EnergyConservation of Energy•Transformation fromTransformation from one form to another•Potential energy of•Potential energy of stretched rubber of slingshotslingshot• Transformed to ki ti f kkinetic energy of rock flying through airConservation of EnergyConservation of Energy•Rock transfers itsRock transfers its kinetic energy to the object it hitsobject it hits• May be transformed to heat upon impactto heat upon impact• Energy cannot be td d t dcreated or destroyedConservation of EnergyConservation of Energy• http://science.howstuffworks.com/roller-coaster.htmConservation of EnergyConservation of EnergyE10000 JE0J•EP= 10000 JEK= 0 J•EP= 7500 JEK= 2500 JEP= 7500 JEK= 2500 J•EP= 5000 J EK= 5000 JPK•EP= 2500 J EK= 7500 J•EP= 0 J EK= 10000 JConservation of EnergyConservation of Energy•Does a car use more energy when itsDoes a car use more energy when its lights are on?• What about when the air conditioner is on?• How about using the radio when the gengine is off?WorkWorkTransference of EnergyTransference of EnergyWork = Force x
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