Second Law of Motion. Force mass. Increasing mass. (Neglect air resistance in this example)

Transcription

1 Newton Law of Motion Moentu and Energy Chapter -3 Second Law of Motion The acceleration of an object i directly proportional to the net force acting on the object, i in the direction of the net force, and i inverely proportional to the a of the object. Mean acceleration ~ orce a orce caue acceleration orce over a Solve for the force: Acceleration = a reely falling object Acceleration of gravity i 9.81 Ue g, a contant 9.81 or lecture calculation round to 10 g = Increaing a g = Weight i a orce Weight = g (Neglect air reitance in thi exaple) Weight i a orce unction of g (acceleration of gravity) Proper unit: Gravity Ma kg orce kg Different planet different weight for the ae a g = g = reely falling object Boulder v. feather Boulder ha ore inertia, but not ore acceleration o how coe it fall fater? Galileo invetigation of otion Surface area change air reitance Object reach terinal velocity due to air reitance In vacuu, thi i not a factor 1

2 Air Reitance Acceleration i le than g due to air reitance riction of air againt falling object Air reitance depend on Speed rontal area expoed to air Air Reitance alling object ha contant a, contant weight Terinal velocity reached when air reitance atche weight Air reitance function of peed and area alling object ha variable frontal area if you deploy a parachute Air Reitance Parachute increae frontal area, increae air reitance Increaed air reitance balanced with lower peed Air Reitance Greater air reitance for elephant, becaue it i larger Greater weight becaue it i ore aive More peed required to gain air reitance to overcoe the greater weight Air Reitance orce of air reitance balance greater a at greater peed Heavier kydiver ha greater terinal velocity than lightweight kydiver Zero Acceleration One cae: Motionle object (no change in velocity) Downward force created by gravity Upward force created by urface Zero Acceleration Another cae: Cart croing roo at contant velocity (no change in velocity) Net force i zero orce applied puhing force = frictional force Zero Acceleration Puh down on pring Spring puhe up on you Each olecule of table act like icrocopic pring puhing up on object riction Work againt force Oppoite direction Not dependent on peed Not dependent on area of contact Only dependent on weight

3 Third Law of Motion Whenever one object exert a force on a econd object, the econd object exert an equal and oppoite force on the firt. orce i an interaction between object orce interaction = a = a = orce in action-reaction pair are equal Different ae ut have different acceleration a orce on object ove it orce by object act on other thing Alway equal Action reaction pair Haer exert force on nail Nail exert equal force on haer = Conider firing a cannon orce on cannonball and on cannon the ae cannonball ha le a than cannon Cannonball ha greater acceleration = a a Rifle ha le acceleration than the bullet Becaue it ha greater a orce are the ae Rocket accelerate upward Recoil fro exhaut ga Bird puh down on air Air puhe up on bird ih puhe backward on water Water puhe forward on fih orce of ato within object are in action reaction pair No net acceleration due to thee olecular force External force needed to ove object 3

4 Bug v. Bu If a bug i platted againt the windhield of a bu on the freeway, i the force the bug exert on the bu the ae a the force the bu exert on the bug? Jutify why the deceleration of the bug i not the ae a the deceleration of the bu with Newton third law. Suary of law of otion Newton irt Law of Motion Object at ret tend to reain at ret Object in otion tend to reain oving Law of Inertia unction of a of object Change in otion occur due to preence of net force acting on object Suary of law of otion Newton Second Law of Motion Acceleration proportional to net force a Suary of law of otion Newton Third Law of Motion Object exert equal and oppoite force upon one another have no net force Moentu Moentu i inertia in otion Ma x velocity Ha both agnitude and direction Large a or high peed can give object great aount of oentu Moentu = v Change oentu by changing a or velocity uually velocity What caue change in velocity? orce Tie i alo iportant Moentu and Ipule Apply force over tie to change velocity and oentu Greater tie of application, greater change in oentu orce x tie interval i IMPULSE orce x tie interval Ipule change oentu Technically: Realitically: Ipule = t t = Δ(v) t = Δv Ipule Increaing Moentu t = Δv Puhing a child on a wing the force Increae oentu Longer puh increae oentu ore than a hort one t = Δ(v) 4

5 Ipule Decreaing Moentu Stopping the out of control car Change oentu to zero Le force if tie i greater t = Δv Ipule Change Moentu Can change force by changing tie Move away increae tie, reduce force Toward decreae tie, increae force Bounce increae ipule There i a change in velocity direction, aking a greater Δv So greater force i required Water change direction, ha greater ipule than a flat paddle t = Δv Conervation of Moentu Conervation of Moentu Conervation of Moentu The yte: the cannon and the ball When it fire, oentu i conerved they both have oentu: in oppoite direction No net force No net oentu Moentu = v In the abence of external force, the oentu of the yte reain unchanged Moentu = v Conider individual ball a individual yte: oentu of each doe change Moentu = v Conervation of Moentu Net oentu i the ae before the colliion A after the colliion Moentu = v Conervation of Moentu a. Yellow ball tart tationary b. Both ball oving oppoite direction c. Green ball oving fater Work Tranference of Energy Work = orce x ditance W=d Moentu = v 5

6 Work W=d Work Work W=d Nothing about tie in definition Slow or fat Sae force, ae ditance = ae work Lifting load againt the force of the weight of the object Twice the ditance reult in twice the work Twice the weight i twice the work Twice the weight Twice the ditance Work W=d Work W=d kg Unit of force : Newton = Unit of work are Joule Work i energy orce x ditance : Newton eter = kg kg = =Joule Work Weight lifter doe work to lift barbell expend energy to keep the potential energy in the barbell But he doe no work on the barbell after it i lifted W=d 6