Newton s Laws of Motion Chapter 2 pages 37 53 Review questions 1 5 10 14 17 21 24 30 Sir Isaac Newton Born 1642 1665 began individual studies Proved universal gravitation Invented the Calculus Reflector telescope 1672 Mathematical Principles of Natural Philosophy Principia First Law of Motion Every object continues in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by forces impressed upon it CONTINUES INERTIA http www physicsclassroom com mmedia newtlaws cci html Mass How much matter Measure of inertia Inertia Bird Inertia Coin Inertia Second Law of Motion The 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 mass say is proportional to Second Law F a m Acceleration is directly proportional to force Acceleration is inversely proportional to mass Increase forces Force on brick creates acceleration Twice force on brick creates more acceleration Double force of bricks requires double force Acceleration equation F a m Double the force Also need to double the mass Direct proportion constant ratio Acceleration Change in velocity over time a change in velocity time F a m Force over mass How can this be Acceleration v t t Change in velocity over time Definition of acceleration Acceleration Force causes acceleration Force over mass F a m Freely falling objects Acceleration of gravity is 9 81 m s2 Use g a constant 9 81 m s2 round to 10 m s2 For lecture calculations F a m F g m Acceleration of Gravity Acceleration same for each second of travel Free falling objects Acceleration of Gravity Free fall of object Time elapsed Speed increases seconds 10 m s for every second of fall 0 10 m s 10m 1 s s2 2 Speed meters second 0 10 20 3 30 4 40 Acceleration of Gravity Upward throw 30 m s Gravity acts against it Slows to stop at 10 m s2 Falls and gains speed at 10 m s2 Neglecting air resistance Increasing mass http www physicsclassroom com mmedia newtlaws efff html Weight is a Force F g m Weight mg Weight is a Force Function of g acceleration of gravity Proper units Gravity m s2 Force kg m s2 Mass kg Different planet Different g different weight for the same mass F g m mg F Freely falling objects Boulder vs feather F a m a F m Boulder has more inertia but not more acceleration so how come it falls faster Galileo s investigation of motion Surface area changes air resistance Objects reach terminal velocity due to air resistance In vacuum this is not a factor Acceleration Neglect air resistance for these equations a v t v at acceleration is velocity divided by time velocity acquired is acceleration multiplied by time Air Resistance Friction of air against falling object Air resistance depends on Speed Frontal area exposed to air Acceleration is less than g due to air resistance Air Resistance Parachute increases frontal area increases air resistance Increased air resistance balanced with slower speed http www physicsclassroom com mmedia newtlaws sd html Air Resistance Greater air resistance for elephant because it is larger Greater weight because it is more massive More speed required to gain air resistance to overcome the greater weight http www physicsclassroom com mmedia newtlaws efar html Air Resistance Force of air resistance balances greater mass at greater speed Heavier skydiver has greater terminal velocity than lightweight skydiver http www physicsclassroom com mmedia newtlaws efar html Zero Acceleration Motionless objects Downward force created by gravity Upward force created by surface Zero Acceleration Push down on spring Spring pushes up on you Each molecule of table acts like microscopic spring pushing up on object Zero Acceleration Cart crossing room without acceleration Net force is zero Force applied pushing force frictional force Friction Works against forces Opposite direction Not dependent on speed Not dependent on area of contact Only dependent on weight Third Law of Motion Whenever one object exerts a force on a second object the second object exerts an equal and opposite force on the first Force is an interaction between objects Action reaction pairs Force interaction F a m F m a F m a Action reaction pairs Force on object moves it Force by object acts on other things Always equal Action reaction pair Hammer exerts force on nail Nail exerts equal force on hammer Action reaction pairs Consider firing a cannon Force on cannonball and on cannon the same cannonball has less mass than cannon Cannonball has greater acceleration F F m F m a Action reaction pairs Rifle has less acceleration than the bullet Because it has greater mass Forces are the same Action reaction pairs Rocket accelerates upward Recoil from exhaust gas Action reaction pairs Birds push down on air Air pushes up on bird Fish pushes backward on water Water pushes forward on fish http en wikipedia org wiki Fish Action reaction pairs Forces of atoms within objects are in action reaction pairs No net acceleration due to these molecular forces External force needed to move object Bug vs Bus If a bug is splatted against the windshield of a bus on the freeway is the force the bug exerts on the bus the same as the force the bus exerts on the bug Justify why the deceleration of the bug is not the same as the deceleration of the bus with Newton s third law Summary of laws of motion Newton s First Law of Motion Object at rest tends to remain at rest Objects in motion tend to remain moving Law of Inertia Function of mass of object Changes in motion occur due to presence of net force acting on object Summary of laws of motion Newton s Second Law of Motion Acceleration proportional to net force F a m Summary of laws of motion Newton s Third Law of Motion Objects exert equal and opposite forces upon one another Action reaction pairs have no net force
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