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2. Newtons Laws of MotionReview 3. BackgroundSir Isaac Newton (1643-1727) an Englishscientist and mathematician famous for hisdiscovery of the law of gravity alsodiscovered the three laws of motion. Hepublished them in his book PhilosophiaeNaturalis Principia Mathematica(mathematic principles of naturalphilosophy) in 1687. Today these laws areknown as Newtons Laws of Motion anddescribe the motion of all objects on thescale we experience in our everyday lives. 4. If I have ever made any valuable discoveries, it hasbeen owing more to patient attention, than to anyother talent.-Sir Isaac Newton 5. Newtons Laws of Motion1. An object in motion tends to stay inmotion and an object at rest tends tostay at rest unless acted upon by anunbalanced force.2. Force equals mass times acceleration(F = ma).3. For every action there is an equaland opposite reaction. 6. Newtons First LawAn object at rest tends to stay at restand an object in motion tends to stay inmotion unless acted upon by anunbalanced force. 7. What does this mean?Basically, an object will keep doing what itwas doing unless acted on by anunbalanced force.If the object was sitting still, it will remainstationary. If it was moving at a constantvelocity, it will keep moving.It takes force to change the motion of anobject. 8. What is meant by unbalancedforce?If the forces on an object are equal and opposite, they are saidto be balanced, and the object experiences no change inmotion. If they are not equal and opposite, then the forces areunbalanced and the motion of the object changes. 9. Some Examples from Real LifeA soccer ball is sitting at rest. Ittakes an unbalanced force of a kickto change its motion.Two teams are playing tug of war. They are bothexerting equal force on the rope in oppositedirections. This balanced force results in nochange of motion. 10. Newtons First Law is alsocalled the Law of InertiaInertia: the tendency of an object toresist changes in its state of motionThe First Law states that all objectshave inertia. The more mass an objecthas, the more inertia it has (and theharder it is to change its motion). 11. More Examples from Real LifeA powerful locomotive begins to pull along line of boxcars that were sitting atrest. Since the boxcars are so massive,they have a great deal of inertia and ittakes a large force to change theirmotion. Once they are moving, it takesa large force to stop them.On your way to school, a bugflies into your windshield. Sincethe bug is so small, it has verylittle inertia and exerts a verysmall force on your car (so smallthat you dont even feel it). 12. If objects in motion tend to stay in motion,why dont moving objects keep movingforever?Things dont keep moving forever becausetheres almost always an unbalanced forceacting upon it.A book sliding across a table slowsdown and stops because of the forceof friction.If you throw a ball upwards it willeventually slow down and fallbecause of the force of gravity. 13. In outer space, away from gravity and anysources of friction, a rocket ship launchedwith a certain speed and direction wouldkeep going in that same direction and at thatsame speed forever. 14. Newtons Second LawForce equals mass times acceleration.F = maAcceleration: a measurement of how quickly anobject is changing speed. 15. What does F = ma mean?Force is directly proportional to mass and acceleration.Imagine a ball of a certain mass moving at a certainacceleration. This ball has a certain force.Now imagine we make the ball twice as big (double themass) but keep the acceleration constant. F = ma saysthat this new ball has twice the force of the old ball.Now imagine the original ball moving at twice theoriginal acceleration. F = ma says that the ball willagain have twice the force of the ball at the originalacceleration. 16. More about F = maIf you double the mass, you double the force. If youdouble the acceleration, you double the force.What if you double the mass and the acceleration?(2m)(2a) = 4FDoubling the mass and the acceleration quadruples theforce.So . . . what if you decrease the mass by half? Howmuch force would the object have now? 17. What does F = ma say?F = ma basically means that the force of an objectcomes from its mass and its acceleration.Something very massive (high mass)thats changing speed very slowly (lowacceleration), like a glacier, can stillhave great force.Something very small (low mass) thatschanging speed very quickly (highacceleration), like a bullet, can stillhave a great force. Something verysmall changing speed very slowly willhave a very weak force. 18. Newtons Third LawFor every action there is an equal andopposite reaction. 19. What does this mean?For every force acting on an object, there is an equalforce acting in the opposite direction. Right now,gravity is pulling you down in your seat, butNewtons Third Law says your seat is pushing upagainst you with equal force. This is why you arenot moving. There is a balanced force acting onyou gravity pulling down, your seat pushing up. 20. Think about it . . .What happens if you are standing on askateboard or a slippery floor and push againsta wall? You slide in the opposite direction(away from the wall), because you pushed onthe wall but the wall pushed back on you withequal and opposite force.Why does it hurt so much when you stubyour toe? When your toe exerts a force on arock, the rock exerts an equal force back onyour toe. The harder you hit your toe againstit, the more force the rock exerts back onyour toe (and the more your toe hurts). 21. ReviewNewtons First Law:Objects in motion tend to stay in motionand objects at rest tend to stay at restunless acted upon by an unbalanced force.Newtons Second Law:Force equals mass times acceleration(F = ma).Newtons Third Law:For every action there is an equal andopposite reaction. 22. VocabularyInertia:the tendency of an object to resist changesin its state of motionAcceleration:a change in velocitya measurement of how quickly an object ischanging speed, direction or bothVelocity:The rate of change of a position alonga straight line with respect to timeForce:strength or energy