ARISTOTELIAN PHYSICS

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Magnus Lee
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1 ARISTOTELIAN PHYSICS Aristoteles (Aristotle) ( BC) had very strog ifluece o Europea philosophy ad sciece; everythig o Earth made of (mixture of) four elemets: earth, water, air, fire every elemet has a atural place : u earth at ceter of Earth, u water above earth, u air above water, u fire above air; celestial bodies (stars, plaets, Moo) made from fifth elemet, ether, which also fills space betwee them; ether is perfect, icorruptible, weightless; two kids of motio of thigs o Earth: atural ad violet motio atural motio: thigs ted to move towards their atural place - atural motio happes by itself, eeds o push/pull (e.g. stoe falls). violet motio: = motio cotrary to atural motio; eeds effort (exteral push or pull) celestial motio = atural motio of ether; atural motio of bodies made from ether is circular motio, regular ad perpetual

2 Problems with aristotelia physics: Galileo Galilei's thought experimets ad real experimets: fallig bodies: u accordig to Aristoteles, heavy bodies (cotai more earth elemet) fall faster tha lighter bodies u observatio: fall equally fast if they have same shape ad size u Galilei: differece i speed of differetly shaped fallig bodies due to air resistace thought experimet about two fallig bodies - reductio ad absurdum : u cosider two bodies, oe light (L), oe heavy (H) Aristoteles: L falls more slowly tha H L put uder H should slow dow fall of H; H with L uder it should fall more slowly tha H aloe; but (L + H) heavier tha H aloe should fall faster tha H aloe cotradictio. pedulum: ball suspeded o strig reaches same height as that to which it was lifted to set it i motio (ot quite; - due to frictio); height idepedet of path (pedulum with shorteed strig) ball rollig o iclied plae: u ball rollig dow iclied plae speeds up; u ball rollig up slows dow; rate of slowig dow depeds o steepess of iclie: less steep loger distace travelled; extrapolatio to zero slope of iclie: ball will go o forever

3 GALILEI'S NEW SCIENCE Galileo Galilei ( ) -- fouder of moder sciece; ew methods itroduced by Galilei iclude: u cotrolled experimets desiged to test specific hypotheses u idealizatios to elimiate ay side effects that might obscure mai effects u limitig the scope of equiry - cosider oly oe questio at a time; u quatitative methods - did careful measuremets of the motio of fallig bodies. from observatios ad thought experimets, geeralizes to two ew laws: LAW OF INERTIA: without exteral ifluece (force) actig o it, a body will ot chage its speed or directio of motio; it will stay at rest if it was at rest to begi with. iertia = property of bodies that makes them obey this law, their ability to maitai their speed (or stay at rest) LAW OF FALLING: if air resistace is egligible, ay two objects that are dropped together will fall together; speed of fallig idepedet of weight ad material.

4 NEWTONIAN MECHANICS Startig from law of iertia (Reé Descartes, Galileo Galilei), Isaac Newto developed a ew way of lookig at ature. Pricipia Mathematica Philosophiae Naturalis (1687) (Mathematical Priciples of Natural Philosophy): based o a small umber of cocepts ad priciples,provide a clear ad quatitative explaatio of a vast array of pheomea. give a uificatio of our view of ature - the first major sythesis of sciece explai: motio of bodies o Earth ad i heave (fallig bodies, Moo, plaets, comets,... key cocepts: u velocity u acceleratio, u force u iertial mass, gravitatioal mass key priciples: u law of iertia (Newto's 1st law of motio) u law of motio (forces) (Newto's 2d law of motio) u law of force pairs (actio=reactio) (Newto's 3rd law of motio'') u law of gravity

5 FORCE law of iertia: o force o acceleratio; if acceleratio - there must be force; we say: body exerts force o aother if it forces the other body to accelerate; ote there is some circularity i this defiitio, but defiitio is justified by its usefuless ad predictive power; force is ot a property of a body; if more the oe force actig effects add forces add -- et force ; acceleratio is i directio of et force; two or more forces ca compesate ( balace ) each other (e.g. two equally strog forces actig i opposite directios) kids of forces: u push, pull, shove, kick, tap u frictio, air resistace u gravity u electric u magetic Aristotelia view: forces cause velocity (force ecessary to maitai uiform motio). Newtoia view: forces cause acceleratio (force ecessary to chage motio)

6 Observatios: Forces, Newto s 2d law stroger force larger acceleratio more massive object smaller acceleratio apply more tha oe force et force determies acceleratio iertia = resistace of object agaist beig accelerated; (iertial) mass = measure of amout of iertia, observed to be proportioal to amout of matter -- set them equal; uit of mass = kilogram = kg (origial defiitio: = mass of 1 liter of water) observatios ca be summarized by: Newto's 2d law: F = k m a k = proportioality costat; by choice of uits, ca make k = 1 ote that F, a are vectors, ad acceleratio a is i directio of force F uit of force = ewto; 1 ewto = 1 kg m s -2 i Eglish system: uit of force = poud = N ote: the mass m i Newto's 2d law is the iertial mass weight vs mass: mass of object = quatity of its iertia; weight of a object = et gravitatioal force o a object; depeds o eviromet; our weight o the Moo is 1/6 of that o the surface of the Earth; our weight o a high moutai is smaller tha at sea level; our weight i a satellite i orbit aroud Earth = 0; our mass is always the same.

7 Newto s 3rd law (Law of force pairs - actio ad reactio) actio = reactio' whe a body exerts a force o a secod body, the secod body exerts a equally strog force o the first body, directed opposite to the first force; examples: apple ad Earth: u Earth exerts force o apple apple exerts force o Earth; u Earth's large mass Earth's acceleratio very small book o table: 2 pairs of forces: u Earth exerts gravitatioal force o book, book exerts gravitatioal force o Earth. u book exerts force (= its weight) o table; table exerts equal ad opposite force o book ( cotact force, ormal force ) u et force o book = 0 book stays at rest o table (does ot fly away, does ot fall through table) u (cotact force caused by iteractio of electros i atoms of book with those i table) walkig: exert force o groud groud exerts force o you; rowig, drivig, recoil of a gu, rocket propulsio Note: Newto s 3rd law closely related to mometum coservatio

04 - LAWS OF MOTION Page 1 ( Answers at the end of all questions )

04 - LAWS OF MOTION Page 1 ( Answers at the end of all questions ) 04 - LAWS OF MOTION Page ) A smooth block is released at rest o a 45 iclie ad the slides a distace d. The time take to slide is times as much to slide o rough iclie tha o a smooth iclie. The coefficiet