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Škola: Gymnázium, Brno, Slovanské náměstí 7

Škola: Gymnázium, Brno, Slovanské náměstí 7 Šablona: III/2 – Inovace a zkvalitnění výuky prostřednictvím ICT – Název projektu: Inovace výuky na GSN prostřednictvím ICT Číslo projektu: CZ.1.07/1.5.00/34.0940 Autor: RNDr. Bc. Miroslav Hruška

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Škola: Gymnázium, Brno, Slovanské náměstí 7

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  1. Škola: Gymnázium, Brno, Slovanské náměstí 7 Šablona: III/2 – Inovace a zkvalitnění výuky prostřednictvím ICT – Název projektu: Inovace výuky na GSN prostřednictvím ICT Číslo projektu: CZ.1.07/1.5.00/34.0940 Autor: RNDr. Bc. Miroslav Hruška Tematická oblast: Dynamics of particles (dynamika hmotných bodů) Název DUMu: Friction Kód: VY_32_INOVACE_FY.3.07 Datum: 20. 10. 2013 Cílová skupina: Žáci středních škol s CLIL výukou fyziky v angličtině Klíčová slova: Friction, kinetic (sliding) friction, static friction, coefficient of friction Anotace: Prezentace slouží k uvedení do problematiky smykového a klidového tření. Je vhodné ji použít při probírání nového učiva.

  2. Dynamics of particles 7. Friction

  3. True of false? All frames of reference obey Newton’s laws of motion A body can continue moving only when a force acts on it The acceleration of a body is directly proportional to the net force After an action arises, it takes some time before a reaction reacts

  4. Complete with prepositions Magnitude consists …… value and unit The driver had a look …… the odometer It depends …… the reference frame Earth revolves …… the Sun What does dynamics deal ……? A swimmer usually swims ……1.2 m/s m/s2 is read metre …… second squared He arrived at half …… five

  5. Exercise I Solution I • MV = mv ⇒ V = mv/M • V = 1 × 50 ÷ 100 = 0.5 m/s • http://www.mwit.ac.th/~physicslab/applet_04/physics_classroom/Class/momentum/u4l2d.html pic A 100-kg cannon launched/fired/shot a cannon ball of a mass of 1 kg at a speed of 50 m/s. What was the speed of the cannon’s recoil (zpětného nárazu/zákluzu) after the shot?

  6. Friction I Friction(třecí síla) is a force that is created whenever two surfaces move or try to move across each other Friction is dependent on the texture(struktuře) of both surfaces Friction is also dependent on the contact force pushing the surfaces together http://www.youtube.com/watch?v=VBBeaokDkjQ 12 min

  7. Friction II Contact force: normal force (kolmá tlaková síla) Fnor sometimes N Friction: Ff(in Czech Ft) Ff= μFn(in Czech Ft = fFn), μ (f) is called coefficient of friction (součinitel tření) – it characterizes the surfaces Coefficient of friction is a dimensionless quantity

  8. Exercise II Solution II • The magnitude of the friction acting on the body must be 0.9 N. The normal force has a magnitude of mg = 2.0 NTherefore f = Ft /Fn = 0.9÷2.0 = 0.45 A body of a mass of 200 g is pulled (on a horizontal plane) by a horizontal pull of 0.9 N, and it moves uniformly. Find the coefficient of friction.

  9. Friction III Coefficient of friction in the previous exercise is called coefficient of kinetic or sliding friction (smykového tření) μk(f) At rest there is a coefficient of staticfriction (klidového tření) μs (f0) Static friction resists motion and counteractsthe applied force up to a certain maximum where static frictionis overcome and motion starts

  10. Think over relations between… • … kinetic friction and static friction • Which coefficient is greater? μk < μsf < f0 • … roughness and smoothness of surfaces • What makes the coefficient less? • The smoother surfaces, the less friction? • Yes, but once you reach a certain degree of smoothness, friction startsincreasing!

  11. Exercise III Solution III • Friction was Ft = ma, where a = v/t = … • Normal force was Fn = mg • Thenf = Ft /Fn = a/g = 2.5/10 = 0.25 A body moving horizontally with an initial speed of 10 m/s stopped thanks to friction in 4 seconds. Find the coefficient of kinetic friction.

  12. That is all for now Thank you for your attention

  13. Materiál je určen pro bezplatné používání pro potřeby výuky a vzdělávání na všech typech škol a školských zařízeních. Jakékoliv další využití podléhá autorskému zákonu. Zdroje: • Bednařík, M., Široká, M. Fyzika pro gymnázia, Mechanika. Dotisk 3. vyd. Praha: Prometheus, 2004. 288 s. ISBN 80-7196-176-0 • Hornby, A. S. Oxford Advanced Learner’s Dictionary of Current English. 5. vyd. Oxford: Oxford University Press, 1995. 1428 s. ISBN 0-19-431423-5 Autor nezaručuje funkčnost webových odkazů!

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