lecture8 - • From experiment we ±nd that the frictional...

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Lecture 8 Friction 1 Review from Last Time Tension and Compresion (normal forces) Can understand why tension force is along the string and the normal force is per- pendicular to the surface if we consider all the atoms in a solid connected by springs. This is a reasonable model because the interatomic forces act like springs (approxi- mately obey Hook’s law) for small displacements. Young’s Modulus - review as hook’s law Strain is fractional stretch of the wire: strain = Δ L L Stress is the tension force per unit area: stress = F T A Stress is generally proportional to strain with proportionality constant Y = F T /A Δ L/L PUZZLER: two wires attached. Then work out amount of stretch goes as 1/Y so bigger in aluminum portion. 2 Friction Friction arises from the electric attraction of molecules between two surfaces that are in close contact. The microphysics of it is very complicated and not fully understood.
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Unformatted text preview: • From experiment, we ±nd that the frictional force – is proportional to the normal force – is independent of area of the contact – depends on whether the object is stationary or sliding: static friction: f s ≤ μ s N kinetic friction: f k = μ k N – μ s and μ k are coe²cients of static and kinetic friction respectively. They are mea-sured in experiments • In general, μ s ≥ μ k for a given situation (SHOW SLIDE - PLOT) • That friction increases with the normal force makes some sense. As the normal force increases, the surfaces are brought into closer contact. Any bumpiness is more ³attened out, increasing the microscopic area of contact. • Air resistance is another “dissipative” force: we won’t talk in detail about it here but you will do a homework porblem that explores it a little bit. 30...
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