Unformatted text preview: Springy Things
Restoring Force Oscillation and Resonance Model for Molecules UCSD: Physics 8; 2 Springs: supplying restoring force When you pull on (stretch) a spring, it pulls back (top picture) When you push on (compress) a spring, it pushes back (bottom) Thus springs present a restoring force:
F = k x x is the displacement (in meters) k is the "spring constant" in Newtons per meter (N/m) the negative sign means opposite to the direction of displacement UCSD: Physics 8; 2 Example If the springs in your 1000 kg car compress by 10 cm (e.g., when lowered off of jacks): then the springs must be exerting mg = 10,000 Newtons of force to support the car F = k x = 10,000 N, x = 0.1 m so k = 100,000 N/m (stiff spring) this is the collective spring constant: they all add to this Now if you pile 400 kg into your car, how much will it sink? 4,000 = (100,000) x, so x = 4/100 = 0.04 m = 4 cm Could have taken shortcut: springs are linear, so 400 additional kg will depress car an additional 40% (400/1000) of its initial depression UCSD: Physics 8; 2 Energy Storage in Spring Applied force is kx (reaction from spring is kx) starts at zero when x = 0 slowly ramps up as you push Work is force times distance Let's say we want to move spring a total distance of x would naively think W = k x2 but force starts out small (not full k x right away) works out that W = k x2 Work "Integral" UCSD: Physics 8; 2 Since work is force times distance, and the force ramps up as we compress the spring further...
Force from spring increases as it is compressed further Area is a work: a force (height) times a distance (width) Force Force distance (x) Total work done is area of triangle under force curve Force distance (x) distance (x) takes more work (area of rectangle) to compress a little bit more (width of rectangle) as force increases (height of rectangle) if full distance compressed is k x, then force is k x, and area under force "curve" is (base)(height) = ( x)k x = k x2 area under curve is called an integral: work is integral of force UCSD: Physics 8; 2 The Potential Energy Function Since the potential energy varies with the square of displacement, we can plot this as a parabola Call the low point zero potential Think of it like the drawing of a trough between two hillsides A ball would roll back and forth exchanging gravitational potential for kinetic energy Likewise, a compressed (or stretched) spring and mass combination will oscillate exchanges kinetic energy for potential energy of spring UCSD: Physics 8; 2 Example of Oscillation Plot shows position (displacement) on the vertical axis and time on the horizontal axis Oscillation is clear Damping is present (amplitude decreases) envelope is decaying exponential function UCSD: Physics 8; 2 Frequency of Oscillation Mass will execute some number of cycles per second (could be less than one) This is the frequency of oscillation (measured in Hertz, or cycles per second) The frequency is proportional to the square root of the spring constant divided by the mass: Larger mass means more sluggish (lower freq.) Larger (stiffer) spring constant means faster (higher freq.) UCSD: Physics 8; 2 Natural Frequencies & Damping Many physical systems exhibit oscillation guitar strings, piano strings, violin strings, air in flute lampposts, trees, rulers hung off edge of table buildings, bridges, parking structures Some are "cleaner" than others depends on complexity of system: how many natural frequencies exist a tree has many: many branches of different sizes damping: energy loss mechanisms (friction, radiation) a tree has a lot of damping from air resistance cars have "shocks" (shock absorbers) to absorb oscillation energy elastic is a word used to describe lossless (or nearly so) systems "bouncy" also gets at the right idea UCSD: Physics 8; 2 If you apply a periodic force to a system at or near its natural frequency, it may resonate depends on how closely the frequency matches damping limits resonance Resonance Driving below the frequency, it deflects with the force Driving above the frequency, it doesn't do much at all Picture below shows amplitude of response oscillation when driving force changes frequency UCSD: Physics 8; 2 Resonance Examples Shattering wine glass if "pumped" at natural frequency, amplitude builds up until it shatters Swinging on swingset you learn to "pump" at natural frequency of swing amplitude of swing builds up Tacoma Narrows Bridge eddies of wind shedding of top and bottom of bridge in alternating fashion "pumped" bridge at natural oscillation frequency totally shattered big lesson for today's bridge builders: include damping UCSD: Physics 8; 2 Wiggling Molecules/Crystals Now imagine models of molecules built out of spring connections Result is very wiggly Thermal energy (heat content) manifests itself as incessant wiggling of the atoms composing molecules and crystals (solids) This will be important in discussing: microwave ovens colors of materials optical properties heat conduction UCSD: Physics 8; 2 A model for crystals/molecules We can think of molecules as masses connected by springs Even neutral atoms attract when they are close, but repel when they get too close electrons "see" (and like/covet) the neighboring nucleus but when the electrons start to overlap, repulsion takes over try moving in with the neighbor you covet! The trough looks just like the spring potential so the "connection" is springlike UCSD: Physics 8; 2 Estimation: How fast do they wiggle? A 1 kg block of wood takes 1000 J to heat by 1 C just a restatement of heat capacity = 1000 J/kg/C so from 0 to 300 K, it takes 300,000 J If we assign some kinetic energy to each mass (atom), it must all add up to 300,000 J The velocities are randomly oriented, but we can still say that mv2 = 300,000 J so v2 = 600,000 (m/s)2 characteristic v = 800 m/s (very fast!) This is in the right ballpark for the velocities of atoms buzzing about within materials at room temperature it's what we mean by heat UCSD: Physics 8; 2 Assignments HW1 due today First biweekly question/observation due tomorrow (4/14) 6PM cutoff is strict; half credit for following week Reading: Chapter 10: 302308, 324330 for Tuesday HW2: 7.E.1, 7.E.4, 7.P.1, 7.P.2, 7.P.3, 3.P.2, 3.P.4, plus eight additional required problems available on assignments page ...
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 Fall '10
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 Physics, Energy, Force, Potential Energy

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