Catch a wave And you're sittin' on top of the World Beach Boys

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In Class: ---------------------------------------------------- review: Universe is big, empty, and highly structured changes with time ----------------------------------------------------- because it's big, it's really hard to study can't go out and get a piece of a star to look at even getting to the Sun would take awhile, not to mention the fact that you'd get fried if you tried stars are just out of the question Pioneer 11 and Voyager 1 are the most distant manmade objects - neither has left the solar system though both are further away from us than Pluto At this rate, they will get to the nearest star in 150,000 years the only way to study most of the universe is to look see the light coming from objects - it travels pretty fast - it can travel through empty space and since space is empty, it can travel pretty far - it comes to us for free - all we need to do is collect it (which IS expensive) and figure out how to interpret it (which is tricky) Light, as we'll find out on Monday, can be described as a wave in many situations. This is certainly true in terms of its propagation. So let's try to understand what a wave is by examining more obvious cases of wave-motion. A wave is first and foremost a vibration - it needs a medium <-- something to wiggle - like a rope - what happens if I put a vibration into a rope - hey who knocked down that cup? - wasn't me - I was way over here - my fingers never left my hands - I did shake this part of the rope, but this part stayed here -- the other part knocked down the cup. - So what gives? - Of course I was responsible for that action but I did it in a clever way I didn't shoot a bullet at the cup (or throw a rock) no material went from me to the cup - but energy did - I gave the rope a wiggle, and the wiggle, NOT the rope, travelled down the rope - the wiggle energized the part of the rope down there, and that part of the rope knocked the cup over. - at the end of the event, no material had been transported from here to there - only energy Energy is the essence of a wave - waves are energy propagating along a medium - they typically do not involve bulk transport of material How does it happen in a rope? I pull one part up; that part pulls the other part up, etc. communication between adjacent parts of the rope wave SPEED of wave is related to how quickly adjacent parts can communicate with one another. - can measure speed directly --> watch - can change speed by changing the tension in the rope - tighter rope --> communcation between bits faster What about other waves? same issues appply water waves - no bulk motion - method of propagation uses gravity flows of material mountain of water propagates across a pond not the same material; just the energy - change the speed of the wave by changing the density of the liquid - thick stuff flows more slowly slinky waves same issues apply however, we can make a different kind of wave up til now, we've looked at TRANSVERSE waves displacement perp. to direction of propagation now look at LONGITUDINAL waves -- displacement in the same direction as propagation -- generally compressive compression; rarefaction same kind of idea as water wave; too much sloshing back and forth -- can change the wave speed by increasing tension in slinky: faster communication; faster wave SPEED sound waves -- are longitudinal waves; involve compression of air (or whatever) -- compressions and rarefactions make your eardrum move back and forth. --> you hear sound -- this is why loud sounds can ruin your hearing whack your eardrums with high pressures does damage -- faster communication means faster speed -- examples with function generator ------------ So far, we've talked about pulses individual wiggles on a rope, a single water wave propagating across a pond most waves aren't like that -- ocean waves; not a single wave, but many, in periodic succession -- guitar strings; not a single wiggle, but many -- sound waves; a single pulse of compressed air isn't going to sound like anything waves are better characterized as a string of pulses for sound, and many waves, the FREQUENCY of repetition is very important and characterizes what you hear your ear measures the FREQUENCY of the pulses how many times per minute or second a compression is detected. Measure the frequency of a wave by standing in one place and counting the number of complete waves that go by (demo transverse wave apparatus) complete wave; frequency Frequency = number of waves per second 10 waves per second = 10 Hz period = length of time between waves 10 waves go by in a second; then each of the ten waves take 1/10 of a second to go by. --> time between, say peaks of waves = 1/10th second period = 1/frequency NOTE NOTE NOTE: changing the frequency does not mean changing the SPEED of the wave. The SPEED of a wave is defined by the medium, not the frequency. Frequency is simply how many waves per second Higher frequency means more waves per second, but not faster waves-- they're just more bunched together WAVELENGTH is a measure of how bunched together the waves are WAVELENGTH= distance between waves eg. distance between peaks, valleys, compressions, whatever measuring the wavelength of a wave is tricky one way is to use the period period is the time between waves since all waves travel at the wave speed the wavelength must be the period x speed remember distance = rate x time?