Wave properties Wave properties High School Physics Khan Academy

imagine that I'm standing here holding the end of rope I'm over here on the Left End and while holding the Rope rapidly move my hand up down and back to the starting position if we were to take snapshot of the Rope immediately after finish my motion we're going to see something like this the Rope has squiggly disturbance that mirrors the motion made with my hand up down and back to the middle and the rest of the Rope is still flat you might have seen something like this if you've ever played with jump rope and wiggled it back and forth or slinky and you've seen that oscillate back and forth on the ground or if you've been in the gym and seen somebody doing exercises with large battle ropes slamming them up and down repeatedly and we know that over time this disturbance is actually going to make its way through the Rope if this is what we observe right after my hand motion at some later point in time we will observe that the beginning of the Rope is back to its original shape the squiggly disturbance has made its way further down the rope and it will keep traveling in this direction until it reaches the end of the Rope this is exactly what wave is in physics wave is disturbance in this case the squiggle in the Rope caused by my hand motion and that disturbance can propagate it can travel or move in particular direction so wave is disturbance that can propagate this particular example is called mechanical Wave It's called mechanical wave because the disturbance is traveling through medium in this case the Rope so mechanical waves travel through medium one important point about waves that is worth noting right now is that waves transfer energy without transferring matter so what that means is that the disturbance that is moving here this squiggly shape is moving through the Rope but it isn't moving the Rope to different position any part of the Rope might go up and down as wave travels through that section but the Rope itself is not going anywhere rather it's the kinetic energy imparted to the Rope by my hand that is transferring from particle to particle and making its way through the Rope so waves transfer energy but not matter so in my first example only jerked my hand up and down once which created single wave pulse that moved through my rope If instead were to keep moving my hand up and down consistently would see waveform that looks something like this and when we model wave there are few key characteristics that we need to know about that wave first is the period period is measured in seconds and it tells us how long it takes for one wave cycle to complete next is the wavelength measured in units of distance like meters the wavelength is the distance between identical points of adjacent waves and finally there's frequency so if the waveform that we've drawn here takes 1 second there are four Cycles in that 1 second that means it has frequency of 4 Hertz or four cycles per second so the frequency measured in Cycles per second tells us how many wave Cycles there are every second Now using just these basic anatomical properties of wave we can start to figure out more interesting physical characteristics like speed or distance over time if we want to know how fast wave is traveling we can take its wavelength which is the distance covered by single cycle and multiply that by the frequency which is how many cycles are completed in second given amount of time the Cycles cancel out and we're left with units of distance over time the same as speed and that's our equation for the speed of the wave wavelength times frequency the standard units for Speed are me/ second there are couple factors that can affect the speed of wave the first is the wave type so different types of waves move at different speeds relatable example of different waves moving at different speeds is lightning have you ever seen lightning strike or been in thunderstorm you know that the first thing you see is the flash of lightning and then you hear the thunder associated with that lightning flash so the lightning comes first and the Thunder comes second that's because those are two different waves that are part of the same phenomenon when the lightning strike hits you see the flash first because that's an electromagnetic wave light it travels much faster than the sound associated with the lightning strike electromagnetic waves are special not only because they travel really fast but they also don't need medium to travel through the thunder on the other hand is sound wave traveling slower than the light so you'll see the lightning before you hear the thunder different wave types move at different speeds the second key factor that can affect the speed of wave is the medium through which the wave travels and we'll consider sound as an example here so when someone is talking right we have this talking head creating some vibrations of the particles in front of their mouth that's the sound wave it's the vibration of those particles propagating through the air when you speak your vocal cords exert force on the particles just in front of you they vibrate back and forth creating compression that transfers to the surrounding particles as the vibrations continue to propagate the sound travels you can imagine that if these particles are packed closer together those vibrations are going to transfer lot more quickly because the particles are colliding much faster than if they're further apart so sound travels much faster in water liquid then it doesn't air for that exact reason the particles in the liquid are closer together since they're closer compacted they Collide more and the propagation of the wave happens faster so different waves move at different speeds and the medium through which wave travels can also affect the speed of wave all right so let's try to summarize all this information we have waves wave disturbance that can propagate and it has few key characteristics there's the period or how long it takes takes one cycle to complete there's the wavelength the distance between identical points on two waves that are next to each other and the frequency which is how many wave Cycles complete in 1 second in this case we have two cycles in 1 second for frequency of two Hertz wave speed is found by multiplying wavelength and frequency and that wave speed is affected by the type of wave and the medium through which the wave travels mechanical waves are waves that travel through medium so sound slinkier or rope ocean waves and electromagnetic waves like light are special because they can travel through vacuum they don't have to have medium in order to propagate but all waves no matter what type transfer energy not matter