WAVEWAVE

• So waves are everywhere. But what So waves are everywhere. But what makes a wave makes a wave a wavea wave? What ? What characteristics, properties, or behaviors characteristics, properties, or behaviors are shared by all the phenomenon are shared by all the phenomenon which we typically characterize as which we typically characterize as being a wave? How can waves be being a wave? How can waves be described in a manner that allows us to described in a manner that allows us to understand their basic nature and understand their basic nature and qualities? qualities?

WHAT IS A WAVEWHAT IS A WAVE

• A wave can be described as a disturbance that travels through a medium A wave can be described as a disturbance that travels through a medium from one location to another location. Consider from one location to another location. Consider a slinky wavea slinky wave as an as an example of a wave. When the slinky is stretched from end to end and is example of a wave. When the slinky is stretched from end to end and is held at rest, it assumes a natural position known as the held at rest, it assumes a natural position known as the equilibrium or equilibrium or rest positionrest position. The coils of the slinky naturally assume this position, . The coils of the slinky naturally assume this position, spaced equally far apart. To introduce a wave into the slinky, the first spaced equally far apart. To introduce a wave into the slinky, the first particle is displaced or moved from its equilibrium or rest position. The particle is displaced or moved from its equilibrium or rest position. The particle might be moved upwards or downwards, forwards or backwards; particle might be moved upwards or downwards, forwards or backwards; but once moved, it is returned to its original equilibrium or rest position. but once moved, it is returned to its original equilibrium or rest position. The act of moving the first coil of the slinky in a given direction and then The act of moving the first coil of the slinky in a given direction and then returning it to its equilibrium position creates a returning it to its equilibrium position creates a disturbancedisturbance in the slinky. in the slinky. We can then observe this disturbance moving through the slinky from one We can then observe this disturbance moving through the slinky from one end to the other. If the first coil of the slinky is given a single back-and-end to the other. If the first coil of the slinky is given a single back-and-forth vibration, then we call the observed motion of the disturbance forth vibration, then we call the observed motion of the disturbance through the slinky a through the slinky a slinky pulseslinky pulse. A . A pulsepulse is a single disturbance moving is a single disturbance moving through a medium from one location to another location. However, if the through a medium from one location to another location. However, if the first coil of the slinky is continuously and periodically vibrated in a back-first coil of the slinky is continuously and periodically vibrated in a back-and-forth manner, we would observe a repeating disturbance moving and-forth manner, we would observe a repeating disturbance moving within the slinky which endures over some prolonged period of time. The within the slinky which endures over some prolonged period of time. The repeating and periodic disturbance which moves through a medium from repeating and periodic disturbance which moves through a medium from one location to another is referred to as a one location to another is referred to as a wavewave..

DIFFERENT TYPES OF DIFFERENT TYPES OF WAVESWAVES• Ocean waves: Many things can create waves on the ocean some of those things are Ocean waves: Many things can create waves on the ocean some of those things are

as wind, earthquakes, the current, etc. Also the sun and moon’s gravitational pull as wind, earthquakes, the current, etc. Also the sun and moon’s gravitational pull controls the tide levels. Tide makes the water go up and down, whereas the current controls the tide levels. Tide makes the water go up and down, whereas the current in the water makes the water move horizontally. in the water makes the water move horizontally.

          Heat wave: A heat wave is a basically a really intense hot spell, that would           Heat wave: A heat wave is a basically a really intense hot spell, that would usually not come during the year. In Europe, 2003, 14,802 people died in just one usually not come during the year. In Europe, 2003, 14,802 people died in just one heat wave. In UK, during the same year 2045 people died. Also during the same year heat wave. In UK, during the same year 2045 people died. Also during the same year in Portugal 2099 people died just because of the one heat wave. in Portugal 2099 people died just because of the one heat wave.

          Greeting waves: People wave to each other in greeting. There are many           Greeting waves: People wave to each other in greeting. There are many different types of greeting waves. They usually correspond to the culture the people different types of greeting waves. They usually correspond to the culture the people are in. Ancient Indians raised their right hand like you do when you take an oath and are in. Ancient Indians raised their right hand like you do when you take an oath and said something that sounded like “hou”. said something that sounded like “hou”.

          Sound waves: Sound waves are very similar to light waves. Sound waves have           Sound waves: Sound waves are very similar to light waves. Sound waves have a crest, trough, etc. Just like light waves. Sound waves travel outward from the a crest, trough, etc. Just like light waves. Sound waves travel outward from the source, just like when you throw a stone into a body of flat water. source, just like when you throw a stone into a body of flat water.

           Seismic waves: A seismic wave is basically an earthquake. Just like sound            Seismic waves: A seismic wave is basically an earthquake. Just like sound waves the earthquake travels away from the source. Each ‘ripple’ is an aftershock. If waves the earthquake travels away from the source. Each ‘ripple’ is an aftershock. If you were standing right where the earthquake started then you might you were standing right where the earthquake started then you might

How does a wave workHow does a wave work

• Waves are among the most familiar features in the ocean. All waves work Waves are among the most familiar features in the ocean. All waves work similarly, so although we are talking about ocean waves here, the same similarly, so although we are talking about ocean waves here, the same information would apply to any other waves you might discuss in science information would apply to any other waves you might discuss in science classes. classes.

• Ocean waves transport energy over vast distances, although the water itself Ocean waves transport energy over vast distances, although the water itself does not move, except up and down. This may surprise you, but if you think does not move, except up and down. This may surprise you, but if you think about it, once you are past the breakers on your raft, you pretty much just about it, once you are past the breakers on your raft, you pretty much just bob up and down. (You might drift up the beach….we’ll get to that.) This bob up and down. (You might drift up the beach….we’ll get to that.) This orbital motionorbital motion is explained in the figure below: is explained in the figure below:

• • There are waves of all sizes and shapes rolling into the beach at any given There are waves of all sizes and shapes rolling into the beach at any given

time. If they’re not stopped by anything, waves can travel across entire time. If they’re not stopped by anything, waves can travel across entire ocean basins and so the waves at your beach might be from a storm half a ocean basins and so the waves at your beach might be from a storm half a world away. The most familiar ocean waves are caused by the wind. These world away. The most familiar ocean waves are caused by the wind. These are are wind-driven waveswind-driven waves. This sort of motion is set up anytime two fluids rub . This sort of motion is set up anytime two fluids rub together, and remember that the atmosphere is essentially fluid. Waves together, and remember that the atmosphere is essentially fluid. Waves caused by underwater disturbances such as earthquakes, landslides, or caused by underwater disturbances such as earthquakes, landslides, or volcanic eruptions are called volcanic eruptions are called tsunamistsunamis. These waves are typically tens to . These waves are typically tens to hundreds of kilometers long. The gravitational pull of the sun and moon on hundreds of kilometers long. The gravitational pull of the sun and moon on the earth causes the tides which are actually the earth causes the tides which are actually tidal wavestidal waves. We’ll get back to . We’ll get back to that.that.

Moving wavesMoving waves

Defined wordsDefined words

• crestcrest-the very top of the wave-the very top of the wave• troughtrough-the hollow between two crests-the hollow between two crests• wave heightwave height-the vertical distance between the top of one wave crest and -the vertical distance between the top of one wave crest and

the bottom of the next troughthe bottom of the next trough• wavelengthwavelength-the horizontal distance between any one point on one wave -the horizontal distance between any one point on one wave

and the corresponding point on the nextand the corresponding point on the next• wave steepnesswave steepness-the ratio of height to length -the ratio of height to length • amplitudeamplitude-the maximum vertical displacement of the sea surface from -the maximum vertical displacement of the sea surface from

still water levelstill water level• (half the wave height)(half the wave height)• periodperiod-the time it takes for one complete wavelength to pass a stationary -the time it takes for one complete wavelength to pass a stationary

pointpoint• wave speedwave speed-the velocity with which waves travel-the velocity with which waves travel• deep water wavesdeep water waves-waves that are in water that is deeper than half their -waves that are in water that is deeper than half their

wavelengthwavelength• shallow water wavesshallow water waves-waves that are in water that is shallower than 1/20 -waves that are in water that is shallower than 1/20

their wavelength (the important difference on these last two is whether or their wavelength (the important difference on these last two is whether or not the sea floor influences the motion of the wave)not the sea floor influences the motion of the wave)

TidesTides

• The biggest waves in our oceans are the tides. These are caused by the gravitational forces between the The biggest waves in our oceans are the tides. These are caused by the gravitational forces between the earth and the sun and the moon. The moon has the biggest influence because it is close. It essentially pulls earth and the sun and the moon. The moon has the biggest influence because it is close. It essentially pulls up a bulge in the ocean on the side of the earth closest to it. It actually pulls up the land too, but not as up a bulge in the ocean on the side of the earth closest to it. It actually pulls up the land too, but not as much. There is also a bulge on the side opposite the moon. This one is tougher to understand. I’ve heard it much. There is also a bulge on the side opposite the moon. This one is tougher to understand. I’ve heard it explained two ways that seem to help:explained two ways that seem to help:

• 1. Because of centrifugal force (more an effect of the earth and moon revolving together than an actual 1. Because of centrifugal force (more an effect of the earth and moon revolving together than an actual force), the ocean on the side of the earth opposite the moon is sort of thrown outward, like you are when force), the ocean on the side of the earth opposite the moon is sort of thrown outward, like you are when you go around a bend in your car.you go around a bend in your car.

• 2. Imagine a race car, minivan, and bicycle starting a race. All three accelerate, and from the point of view 2. Imagine a race car, minivan, and bicycle starting a race. All three accelerate, and from the point of view of the minivan, the race car shoots out in front and the bicycle gets left behind. The way they spread out of the minivan, the race car shoots out in front and the bicycle gets left behind. The way they spread out depends on the depends on the differencesdifferences in rate of acceleration. Similarly, the side of the earth nearest the moon gets in rate of acceleration. Similarly, the side of the earth nearest the moon gets pulled out harder than the side away from the moon relative to the earth itself. The nearside shoots out pulled out harder than the side away from the moon relative to the earth itself. The nearside shoots out ahead, and the backside gets left behind.ahead, and the backside gets left behind.

• I don’t care which of these you prefer, as long as you get that there is this bulge on BOTH sides of the I don’t care which of these you prefer, as long as you get that there is this bulge on BOTH sides of the earth even though the moon is only on one side! So this bulge sort of sits there and we rotate around such earth even though the moon is only on one side! So this bulge sort of sits there and we rotate around such that sometimes we’re under the bulge and sometimes we’re not. Since it takes 24 hours for the earth to that sometimes we’re under the bulge and sometimes we’re not. Since it takes 24 hours for the earth to complete a rotation, plus we have to catch up a little because while the earth was rotating, the moon was complete a rotation, plus we have to catch up a little because while the earth was rotating, the moon was revolving around the earth, we are directly under a bulge, or experiencing high tide, about every 6 1/2 revolving around the earth, we are directly under a bulge, or experiencing high tide, about every 6 1/2 hours. hours.

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TidesTides

TidesTides

• Twice daily tides like this are called Twice daily tides like this are called semidiurnal tidessemidiurnal tides. It is . It is also possible to have only one high and one low tide per day. also possible to have only one high and one low tide per day. That would be a That would be a diurnal tidediurnal tide. Partly this depends on your . Partly this depends on your latitude, but it turns out that some 400 variables go into latitude, but it turns out that some 400 variables go into predicting the tide at any one place, so it isn’t nearly this predicting the tide at any one place, so it isn’t nearly this simple.simple.

• The sun tugs on the oceans too, but since it’s so far away, it The sun tugs on the oceans too, but since it’s so far away, it has less influence than the moon. You can see the influence has less influence than the moon. You can see the influence when the moon and sun and earth are all lined up. This would when the moon and sun and earth are all lined up. This would be during a full moon and a new moon. With both the sun and be during a full moon and a new moon. With both the sun and moon pulling the same direction, we get extra high high tides moon pulling the same direction, we get extra high high tides and extra low low tides (a big and extra low low tides (a big tidal rangetidal range). These happen ). These happen twice a month and are called twice a month and are called spring tides.spring tides. In between In between these, during the quarter phases of the moon, we get tides these, during the quarter phases of the moon, we get tides with the lowest ranges. These are called with the lowest ranges. These are called neap tidesneap tides. .

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THE ENDTHE END