Please see the link in the description
to download a worksheet for this video. Experiments can be dangerous. If you are a
child: never do experiments unless your parent, guardian or adult educator says it's alright and
is there with you the whole time. If you've not already done so, we suggest that you watch the
videos "Overview of 5 wave types in science" and "Intro to electromagnetic and mechanical waves"
before watching this video. Here are some waves on a beach. Here are some waves in a
pond.
Although all of these waves are made of water, they're very different in many ways. For example,
compared with those on the pond, the waves on the beach are much taller, they're moving much
faster and the distance between each wave's crest is much larger. In physics, we use specific
words to describe each of these characteristics, and collectively we call those words "wave
properties." In this video we'll introduce these wave properties and how to solve some related
problems: amplit
ude, wavelength, wave speed and velocity, wave frequency, wave period and
reciprocal relationships. We'll begin with wave amplitude. Wave amplitude is the distance between
a wave's crest and the equilibrium. To measure the amplitude of these waves, we first need to know
the water's level if there were no waves . W call that level the medium's equilibrium. We'll put
some black tape just below the equilibrium. Then we get a picture when the there are waves. We call
this picture a snapshot. Ne
xt, we find a crest. That's the topmost part of a wave. And we measure
the distance from the crest to the equilibrium. That distance is the wave's amplitude. For this
water wave, the amplitude is 3.5 centimeters. Then we find the trough, which is the lowest point
on the wave. We measure its distance from the equilibrium. This is the negative amplitude. For
this water wave, the negative amplitude's value is negative 3.5 centimeters. To graph the snapshot,
we first draw the y-axis to represen
t displacement from the equilibrium. A value of zero represents
the equilibrium. Then we draw an x-axis which represents distance. We find the value of the
amplitude and negative amplitude and draw those in as dotted lines. Then we sketch the wave so that
the crest and trough touch those dotted lines. The units of displacement are often in centimeters
or meters, as in the case of this water wave and for many waves we can see. But the units are in
pressure when we're graphing a sound wave. T
his animation shows a pressure wave in blue that
corresponds with the oscillations of gray dots, which represent molecules in the air. When a
sound wave propagates, it creates high pressure regions called compressions which are followed by
low pressure regions called rarefactions. Please note that the pressure graph is shaped like a sine
wave, as are most waves we graph in K12 education. As we turn up the volume of our speakers, the
pressure in the air with each wave gets higher, so the amp
litude gets bigger. One way to remember
that louder sound waves have waves with a bigger amplitude, is to think of an amplifier. Both
of these come from the root word "amplify," which means to make something bigger. Amplitude
is a major factor in how much energy a wave has. That's a primary reason why these ocean waves with
large amplitudes have much more energy than those on a pond that have small amplitudes. Next, we'll
discuss wavelength. As we noted in the overview video, wavelength is
the length of one wave. It's
easiest to measure this as the distance between two adjacent wave crests. The wavelength for these
water waves is 6 centimeters. We use the Greek letter lambda to symbolize a wave's wavelength.
A snapshot graph is useful when we want to know a wave's wavelength. In this case, the wavelength is
100 centimeters. Each key on a piano makes a sound with a specific wavelength. Low pitch notes have
long wavelengths and are made by long strings. High pitch notes have sh
ort wavelengths and are
made by short strings. Next, we'll introduce wave speed. A wave speed is how far its crest goes in
a certain time. These ocean waves are traveling at about 5 miles per hour. Sound waves go
at different speeds based on what they're traveling through. In air sound waves travel at
about 750 miles per hour, which which is slightly faster than commercial airplanes. In water, sound
waves travel at about 3,000 miles per hour, which is faster than the fastest plane that ever
flew.
In solids like a diamond, sound waves travel at about 40,000 miles per hour. If the Earth's
core was made of rocks similar to diamonds, a sound wave could go around the entire Earth
in about half an hour. Electromagnetic waves also travel at different speeds depending on
what they're traveling through. For example, in the air light travels at about 670 million
miles per hour. That means light could go all around the Earth in about the time it takes us
to blink. Compared to its speed
in air, light goes about 20% slower in water. Light travels at
its fastest speed when it's traveling in a vacuum, but it slows to less than half that speed in a
diamond. We use the word "velocity" when we want to describe a wave's speed and the direction the
waves are going. For example, we would say these waves have a velocity of 2 miles per hour in the
southern direction. We often write this with a "v" that stands for velocity, and sometimes with
a small subscript "w" that stands for wav
e. Next, we'll discuss wave frequency. Wave frequency is
the number of waves that propagate per second. To measure a wave's frequency, we count how
many waves propagate in a second. We'll use this stem as a reference point. There are
two waves that pass this stem every second, so the wave frequency is two waves per second.
Instead of saying the phrase "waves per second" we use the term "Hertz." It's written like
this. It's named in honor of Heinrich Hertz who discovered radio waves. We typi
cally use a
lowercase "f" to represent frequency. This is a history graph of the lake waves passing
the stem. It shows that every second, two waves pass the stem. We use a history graph
like this to analyze a wave's frequency. Please note that instead of distance on the x-axis, as
with a snapshot graph, we've written time on the x axis. You may be given a test question
like this. What's the frequency of this wave? In this case, there is not a wave crest
above the 1 second mark, so we find
a crest that's above a number, then we count the number of
waves in the interval between zero and the mark we selected. We know that "frequency" is defined as
the number of waves per second. To calculate the number of waves per second, we divide the number
of waves by the seconds. In this case, we get 0.6 waves per second, which we write as 0.6 Hz. To
make a history graph, we plot the displacement from the equilibrium of one point in a medium
over the course of several seconds. For example,
this history graph corresponds to the activity of
the red dot in the medium during several seconds. In contrast, a snapshot graph is a picture of all
the waves in a medium at one instant in time. Most of the time, the wave frequencies we experience
are high numbers. You may recognize this frequency as middle C on a piano. This tuning fork is making
a sound with that same frequency. It does that by vibrating exactly 256 times per second. With
every vibration, the Tuning Fork oscillates the
air molecules back and forth to produce sound
waves with that frequency. This radio wave tower sends out radio waves with a wide range of
frequencies. When we tune our radios to a station, we're selecting what frequency waves we want
the radio to process. FM frequencies are in the millions of waves per second. In this case,
we've tuned the radio to only process radio waves reaching this antenna if they're arriving
at 104.5 million waves per second. Our phones and cell towers use even higher
frequency radio
waves. Those waves have frequencies of about 2 billion waves per second, which we abbreviate by
writing 2 GHz. Next, we'll discuss wave period. Wave period is the number of seconds for one
wavelength to propagate. These ocean waves have a wavelength that spans the distance between
adjacent crests. To measure the period, we'll make a vertical line at the front end of a wave, then
count how many seconds elapse until the back end of the wave crosses that line. In this case, th
e
period is 6 seconds. What's the period for this wave? We can find the length of one wave and
estimate that it's about one and a half seconds. But to be more precise, we find a crest that
is directly above a hash mark. In this case, there's one above the 5 second hash mark. Then we
count the number of waves in the interval from 0 to 5 seconds. Since the definition of a wave
period is the number of seconds in one wave, to calculate the number of seconds per
wave we divide the number of sec
onds in the interval by the number of waves. In this
case, that's 5 seconds divided by 3 waves, which equals 1.67 seconds per wave. The symbol
for period is a capital T. Our final topic is the reciprocal relationship between a wave's frequency
and its period. We use the same history graph to calculate a wave's frequency and to calculate its
period. For the frequency calculation, we divided the number of waves by the seconds. For the period
calculation, we divided the number of seconds by th
e waves. This tells us that a wave's frequency
and period are reciprocals of each other. We can express a reciprocal relationship in words by
using a division bar, and by using a division sign. We can switch between frequency and period
by dividing the number one by the other's value. Here's a tuning fork with a frequency of 512 Hz.
What's the period of the waves this tuning fork makes? To convert from frequency to period,
we divide one by the frequency. In this case, 1 divided by 512 gives
a wave period of two
thousandths of a second. Here's a summary of this topic plus some additional information.
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