00:00Hi everyone. So in the
00:02last lesson we looked at
00:03sinusoidal graphs and we looked
00:05at their shape and we
00:06came up with some formulae
00:07that were not in the
00:08formulae booklet but we're going
00:10to use here. In this
00:12lesson we're going to see
00:15how we can turn a
00:16real life situation into a
00:19model and specifically into a
00:21sinusoidal model. Okay the example
00:23is tied. So I mentioned
00:25in the last lesson we
00:26can model tides using the
00:28these sinusoidal ones because the
00:29tide comes in and it
00:30goes out, then it comes
00:31in, then it comes out,
00:32and it's periodic. This question
00:36says, at low tide, the
00:37depth of the water is
00:38two meters. At high tide,
00:40six hours later, the depth
00:41is 6 .8 meters. Find
00:43a function that models the
00:44depth of water D in
00:45the harbor, T hours after
00:47low tide. When you first
00:49read that, that's like, hang
00:52on, how am I going
00:53to do that? I have
00:55no idea what to do.
00:56But, well, maybe you do.
00:59I don't yet. If I
01:02graph it, that's what I'm
01:04going to ask you to
01:04do. If you can graph
01:06this, get some idea what
01:07it looks like, then that's
01:11really going to help us
01:12create the model, the sinusoidal
01:15model. So this is going
01:16to be t, time, and
01:18this is going to be
01:20d, the depth of water.
01:23Now it says,
01:24At low tide the depth
01:25of water in the harvest
01:26two meters. That means we
01:29are never going below two
01:32meters. So I'm going to
01:33draw a line here at
01:36two. So this is at
01:39two and my graph we're
01:41never going to get below
01:41two. And the maximum is
01:44at 6 .8. So let's
01:46say two for 6 .8
01:49this certainly doesn't have to
01:50be accurate.
01:52it's never going to get
01:53higher than 6 .8. And
01:58it says, high tide happens
02:00six hours later. And it
02:03says the function models the
02:04depth of water, D in
02:08the harbor, T hours after
02:09low tide. So when T
02:11is zero, we're at low
02:13tide. So we're starting here.
02:16And we're going to get
02:17to high tide at six.
02:20We're going to get to
02:21high tide at six meters,
02:24sorry six hours. So this
02:27is actually going to be
02:29six. This is six hours.
02:32So what's happening is we're
02:35at low tide and then
02:36we go up to high
02:38tide and then we come
02:41down to low tide and
02:45then we go up to
02:47high tide and the whole
02:48thing
02:48and it's just continues with
02:50this cycle. Now if this
02:52is six meters here, sorry,
02:54I mean not meters, if
02:55this is six hours there,
02:58what is this? Well, six
03:01hours to get to high
03:02tide, six hours to get
03:03to low tide. So this
03:04is another six. So this
03:05is 12. So what's the
03:07period of this function? Well,
03:09the period is going to
03:10be 12 because that's when
03:12it gets back to low
03:13tide. Okay.
03:16question. Is this going to
03:18be a sine function or
03:19cosine function? Well, it's going
03:23to be cosine because its
03:25sine starts in the middle
03:27and goes up or down
03:28depending on the negative of
03:29a positive, whereas cosine starts
03:31either at the top or
03:32the bottom. So this starts
03:33at the bottom, so it's
03:34going to be cosine, and
03:35it's going to be a
03:35negative cosine. So it's going
03:37to be of the form
03:38d equals a
03:44cos bt plus c and
03:50a is going to be
03:51negative. Now let's try and
03:52find a, b, and c.
03:54So I know a, the
03:56size of a is, it's
04:00the size of the height
04:01of the wave. Now I
04:03can actually put this line
04:07in here. This is the
04:08line that's right in the
04:09middle. And this, what is
04:12this value?
04:12It's halfway between 6 .8
04:15and 2, which is actually
04:186 .8 plus 2 is
04:218 .8 divided by 2
04:22is 4 .4. So I
04:23just did, I just found
04:25C there without even realizing
04:27it. I'll show you that
04:29more formally in a second.
04:30But that line is at
04:314 .4. And this height
04:34here, the height of the
04:38wave, if you like, is
04:39the amplitude.
04:40So that's why this size
04:43of a is max minus
04:46min over 2. What's the
04:50max? 6 .8. What's the
04:53min to divide by 2?
04:566 .8 minus 2 is
05:002 .8 minus 2 is
05:022 .4. So it's 2
05:04.4. That's the size of
05:06the amplitude. But we know
05:07because it's starting at the
05:08bottom
05:08that A is negative, so
05:12I have to say A
05:13is equal to negative 2
05:16.4. Okay, fine, let's say,
05:21let's find B. B, another
05:27formula that we had in
05:30the previous lesson was 360
05:32divided by B equals the
05:35period.
05:36Now we know from this
05:40graph that we've made the
05:41period is 12. So I'm
05:43going to say 360 divided
05:44by b equals 12. And
05:49then I'm going to do
05:50this in my head. You
05:51can use a calculator if
05:52you want. 360 divided by
05:5412 is 30. So b
05:56equals 30. And then finally,
06:00finally, let's get c.
06:04the formula was max plus
06:09min over 2. And you
06:11saw I actually did that
06:13when I drew the screen
06:15line. I did 6 .8
06:16plus 2 over 2, which
06:20is 8 .8 over 2,
06:22which is 4 .4. So
06:23C is 4 .4. Therefore,
06:27the equation is D equals
06:30negative 2 .4.
06:33negative 2 .4, cos of
06:3630t plus 4 .4. Let's
06:42draw that with the calculator
06:44just to make sure that
06:47we are right. So it's
06:49negative 2 .4. Let's do
06:53times, cos of 30x we
06:58have to use here. 30x
07:01plus 4 .4 and we
07:07get this graph which is
07:09even the same color as
07:10my graph. How nice is
07:12that? Let's just do trace,
07:14many trace, graph trace. And
07:18we see the minimum is
07:19at zero two, so the
07:21height is two, great. We
07:23continue on our journey. The
07:24tide continues this journey coming
07:26in, keeps coming in and
07:28it gets to a maximum
07:29of 6 .8 lovely and
07:31then it goes out at
07:3212 it goes it's now
07:34we're at low tide then
07:35we're at high tide then
07:37we're at low tide so
07:43now I can actually find
07:45I can actually find when
07:49the tide is going to
07:50be in the next I
07:52could find it in the
07:54next I don't know 50
07:55hours or
07:57300 hours or I can
07:58actually predict it for quite
08:00a long time. And for
08:01me personally, this is really
08:02useful because I go kite
08:05surfing and I can only
08:07go kite surfing in this
08:08particular spot where I live
08:10when the tide is in
08:11because it comes in and
08:12it kind of fills up
08:13a lagoon. So I can
08:15use this model to predict
08:18when I can go kite
08:18surfing. How cool is that?
08:21Okay. That's sinusoidal modeling.
08:25look that's not easy. The
08:29other modeling that we've been
08:30doing like quadratic exponential cubic,
08:33I'm not going to say
08:34that's easy either, but this
08:36is the more difficult of
08:38the models in my opinion.
08:40So hopefully it makes sense.
08:43Remember the calculator is your
08:44friend, but if they ask
08:49you to create a model,
08:50I always find actually drawing
08:51the graph
08:53I'll try out a lot.
08:54Okay, see you in the
08:55next lesson.