AAHL 3.18.4 Angle between a line and a plane
Learn AAHL 3.18.4 Angle between a line and a plane in this free IB Mathematics Analysis and Approaches (AA) video lesson for AHL 3.18—Intersections of lines & planes.
Learn AAHL 3.18.4 Angle between a line and a plane in this free IB Mathematics Analysis and Approaches (AA) video lesson for AHL 3.18—Intersections of lines & planes.
Learn AAHL 3.18.4 Angle between a line and a plane in this free IB Mathematics Analysis and Approaches (AA) video lesson for AHL 3.18—Intersections of lines & planes.
In this lesson, the focus is on finding the angle between a line and a plane. The angle of interest is defined as the angle between the line and the plane, which can be determined using the normal vector of the plane.
To calculate this angle, the following steps are taken:
The lesson emphasizes the importance of understanding the geometric relationships involved rather than just memorizing formulas.
00:00Hi guys, so in this
00:02next lesson we are going
00:03to find the angle between
00:05a line and a plane.
00:06So I think of a
00:06line coming straight down and
00:09going I don't know through
00:10a plane like this. It's
00:11this angle that we're trying
00:13to find here. It might
00:15necessarily be right here, but
00:18I think of it. I
00:19like to think of it.
00:20Get a piece of string
00:22
00:24the table at an angle.
00:25That's the angle we're looking
00:26for. Or this example
00:28I was a ray of
00:28light coming from a point
00:30coming down and maybe hitting
00:31the ground. On geodebra, it
00:34looks something like this. So
00:35here's our plane. Now, if
00:39you zoom in like when
00:41we're, where I first had
00:42it here, it doesn't even
00:43touch the plane, but a
00:45line will always, and we
00:46talked about this when we
00:47got the intersecting point, but
00:48a line's always going to
00:49touch a plane. It's always
00:50going to intersect a plane
00:52unless, unless they're
00:56parallel. So when they talk
00:57about when he asks for
00:58the angle between the line
01:00and the plane, he's looking
01:00at this angle here. And
01:05of course, there's this angle
01:06too. So there's an acute
01:07angle and an obtuse angle.
01:08So just be careful what's
01:12he asking for. OK, so
01:14let's do an example. Well,
01:17actually, before I do this,
01:18let me show you how
01:19we do it. So how
01:21am I supposed to get
01:22the angle between
01:24line and a plane if
01:25I like I have the
01:26plane but I don't have
01:28I don't have a vector
01:29here I don't have this
01:31vector I don't have any
01:32vector on the line but
01:34hopefully you've started to see
01:36now that when it when
01:37it comes to planes the
01:38most important the most important
01:41part of the plane or
01:43part of the equation of
01:44the plane is the normal
01:45because the normal kind of
01:46gives us it's actually perpendicular
01:48to the direction but it's
01:51it's our way of kind
01:52of looking
01:52at directions because it's very
01:53easy to see when it's
01:56in this form. So the
01:58normal is the normal comes
02:02straight out of the plane
02:03like this. It goes straight
02:05up now you have to
02:05imagine this is 3D, this
02:07is a table and the
02:08normal is coming straight out
02:10of the table. This is
02:11called n for normal. Well
02:15the angle between
02:20the angle between the line
02:22and the plane, let's call
02:23theta. Now again, as I
02:25say, I don't know what
02:26this, I don't have a
02:27vector on the plane. So
02:29I just don't know what
02:29this vector is. I can't
02:31use it to find the,
02:33um, this angle. But I
02:36can find this angle and
02:38we often call this theta
02:40and this angle phi. So
02:42this angle is phi. I
02:44can find phi because phi
02:45is the angle between the
02:47normal and
02:48the line. And I know
02:49how to find the the
02:50angle between a vector and
02:52a line. I just use
02:53the direction vector of the
02:55line. So I can find
02:57phi. And then theta, once
02:59I have phi, I can
03:01just get theta because theta
03:02is just, I don't know
03:04if we're, often we use
03:05degrees for vectors, but so
03:07theta would just be 90
03:08degrees minus phi. That's obvious
03:12because this is a right
03:12angle. And then I'm not
03:14for it 90. So if
03:16this is
03:16If I have 5, this
03:18is 30, then this is
03:20obviously 60. And that's it.
03:23So to get the angle
03:25between a line and a
03:26plane, we get the angle
03:27between the normal and the
03:30line, and then subtract that
03:32from 90 degrees. If you
03:34end up getting 5 and
03:35you end up getting an
03:35obtuse angle, make sure you're
03:37using the acute angle because
03:38of this diagram that I've
03:39just drawn. Okay, let's do
03:41this example. A ray of
03:42light coming from the point,
03:44is traveling in the direction
03:45of this. So our line,
03:47our line, let's call it
03:49L, is R equals point
03:53or position vector negative one,
03:56three, two. Let me answer
03:59my pen. So the light,
04:01the position vector is negative
04:04one, three, two. And the
04:05direction vector, it gives us
04:06here, is four negative one,
04:09two. So let me write
04:09that again. Negative one, three,
04:12two,
04:12plus direction vector t times
04:154 1 negative 2. And
04:19then the plane is given
04:26as this. Now, as I
04:28always say, when you can
04:31with vectors draw a diagram.
04:33So let's say, and again,
04:34it doesn't have to be
04:35flat. Let's say this is
04:36the plane. Remember, you're kind
04:38of looking at it from
04:40the side. So this is
04:41the plane pi and this
04:43is my straight line, my
04:46straight line coming like this.
04:48You can draw in any
04:50way you want. This is
04:52my line L and then
04:54the normal comes straight out,
04:59straight out of the plane
05:01like this normal. Now my
05:05normal, so the normal N
05:08is
05:08is equal to 1, 3,
05:142. So hopefully, guys, you
05:15can see that now. If
05:18you've got the equation of
05:18a plane, the normal is
05:20just 1, 3, 2. It's
05:22the A, B, C, and
05:24the A, X plus B,
05:25Y plus C, Z equation.
05:28So this is my normal.
05:29So this angle here, this
05:31is the right angle, this
05:32is phi, and this is
05:34theta.
05:36That's phi. So the cos
05:40cos of phi is equal
05:44to this dot this. So
05:46it's the direction vector for
05:481 negative 2 dot 1
05:523 2. So the direction
05:56vector or the normal direction
05:58vector dot the normal divided
06:01by x, this is just
06:03the
06:04the cosine angle formula divided
06:08by the magnitude, the magnitude
06:11of the direction vector, which
06:14is 4 squared plus 1
06:16squared plus 2 squared times
06:19the magnitude of 1 squared
06:24plus 3 squared plus 2
06:28squared. And this equals, let's
06:31do it here, this dot
06:32this is
06:33four, four, plus three, minus
06:38four is three. So it's
06:40three over the square root
06:45of, and I don't need
06:47that much, 16, 17, 21,
06:5121, the square root of
06:54nine, 10, 14. This is
06:56cos of theta, no sorry,
06:58this is cos of five,
06:59be careful. So then five,
07:01is equal to phi, which
07:05I'll have proper phi, phi
07:07is equal to the inverse
07:09cos, so I need to
07:10do this menu. That's our
07:12trig. Let me get out
07:13of that. Trig inverse cos,
07:17and I'm going to put
07:18this in just like I
07:19see it, so I have
07:20this, I have three over
07:24the square root of 21
07:26multiplied by
07:29by the square root of
07:3014. Press enter and I
07:34get 79 .9235 .79 .9235.
07:43That's 5. Therefore, theta equals
07:4890 minus 79 .9235, which
07:54equals, I'm just going
07:57going to go minus 90
07:58forgive me for this guys
07:59but I think that's I'm
08:00okay to do that it's
08:02obviously positive so 10 .0765
08:0510 .0765 degrees that is
08:11the angle that is the
08:13acute angle between the line
08:14and the plane the up
08:17to angle would obviously be
08:18180 minus 10 .0765 degrees
08:22okay hope that makes sense
08:25Definitely, definitely, definitely, this is
08:27one of those situations where
08:28you're much better off having
08:30a good understanding about what's
08:31going on than just learning
08:34the rule. If you can
08:36replicate this diagram in an
08:39exam, it will really set
08:40your mind at ease, because
08:42you can show the examiner
08:44that you know exactly what
08:45you're doing. Okay, I hope
08:47you understood everything that lesson.
08:49If not, let me know,
08:50and I'll see you in
08:51the next video.
08:53you