Tangent and normal vectors can help us make interesting parametric plots.
A sine curve on a circle
Suppose you wish to draw a sine curve on a circle like this:
How do you do this? Well, a general method for placing one curve along another is to
use unit tangent and unit normal vectors!
Plot the curve “wrapped” around a circle
of radius .
To do this, start by setting: The function will draw our circle of radius .
Now we need to add our sine curve. To do this, we compute the unit tangent vector
and the unit normal vector:
We’ve plotted our circle of radius with some unit tangent and unit normal vectors
for your viewing pleasure:
To add the sine curve, quite literally, add it in: where draws the circle,
and draws the sine curve. Note, breaking in to components we have:
We can confirm our construction by making a graph:
Thickening a curve
Suppose you have a vector-valued function that defines a curve in space,
and you want to build a parameterized surface that looks like a “thickened”
version of the curve. In other words, we want to convert a curve like
into a thickened “tube” like
To plot a “tube” around a vector-valued function , we need three handy vectors:
- The unit tangent vector:
- The unit normal vector:
- The unit binormal vector:
Let’s see these vectors in action with our next example.
You have been given a curve in space, say We want to make a tube around it of
radius .
Here is the idea, we want to take our curve and attach two orthogonal
vectors to every point
We’ll start by computing the
unit tangentunit normalunit binormal
vectors. Write with me From these vectors we can obtain the
unit tangentunit
normalunit binormal
vectors by differentiating: Finally we can obtain the
unit tangentunit normalunit binormal
vectors via the cross product: Looking again at
we see the unit normal vectors in red, and the unit binormal
vectors in blue. So we now can plot our tube where runs from to :
Here the function runs along the center of the tube, and and create a moving axis,
where a circle is drawn. Putting this all together we get a tube drawn around . We
can check our work with the following interactive:
