- 3.3.1 Determine the length of a particle’s path in space by using the arc-length function.
- 3.3.2 Explain the meaning of the curvature of a curve in space and state its formula.
- 3.3.3 Describe the meaning of the normal and binormal vectors of a curve in space.
In this section, we study formulas related to curves in both two and three dimensions, and see how they are related to various properties of the same curve. For example, suppose a vector-valued function describes the motion of a particle in space. We would like to determine how far the particle has traveled over a given time interval, which can be described by the arc length of the path it follows. Or, suppose that the vector-valued function describes a road we are building and we want to determine how sharply the road curves at a given point. This is described by the curvature of the function at that point. We explore each of these concepts in this section.
Arc Length for Vector Functions
We have seen how a vector-valued function describes a curve in either two or three dimensions. Recall Arc Length of a Parametric Curve, which states that the formula for the arc length of a curve defined by the parametric functions is given by
In a similar fashion, if we define a smooth curve using a vector-valued function where the arc length is given by the formula
In three dimensions, if the vector-valued function is described by over the same interval the arc length is given by
- Plane curve: Given a smooth curve C defined by the function where t lies within the interval the arc length of C over the interval is
- Space curve: Given a smooth curve C defined by the function where t lies within the interval the arc length of C over the interval is
The two formulas are very similar; they differ only in the fact that a space curve has three component functions instead of two. Note that the formulas are defined for smooth curves: curves where the vector-valued function is differentiable with a non-zero derivative. The smoothness condition guarantees that the curve has no cusps (or corners) that could make the formula problematic.
Calculate the arc length of the parameterized curve
We now return to the helix introduced earlier in this chapter. A vector-valued function that describes a helix can be written in the form
where R represents the radius of the helix, h represents the height (distance between two consecutive turns), and the helix completes N turns. Let’s derive a formula for the arc length of this helix using Equation 3.12. First of all,
This gives a formula for the length of a wire needed to form a helix with N turns that has radius R and height h.
We now have a formula for the arc length of a curve defined by a vector-valued function. Let’s take this one step further and examine what an arc-length function is.
If a vector-valued function represents the position of a particle in space as a function of time, then the arc-length function measures how far that particle travels as a function of time. The formula for the arc-length function follows directly from the formula for arc length:
If the curve is in two dimensions, then only two terms appear under the square root inside the integral. The reason for using the independent variable u is to distinguish between time and the variable of integration. Since measures distance traveled as a function of time, measures the speed of the particle at any given time. Since we have a formula for in Equation 3.13, we can differentiate both sides of the equation:
If we assume that defines a smooth curve, then the arc length is always increasing, so for Last, if is a curve on which for all t, then
which means that t represents the arc length as long as
Let describe a smooth curve for Then the arc-length function is given by
Furthermore, If for all then the parameter t represents the arc length from the starting point at
A useful application of this theorem is to find an alternative parameterization of a given curve, called an arc-length parameterization. Recall that any vector-valued function can be reparameterized via a change of variables. For example, if we have a function that parameterizes a circle of radius 3, we can change the parameter from t to obtaining a new parameterization The new parameterization still defines a circle of radius 3, but now we need only use the values to traverse the circle once.
Suppose that we find the arc-length function and are able to solve this function for t as a function of s. We can then reparameterize the original function by substituting the expression for t back into The vector-valued function is now written in terms of the parameter s. Since the variable s represents the arc length, we call this an arc-length parameterization of the original function One advantage of finding the arc-length parameterization is that the distance traveled along the curve starting from is now equal to the parameter s. The arc-length parameterization also appears in the context of curvature (which we examine later in this section) and line integrals, which we study in the Introduction to Vector Calculus.
Finding an Arc-Length Parameterization
Find the arc-length parameterization for each of the following curves:
- First we find the arc-length function using Equation 3.14:
which gives the relationship between the arc length s and the parameter t as so, Next we replace the variable t in the original function with the expression to obtain
This is the arc-length parameterization of Since the original restriction on t was given by the restriction on s becomes or
- The arc-length function is given by Equation 3.14:
Therefore, the relationship between the arc length s and the parameter t is so Substituting this into the original function yields
This is an arc-length parameterization of The original restriction on the parameter was so the restriction on s is or
Find the arc-length function for the helix
Then, use the relationship between the arc length and the parameter t to find an arc-length parameterization of
An important topic related to arc length is curvature. The concept of curvature provides a way to measure how sharply a smooth curve turns. A circle has constant curvature. The smaller the radius of the circle, the greater the curvature.
Think of driving down a road. Suppose the road lies on an arc of a large circle. In this case you would barely have to turn the wheel to stay on the road. Now suppose the radius is smaller. In this case you would need to turn more sharply to stay on the road. In the case of a curve other than a circle, it is often useful first to inscribe a circle to the curve at a given point so that it is tangent to the curve at that point and “hugs” the curve as closely as possible in a neighborhood of the point (Figure 3.6). The curvature of the graph at that point is then defined to be the same as the curvature of the inscribed circle.
Let C be a smooth curve in the plane or in space given by where is the arc-length parameter. The curvature at s is
Visit this website for more information about the curvature of a space curve.
The formula in the definition of curvature is not very useful in terms of calculation. In particular, recall that represents the unit tangent vector to a given vector-valued function and the formula for is To use the formula for curvature, it is first necessary to express in terms of the arc-length parameter s, then find the unit tangent vector for the function then take the derivative of with respect to s. This is a tedious process. Fortunately, there are equivalent formulas for curvature.
Alternative Formulas for Curvature
If C is a smooth curve given by then the curvature of C at t is given by
If C is a three-dimensional curve, then the curvature can be given by the formula
If C is the graph of a function and both and exist, then the curvature at point is given by
The first formula follows directly from the chain rule:
where s is the arc length along the curve C. Dividing both sides by and taking the magnitude of both sides gives
Since this gives the formula for the curvature of a curve C in terms of any parameterization of C:
In the case of a three-dimensional curve, we start with the formulas and Therefore, We can take the derivative of this function using the scalar product formula:
Using these last two equations we get
Since this reduces to
Since is parallel to and is orthogonal to it follows that and are orthogonal. This means that so
Now we solve this equation for and use the fact that
Then, we divide both sides by This gives
Find the curvature for each of the following curves at the given point:
- This function describes a helix.
The curvature of the helix at can be found by using Equation 3.15. First, calculate
Last, apply Equation 3.15:
The curvature of this helix is constant at all points on the helix.
- This function describes a semicircle.
To find the curvature of this graph, we must use Equation 3.16. First, we calculate and
Then, we apply Equation 3.17:
The curvature of this circle is equal to the reciprocal of its radius. There is a minor issue with the absolute value in Equation 3.16; however, a closer look at the calculation reveals that the denominator is positive for any value of x.
Find the curvature of the curve defined by the function
at the point
The Normal and Binormal Vectors
We have seen that the derivative of a vector-valued function is a tangent vector to the curve defined by and the unit tangent vector can be calculated by dividing by its magnitude. When studying motion in three dimensions, two other vectors are useful in describing the motion of a particle along a path in space: the principal unit normal vector and the binormal vector.
Let C be a three-dimensional smooth curve represented by r over an open interval I. If then the principal unit normal vector at t is defined to be
The binormal vector at t is defined as
where is the unit tangent vector.
Note that, by definition, the binormal vector is orthogonal to both the unit tangent vector and the normal vector. Furthermore, is always a unit vector. This can be shown using the formula for the magnitude of a cross product
where is the angle between and Since is the derivative of a unit vector, property (vii) of the derivative of a vector-valued function tells us that and are orthogonal to each other, so Furthermore, they are both unit vectors, so their magnitude is 1. Therefore, and is a unit vector.
The principal unit normal vector can be challenging to calculate because the unit tangent vector involves a quotient, and this quotient often has a square root in the denominator. In the three-dimensional case, finding the cross product of the unit tangent vector and the unit normal vector can be even more cumbersome. Fortunately, we have alternative formulas for finding these two vectors, and they are presented in Motion in Space.
Finding the Principal Unit Normal Vector and Binormal Vector
For each of the following vector-valued functions, find the principal unit normal vector. Then, if possible, find the binormal vector.
- This function describes a circle.
To find the principal unit normal vector, we first must find the unit tangent vector
Next, we use Equation 3.18:
Notice that the unit tangent vector and the principal unit normal vector are orthogonal to each other for all values of t:
Furthermore, the principal unit normal vector points toward the center of the circle from every point on the circle. Since defines a curve in two dimensions, we cannot calculate the binormal vector.
- This function looks like this:
To find the principal unit normal vector, we first find the unit tangent vector
Next, we calculate and
Therefore, according to Equation 3.18:
Once again, the unit tangent vector and the principal unit normal vector are orthogonal to each other for all values of t:
Last, since represents a three-dimensional curve, we can calculate the binormal vector using Equation 3.17:
Find the unit normal vector for the vector-valued function and evaluate it at
For any smooth curve in three dimensions that is defined by a vector-valued function, we now have formulas for the unit tangent vector T, the unit normal vector N, and the binormal vector B. The unit normal vector and the binormal vector form a plane that is perpendicular to the curve at any point on the curve, called the normal plane. In addition, these three vectors form a frame of reference in three-dimensional space called the Frenet frame of reference (also called the TNB frame) (Figure 3.7). Last, the plane determined by the vectors T and N forms the osculating plane of C at any point P on the curve.
Suppose we form a circle in the osculating plane of C at point P on the curve. Assume that the circle has the same curvature as the curve does at point P and let the circle have radius r. Then, the curvature of the circle is given by We call r the radius of curvature of the curve, and it is equal to the reciprocal of the curvature. If this circle lies on the concave side of the curve and is tangent to the curve at point P, then this circle is called the osculating circle of C at P, as shown in the following figure.
To find the equation of an osculating circle in two dimensions, we need find only the center and radius of the circle.
Finding the Equation of an Osculating Circle
Find the equation of the osculating circle of the helix defined by the function at
Figure 3.9 shows the graph of
First, let’s calculate the curvature at
This gives Therefore, the radius of the osculating circle is given by Next, we then calculate the coordinates of the center of the circle. When the slope of the tangent line is zero. Therefore, the center of the osculating circle is directly above the point on the graph with coordinates The center is located at The formula for a circle with radius r and center is given by Therefore, the equation of the osculating circle is The graph and its osculating circle appears in the following graph.
Find the equation of the osculating circle of the curve defined by the vector-valued function at
Section 3.3 Exercises
Find the arc length of the curve on the given interval.
This portion of the graph is shown here:
This portion of the graph is shown here:
over the interval Here is the portion of the graph on the indicated interval:
Find the length of one turn of the helix given by
Find the arc length of the vector-valued function over
A particle travels once around a circle with the equation of motion Find the distance traveled around the circle by the particle.
Set up an integral to find the circumference of the ellipse with the equation
Find the length of the curve over the interval The graph is shown here:
Find the length of the curve for
The position function for a particle is Find the unit tangent vector and the unit normal vector at
Given find the binormal vector
Given determine the tangent vector
Given determine the unit tangent vector evaluated at
Given find the unit normal vector evaluated at
Given find the unit binormal vector evaluated at
Given find the unit tangent vector The graph is shown here:
Find the unit tangent vector and unit normal vector at for the plane curve The graph is shown here:
Find the unit tangent vector for
Find the principal normal vector to the curve at the point determined by
Find for the curve
Find for the curve
Find the unit normal vector for
Find the unit tangent vector for
Find the arc-length function for the line segment given by Write r as a parameter of s.
Parameterize the helix using the arc-length parameter s, from
Parameterize the curve using the arc-length parameter s, at the point at which for
Find the curvature of the curve at (Note: The graph is an ellipse.)
Find the x-coordinate at which the curvature of the curve is a maximum value.
Find the curvature of the curve Does the curvature depend upon the parameter t?
Find the curvature for the curve at the point
Find the curvature for the curve at the point
Find the curvature of the curve The graph is shown here:
Find the curvature of
Find the curvature of at point
At what point does the curve have maximum curvature?
What happens to the curvature as for the curve
Find the point of maximum curvature on the curve
Find the equations of the normal plane and the osculating plane of the curve at point
Find equations of the osculating circles of the ellipse at the points and
Find the equation for the osculating plane at point on the curve
Find the radius of curvature of at the point
Find the curvature at each point on the hyperbola
Calculate the curvature of the circular helix
Find the radius of curvature of at point
Find the radius of curvature of the hyperbola at point
A particle moves along the plane curve C described by Solve the following problems.
Find the length of the curve over the interval
Find the curvature of the plane curve at
Describe the curvature as t increases from to
The surface of a large cup is formed by revolving the graph of the function from to about the y-axis (measured in centimeters).
[T] Use technology to graph the surface.
Find the curvature of the generating curve as a function of x.
[T] Use technology to graph the curvature function.