cp ~bfarr/Curves2D_start.mws ~

You can copy the worksheet now, but you should read through the lab
before you load it into Maple. Once you have read to the exercises,
start up Maple, load
the worksheet `Vec2D_start.mws`, and go through it
carefully. Then you can start working on the exercises.

where and are functions defined on some interval . From our definition of a parametric curve, it should be clear that you can always associate a parametric curve with a vector-valued function by just considering the curve traced out by the head of the vector.

For this lab, we will assume that we have a
vector-valued function that gives the position at time
of a moving point in the plane. The velocity of this point is
given by the derivative
and the acceleration is given
by the second derivative,
.
In many applications of
curvilinear motion, we need to know the magnitude of the velocity, or
the speed. This is easy to compute - just take the magnitude
. If you think of the speed as the rate of change
of distance along the curve, and recall that arc length is distance
measured along the curve, then you have the following interpretation
of the speed

where is arc length.

If the speed is not zero for any value of in the interval ,
then it is possible to define a unit vector, that is
tangent to the curve as follows.

Using this definition, you can write the velocity in the following form.

This is not the most useful form for calculating the velocity, but it does lead to a useful way of thinking about the acceleration experience by a particle moving in a curvilinear path. If the path is a straight line, acceleration depends only on whether the particle is speeding up or slowing down. In a curve, however, there is an additional acceleration, called the centripetal acceleration, that is needed to keep the particle moving on the curve. The magnitude of this acceleration depends on the speed of the car and how much the path is curving. It turns out that you can quantify this with an intrinsic property of the curve called the curvature, usually denoted , defined by the following equation.

That is, the curvature is the magnitude of the rate of change of the tangent vector with respect to arc length. For example, the curvature of a straight line is zero and it can be shown that the curvature of a circle of radius is the same for every point on the circle and is given by .

If the curvature is never zero for a particular curve, then we can
define another intrinsic property of curve, the unit normal vector
by the following equation.

It can be shown that at each point on the curve the vector defined by this equation is a unit vector that is always perpendicular to the tangent vector at that point. Furthermore, the unit normal vector always points in the direction of the centripetal acceleration required to keep a particle moving on the curve. In your text, the following important relation is derived.

To see why this equation is useful, recall that is the speed, so is the rate of change of the speed. That is, this term measures whether the particle is speeding up or slowing down. Because this component of the acceleration is in the direction of the tangent vector it is often called the tangential acceleration, denoted by the symbol . The component of the acceleration in the direction of the normal vector is called the normal acceleration, denoted . In the case of motion on a circular path, the curvature is the reciprocal of the radius, so this term should be easily recognizable as the centripetal acceleration for uniform circular motion.

Computing these quantities is generally not an
easy task. The **Getting started** worksheet for this lab
describes commands from the `CalcP` package that simplify these
calculations and provides examples for you to work from.

- Consider the parametric curve , . Plot
the graph of this curve for
. Identify the points
on your graph that correspond to the values and and indicate the direction of motion on the graph as
increases. This is probably best done by hand on the printed copy.
- Consider the curve in the plane given by for
and the two different parametrizations given below. In each
case is restricted to the interval
.

- Describe the difference between these two different parametrizations.
- Compute the unit tangent vector at the point , for each parametrization and explain your results.
- Compute the curvature and the unit normal vector at the point , for each parametrization and explain your results.

- Consider . What is the normal vector at ? At ? What about at ? What goes wrong?

2003-03-18