Calculus Animations,Graphics and Lecture Notes

Parametric Equations 3-space

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Parametric Equations

The only differences in parametric equations in 3 space are:


1. There are 3 equations x =x(t)  y =y(t) and z = z(t)

2. We have to determine the surface or surfaces the trajectory is restricted to.
We'll see several egs to see how this is done

Example 1 ---  A Helix

Here we let x = cos(t)   y -sin(t) and z = t

1 We see that x2 + y2 =1 so the trajectory is on the surface of a vertical cylinder

2. The inititial point is (1,0,0)

3. x'(t) =-sin(t)   y' (t) = cos(t)  and  z'(t) = 1

therefore the trajectory is smooth and initially the particle moves back, to the right and up.

In other words the particle travels counterclockwise with a constant rate of increase in the +z direction

Helix z increasing

Example 2 -- Another Helix

Let x(t) =cos(t) and y(t) =sin(t) as before but Let z(t) = e -t .

1. The initial pt is now (1,0,1)

2. z '(t) = - e -t  

Therefore the particle winds down the cylinder approaching circular motion in the x-y plane

Helix z decreasing

Example 3 Motion on a Parabaloid

Let x(t) = t    y(t)= t     z(t) = 1-2t2 .


1 This is a little tricky but notice z = 1 - x2 - y2 the eqn of a parabaloid with vertex (0,0,1) opening down. Futher y = x which is a vertical plane; therefore the trajectory is on the curve of
 intersection of the  parabaloid and  and the vertical plane

2. Initial pt (0.0.1)

3  x' = 1     y' =1    z' = -4t
Therefore the motion is forward, right, and down

Motion on a Parabaloid

Inducing a Parameterization

Of course
we want to be able to create parametric equations, not just determine a trajectory from given eqns.

Suppose we think of z = f(x,y) as the temperature at any pt in the x-y plane.

In our previous example we can think of the trajectory as the temperature as we move along the line y =x in the xyplane.

Suppose we choose any path to travel in the plane say along the curve y = x2 .

Then we choose x = t  and    y = t2 . since we know z = 1 - x2 - y2 we then define
z(t) = 1 - t2 - t as our 3d eqn.

This is called inducing a parameterization.
See the animation below

Motion on a Parabaloid 2

Motion on a saddle

Suppose we have the saddle z = 1 - x2 +y2 .

For our path in the plane le's take the circle x = cos(t)  y = sin(t)

Then the induced parameterization for z is z = 1 - cos2(t) + sin2(t) = 1 - cos(2t)

View below

Motion on a Saddle

The following Animation shows if you move along a level curve f does not change

Motion on a Level Curve

In the following Animation and Notes we solve the problem of dropping a rock off

of a spinning merry-go-round.

Animation- Merry-go-Round

Notes- Solution to Merry-go-Round Problem

The following notes and animation deals with projectile motion in 3 dimensions.

The example used is a rocket fired from ground level. 

Notes- Projectile motion in 3-D

Animation - 3D Projectile Motion

There is an intimate relationship between parametric equations  and vector valued functions. You may want to see the Vevtor value page at this point.

For a discussion of the parameterization of lines in 2 and 3spce go to the vector valued function page



The Smart Bunny-A very short story by Kurt Vonnegut Jr.

©2008-2010 Kelly Liakos