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Thus, the vectors I discuss in the next few lines can be considered position vectors, originating from the origin.
I.e., creates a zero volume parallelpiped, because it's living in the plane of and .
This gives us our first equation of a plane: if the coordinates of are given by and the coordinates of are given by (that is is a normal vector) then the equation becomes
This is one form of the equation of a plane (through the origin). We say that the cross product is normal to the vectors that live in the plane (or normal to the plane): that is, that it is perpendicular to the plane.
Notice that we can instantly see a normal vector to the vectors in the plane in this form: just pick off the coefficients of the variables (a, b, and c) and turn those into a normal vector.
Now (for the second equation!): notice that satisfies the equation of the plane , since and are both perpendicular to their cross product. Hence we have another (parametric) equation of a plane (through the origin):
which leads to
where
which leads to
A good introduction to parametric curves is given by ballistics. If we shoot a bullet into the air with speed v horizontally, and we neglect all forces but gravity, then the bullet will trace out a parabola (some bullets are larger than others):
Now how might we characterize the path of the bullet? The answer is a parametric curve, of the form C(t)=(x(t), y(t)).
or | ||
If we wish, we can solve for t in the equation for x and use that to eliminate the parameter t from the equation for y, hence getting an equation for the parabola traced out: | ||
Orbits of planets in the heavens, movements of ants on a hill, a robotic arm in an assembly plant: all these can be described by parametric curves.