MAT 1236 Calculus III Section 12.4 The Cross Product
HW… WebAssign 12.4 (18 problems, 98 min.) Read 12.5 (Seriously!): The first not-too- easy section in Calculus
Preview Define a new operation on vectors: The Cross Product Unlike the dot product, the cross product of two vectors is a vector. Properties of the cross product.
Classwork of the last section... We did not have time to work on the last classwork....
The Right Hand Rule FBI
We are Interested in … Given 2 vectors, they “span” a plane Find a vector perpendicular to this plane
The Cross Product If and, the cross product of a and b is the vector
The Cross Product The formula is traditionally memorized by using (formal) determinant expansions
2x2 Determinant Expansions
3x3 Determinant Expansions
The Cross Product The formula is traditionally memorized by using (formal) determinant expansions
Example 1
Expectations You are expected to use the above standard procedure to find the cross product. You are expected to show all the steps. Keep in mind, good practices are key to minimize the chance of making mistakes.
Property A
Property B
In addition, the cross product obeys the Right Hand Rule.
Property B (Why?)
Example 1 (Verify Property B)
Property C
Property C (Why?)
In Particular
is in the same direction of k and
Property D Two nonzero vectors and are parallel if and only if
Property D (Why?) Two nonzero vectors and are parallel if and only if
Property E The length of the cross product axb is equal to the area of the parallelogram determined by a and b.
Example 2 Find a vector perpendicular to the plane that passes through the points P(6,0,0), Q(1,1,1), R(0,0,2)
Example 3 Find the area of the triangle with vertices P(6,0,0), Q(1,1,1), R(0,0,2)
Other Properties Reference only Default Right Hand Rule