Constant vector and cross product?

In summary, the conversation was about evaluating grad x (k x r) where k is a constant vector. The individual asking the question did not know the value of k but knew how to do the cross product and the gradient. Another person suggested assuming an arbitrary k vector and solving for the answer, which they both did and got slightly different results. The individual asking the question then asked for clarification on how the other person got their answer.
  • #1
jlmac2001
75
0
Okay, here's the question:

For r=xi + yj +zk evaluate grad x (k x r), where k is a constant vector.

I know how to do the cross product of k x r and I know how to do
grad x (k x r). But I don't know what k is. What's k?
 
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  • #2
Originally posted by jlmac2001
... But I don't know what k is. What's k?
Try assuming an arbitrary k vector: ai + bj + ck (or whatever). Then see what happens.
 
  • #3
does this look right?

The answer I got is: -2ai-2bj-2ck
 
  • #4


Originally posted by jlmac2001
The answer I got is: -2ai-2bj-2ck
I got 2k (where k is the constant vector) but I did it quick. (So my answer differs by a minus sign from yours.) My main point was that you didn't have to know the value of k (the a, b, & c) to do the problem.
 
  • #5
question

how did you just get 2k? i don't understand
 
  • #6


Originally posted by jlmac2001
how did you just get 2k? i don't understand
By "2k" I mean: 2ai + 2bj + 2ck. (The same as you, but with a + instead of -.) [itex]2\vec{k}[/itex] is what I should have written. Make sense?
 
  • #7
thanks

i get it now
 

1. What is a constant vector?

A constant vector is a vector that has a fixed magnitude and direction. It does not change in value or direction, regardless of the coordinate system it is being referenced in.

2. How is a constant vector represented mathematically?

A constant vector is represented by a boldface letter with an arrow on top, such as v or F. It can also be represented by its components, (a,b), where a represents the horizontal component and b represents the vertical component.

3. What is the purpose of a cross product?

A cross product is used to find a vector that is perpendicular to two given vectors. It is also used to calculate the area of a parallelogram formed by two vectors.

4. How is a cross product calculated?

A cross product is calculated by taking the determinant of a 3x3 matrix formed by the two given vectors and the unit vectors i, j, and k. The resulting vector is perpendicular to the given vectors and has a magnitude equal to the area of the parallelogram formed by the two vectors.

5. Can a constant vector and cross product be negative?

Yes, a constant vector and cross product can both have negative values. The direction of a vector can be negative if it points in the opposite direction of a given reference point. The magnitude of a cross product can also be negative if the two given vectors form a clockwise rotation, rather than a counterclockwise rotation, as seen from a reference point.

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