Dot Product Please Help?

Use lower case 'i' for the x-axis unit vector.

We want (2.0x²i – 4.0xj + 5.0k) • (11.0i + 2.5xj + 0k)

Multiply the i coefficients: 2.0x² * 11.0 = 22.0x²

Multiply the j coefficients: -4.0x * 2.5x = -10.0x²

Multiply the k coefficients: 5.0 * 0 = 0

Add these: 22.0x² + (-10.0x²) + 0 = 12.0x²

And that's the answer: 12.0x²

Of all the vector operations, the dot product may be the easiest to do. But you have to know the operation rules.

A dot B = <a1i + a2j + a3k> dot <b1i + b2j + b3k> = a1i*b1i + a2j*b2j + a3k*b3k = a1b1 i^2 + a2b2 j^2 + a3b3 k^2 = a1b1 + a2b2 + a3b3 . Note that each unit vector, i, j, k, = 1.0 when squared.

Follow the dot rule above and you can do your problem. But learn the rule; this is something that needs to be memorized and understood.

EXAMPLE:

<2.0x^2I – 4.0xj +5.0k> dot <11.0I +2.5xj + 0k> = 22x^2 - 10x^2 + 0 = 12x^2 ANS.

that look like i,j and k are mutually orthogonal unit vectors and if you order them [ i,j,k ] you are asking what is [2,-4,5] dot [11,2.5,0] ... but maybe you are asking something else

http://en.wikipedia.org/wiki/Dot_product says

[2,-4,5] dot [11,2.5,0] = 22 - 10 +0 = 12