How do you multiple vectors?

[RESmonkey]

u = <7,-2> and v = <3,5>

u * v = ???

 

[Cogman]

Originally posted by: RESmonkey
u = <7,-2> and v = <3,5>

u * v = ???

I believe you mean multiply.

There are two methods, the dot product and the cross product. From your syntax I believe you want the dot product.

 

[RESmonkey]

For dot product, I find their magnitudes, and just use the formula lullvlcos(x) ? That doesn't seem right. Shouldn't the answer be another vector?

 

[BassBomb]

cross product will give you another vector

 

[Cogman]

For dot product, I find their magnitudes, and just use the formula lullvlcos(x) ? That doesn't seem right. Shouldn't the answer be another vector?

No, Dot product always returns a magnitude and not a vector. The cross product is what you are looking for to get another vector (but yes, the dot product is just basically multiplying the magnitudes together) and multiplying by the cos of the angle between the two)

 

[RESmonkey]

I can multiple the magnitudes together, take the cosine of that to find the x component and the sine of that to find the y component?

 

[Cogman]

Wha? What are you taking the cosine of? You only get a magnitude. Saying you will take the cosine and sin of a magnitude makes no sense.

 

[RESmonkey]

^ I thought I could turn the answer I get from multiplying the magnitudes out into vector form by taking cosine and sine of it

 

[Cogman]

Originally posted by: RESmonkey
I can multiple the magnitudes together, take the cosine of that to find the x component and the sine of that to find the y component?

Unless, you are trying to do the cross product, in which case the vector would be the result of the multiplication. However, if your homework said u*v then it wants the dot product. If it said u x v then it wants the cross product.

 

[Kirby]

7*3 + -2*5 = 11

That's the dot product.

 

[Cogman]

Originally posted by: RESmonkey
^ I thought I could turn the answer I get from multiplying the magnitudes out into vector form by taking cosine and sine of it

It makes no sence. I could have a rod that is 500 ft long, but if I took the sin and cosine of 500 what would I have? The direction the rod is pointing? Nope, just the sin of 500 and the cosine of 500. Dot product returns a magnitude, nothing more. Trying to detrimine what direction the magnitude is pointing is like trying to determine where a rod is pointing knowing only the length of said rod.