Question

Given vectors a and b, ware a = 5, b = 8 and angel between them is 60 degrees. calculate a . b (a DOT b)

Answer 1

Here is the acutal question...

Answer 2

the dot product of two vectors is given by the magnitudes times the cosine of the angle between them\[\vec A\cdot\vec B=|A||B|\cos\theta\]you are given all the info, so just plug the numbers into the formula

Answer 3

*the product of the magnitudes times the cosine of the angle between them I mean

Answer 4

ok so i get 20 .. and the next question says to do it b dot a... so do i just switch the a and b in the formula?

Answer 5

becaosue i go the same answer and that dosent feel right

Answer 6

a . b = 20 and b . a = 20

Answer 7

yes, dot products are commutative\[\vec A\cdot B=\vec B\cdot\vec A\]so they should be the same

Answer 8

and that would be the same even if i change the angle?

Answer 9

if you change the angle, theta in your formula changes, so that will change the value of the dot product

Answer 10

but it would still be commutative if that's what you mean

Answer 11

but if the angle is same between b . a and a . b then the shuld be the same right?

Answer 12

yes, the dot product is always commutative

Answer 13

even if the angle is 90 both answers shuld be 90.. so =

Answer 14

what is the cosine of 90 degrees?

Answer 15

0 so it would cause everything to be 0

Answer 16

yep

Answer 17

cool, you are awesome! you just clarified somuch for me... thx.. I wish i had someone like you for my entire assignment...

Answer 18

Thanks, and you're welcome!
I'm not hard to find here, and there are plenty of good tutors on this site
welcome to OS, hope to see you around

Answer 19

I will definately stay around... I was looking through many sites for help and got none and i saw an ad for OS and i came here and registed.

Answer 20

for this one (see attachment) when i do a . a and b .b i get totally different results so what can i conclude about U . U ? - I dont even understand what they are asking.