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# c4 scalar products quick q watch

1. find the angle that the vector 9i - 5j +3k makes with
a) the positive x axis
b) the positive y axis

how do i solve this?

do i use the a.b = |a||b|cosX formula?
if so, how?
cos it seems like it would have a lot of zeros in it...

ta,

JB
2. (Original post by jumblebumble)
find the angle that the vector 9i - 5j +3k makes with
a) the positive x axis
b) the positive y axis

how do i solve this?

do i use the a.b = |a||b|cosX formula?
if so, how?
cos it seems like it would have a lot of zeros in it...

ta,

JB
Well, any vector which has a direction vector in the direction of the positive x-axis, for example, 5x +0y+0z or 854x + 0y+0z.
3. (Original post by Clarity Incognito)
Well, any vector which has a direction vector in the direction of the positive x-axis, for example, 5x +0y+0z or 854x + 0y+0z.
what do you mean?
4. (Original post by jumblebumble)
what do you mean?
There are infinitely many vectors that can describe the direction of the positive x axis, but you don't want them moving into the y or z plane because then it wouldn't be describing the direction of the +ve x axis.
5. For the first one.

I'd forget about the dot product. Just drop a perpendicular from the point specified by the vector down to the x-axis. You now have a right angled triangle.

What's the length of the hypotenuse?

What's the length along the x axis?

Hence cosine of your angle between the vector and the x-axis is?

Hence....

In this case the x co-ordanate is positive so you have found the angle with the postive x-axis.

In the second one you'll have to work a little harder.
6. (Original post by ghostwalker)
For the first one.

I'd forget about the dot product. Just drop a perpendicular from the point specified by the vector down to the x-axis. You now have a right angled triangle.

What's the length of the hypotenuse?

What's the length along the x axis?

Hence cosine of your angle between the vector and the x-axis is?

Hence....

In this case the x co-ordanate is positive so you have found the angle with the postive x-axis.

In the second one you'll have to work a little harder.
Thanks. I've worked out both parts now

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