Question

http://answers.yahoo.com/question/index?qid=20110827200549AAtiUiO

Answer 1

I don't understand on how he derived the angle?

Answer 2

Tan x= opp/adj

Answer 3

"A wrench 30 cm long lies along the positive y-axis..."
"... A force is applied in the direction < 0 , 3 , -4 >..."

Can you write vector r and vector F ??

Answer 4

R= <0,0.3,0> and F=<0,3,-4>

Answer 5

Then it is best to calculate cross product by determinant method.
You can still calculate tan (with a bit of imagination); so if you want to use the angle approach I can assist you :)

Answer 6

Ya, can u help me to find the angle plz

Answer 7

Consider the yz plane -


Can you calculate tan(theta) ??

Answer 8

Tan-1(-4/3) =-53.13

Answer 9

But, isn't 3 and -4 in y and z-axis

Answer 10

Now, do you understand how theta is the angle b/w F and r ?

Answer 11

Never mind yes, thx. Another way is by using determination right?

Answer 12

I got -1.2 j

Answer 13

"Never mind"
Looks like you're still not satisfied with - "why I chose the yz plane", right ?

Answer 14

Yes I got that part u choose that because x=0

Answer 15

I wonder what to do if x is not equal to 0

Answer 16

"...u choose that because x=0"
Correct.

"I wonder what to do if x is not equal to 0"
In general, to find the angle we chose a plane which contains both the vectors.

Answer 17

Okay, can you plz describe another method. I will try by own

Answer 18

Using determination

Answer 19

I would like to point here that only the direction of force is given. We need to find the magnitude of force.

"Never mind yes, thx. Another way is by using determination right?"
"I got -1.2 j"
Can you explain this ^^