Multiplication using dot product.

[AryRezvani]

Homework Statement

Homework Equations

Tan θ=A.B/|A|*|B|

The Attempt at a Solution

A=4i-9j
B=9i-6j

A.B=-18
Magnitude of A = √97
Magnitude of B = √117

Solve that out using Cos^-1 (-18/Sqr97*Sqr117) and I keep getting the wrong answer.

 

[gneill]

Check the value you've calculated for A.B

 

[AryRezvani]

Check the value you've calculated for A.B

Ahh just caught that. Replugged in the values, and still came out wrong.

Work:

Plugged in 90/(Sqr97*Sqr117) = .8448190756

Tan^-1 (.84481...) = 40.191 degrees.

Plugged that into online homework and it was wrong apparently.

 

[gneill]

Check you definition of A.B in terms of magnitudes and angle. You used a correct method in the final line of your first post.

 

[AryRezvani]

Check you definition of A.B in terms of magnitudes and angle. You used a correct method in the final line of your first post.

Could you specify? Are you referring to A.B=|A||B|Cosθ?

Edit: nevermind, got it!

 

[gneill]

Could you specify? Are you referring to A.B=|A||B|Cosθ?

Yup. The formula involves a cosine, so why are you employing $tan^{-1}$?

 

[AryRezvani]

Yup. The formula involves a cosine, so why are you employing $tan^{-1}$?

Hahaha thanks, took me a while to catch that.. sadly.

 

[gneill]

Hahaha thanks, took me a while to catch that.. sadly.

Glad I could help