# Finding angle between two unit vectors

Printable View

• Dec 2nd 2009, 10:18 PM
Martin Billado
Finding angle between two unit vectors
Hello

Please help me out with the question.

Given two unit vectors A,B and their sum (vector C) which is also a unit vector, find the angle between B and C?

Thanks in advance
• Dec 2nd 2009, 10:29 PM
qmech
When working with vectors, you usually compute the angle by
$
cos \theta = \frac {\vec A \cdot \vec B}
{ A B }
$

where the denominators refer to the magnitude of the vectors.

In this case, since C = A + B is also a unit vector, are you working with an equilateral triangle?
• Dec 2nd 2009, 11:44 PM
Martin Billado
Thats what I thought and therefore wouldnt the answer be 60 degrees. But talking to my friend, she was arguing the answer is 120 degrees.
• Dec 3rd 2009, 03:43 AM
mr fantastic
Quote:

Originally Posted by Martin Billado
Thats what I thought and therefore wouldnt the answer be 60 degrees. But talking to my friend, she was arguing the answer is 120 degrees.

Unless the two vectors are perpendicular, there is obviously both an acute angle and an obtuse angle between them ....