Mastering Physics Vector Question

AI Thread Summary
The discussion centers around solving a physics problem involving vectors A, B, and C, where C needs to be expressed in terms of A, B, and the angle theta. The initial solution using the law of cosines was C = sqrt(A^2 + B^2 - 2ABcos(θ), but feedback indicated a sign error, leading to the correction C = sqrt(A^2 + B^2 - 2ABcos(π - θ). The second part of the problem requires finding the angle phi that vector C makes with vector A, with an attempt using arcsin(ABsin(π - theta) / Csin(a). The user is seeking clarification on whether their approach or input is incorrect.
fishguts
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Homework Statement



I am having a problem with a mastering physics problem. I am given this image
41119_b.jpg


I am to Express C in terms of vector A, vector B, and angle theta, using radian measure for known angles. I know theta is equal to pi - c.

Homework Equations



law of cosines
render?tex=C%5E2%3DA%5E2+%2B+B%5E2+-+2AB+%5Ccos%28c%29.gif



The Attempt at a Solution



By using the law of cosines the answer i got was

C = sqrt(A^2 + B^2 - 2ABcos(θ))

The feedback I got back from the program was to check my signs but everything apperas to be correct. Can someone help me out?
 
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Hi,
Should'nt it be C = sqrt(A^2 + B^2 - 2ABcos(Pi-θ)) ?
As c is the angle opposite to C.
 
muscaria said:
Hi,
Should'nt it be C = sqrt(A^2 + B^2 - 2ABcos(Pi-θ)) ?
As c is the angle opposite to C.

Thats right! I do not know how I missed that
 
I am having trouble with the second part of the problem. I do not know if it is my input to the problem or if I have the problem totally wrong. I am told to:

Find the angle phi that the vector C_vec makes with vector A_vec.

I get arcsin(ABsin(pi - theta) / Csin(a))

For earlier solutions in the problem I found C = to pi - theta so U used that instad of c. Can anyone help me out?
 
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