Mastering Physics Vector Question

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Homework Help Overview

The discussion revolves around a physics problem involving vectors, specifically focusing on expressing vector C in terms of vectors A and B and an angle theta. The original poster mentions using the law of cosines and has received feedback regarding the signs in their calculations.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the application of the law of cosines and question the correctness of the angle used in the formula. There is also a mention of deriving the angle phi that vector C makes with vector A, leading to further exploration of trigonometric relationships.

Discussion Status

Some participants have provided alternative formulations for vector C and are exploring the implications of using different angles. The conversation reflects a collaborative effort to clarify the problem setup and assumptions without reaching a definitive conclusion.

Contextual Notes

There is mention of potential confusion regarding the input to the problem and the relationships between the angles involved, particularly with respect to the angle opposite to vector C. The original poster is also navigating feedback from a program regarding their calculations.

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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