Express VectorU in Terms of VectorA, VectorB, ScalarF

  • Context: Graduate 
  • Thread starter Thread starter starbaj12
  • Start date Start date
  • Tags Tags
    Scalars Vectors
Click For Summary

Discussion Overview

The discussion revolves around expressing an unknown vector, vector U, in terms of known vectors vector A, vector B, and scalar F, along with the magnitude of vector A. The context includes mathematical reasoning and vector relationships.

Discussion Character

  • Mathematical reasoning
  • Technical explanation
  • Exploratory

Main Points Raised

  • Post 1 introduces the problem of expressing vector U given the equations vectorA (cross) vectorU = vectorB and vectorA (dot) vectorU = F.
  • Post 2 discusses the implications of the equations, noting that vector B is perpendicular to the plane spanned by vector A and vector U, and that the dot product provides a relationship involving the angle between vector A and vector U.
  • Post 2 further suggests that vector U can be expressed as a linear combination of orthonormal vectors derived from vector A and a unit vector perpendicular to the plane spanned by vectors A and B.
  • Post 2 concludes that the magnitude of vector U can be determined from the relationship involving the magnitudes of vectors A and B and the sine of the angle between them.
  • Post 3 requests clarification on a specific statement regarding the positivity of sin(t) and its relationship to cos(t).
  • Post 4 provides a mathematical clarification that sin(t) can be expressed in terms of cos(t), suggesting that all necessary components are known.

Areas of Agreement / Disagreement

Participants engage in a technical exploration of the problem, with some agreement on the relationships between the vectors and the implications of the equations. However, the discussion does not reach a consensus on the final expression for vector U.

Contextual Notes

The discussion includes assumptions about the angles and relationships between the vectors, which may not be fully resolved. The dependence on the definitions of the vectors and the conditions under which the equations hold is acknowledged.

starbaj12
Messages
48
Reaction score
0
vector U is an unkown vector; known vectors are vectorA and vectorB and scalarF. vectorA (cross) vectorU = vectorB and vectorA (dot) vectorU = F. Express vectorU in terms of vectorA, vectorB, scalarF and the magnitude of vectorA.

Thank you
 
Physics news on Phys.org
given A,B,f, such that AxU = B, and A.U = f, find U in terms of A,B,f.

well let's see what we have. AxU = B means that B is perpendicular to the plane spanned by A and U. A.U = f, means that |A||U|cos(t) = f where t is the angle between A and U. Then B/|B| x A/|A| = C is a unit vector perpendicular to the plane spanned by A and B, hence lies in the plane spanned by A and U. So U is in the plane spanned by A and C, which are perpendicular to each other, but the angle t between A and U has cosine equal to f/(|A||U|. So C and A/|A| =V are orthonormal vectors and U is a linear combination of them. Indeed U/|U| = cos(t)A + sin(t)C.

now all we need is the length of U. But |B| = |A||U| sin(t), so |U| = |B|/|A|sin(t).

that is pretty close if you note that sin(t) is positive, hence determined by cos(t).

does that help?
 
Thank you for your reply,

But could you elaborate on, "that is pretty close if you note that sin(t) is positive, hence determined by cos(t)"

Thank you for your help
 
i mean sin(t) = +sqrt(1-cos^2(t)), so everything is known.
 

Similar threads

  • · Replies 5 ·
Replies
5
Views
12K
  • · Replies 3 ·
Replies
3
Views
5K
Replies
4
Views
2K
  • · Replies 8 ·
Replies
8
Views
1K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 4 ·
Replies
4
Views
4K
  • · Replies 6 ·
Replies
6
Views
3K
  • · Replies 6 ·
Replies
6
Views
3K
  • · Replies 7 ·
Replies
7
Views
2K
Replies
1
Views
2K