Solving Problem Involving Cross Product

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SUMMARY

The discussion centers on solving the equation u = v x w, where u, v, and w are 3x1 vectors, and x denotes the cross product. The user seeks to express vector v in terms of vectors u and w but realizes that a unique solution for v is not possible without additional information. The problem is further complicated by the context of measuring moments (M) and forces (F) in a physical setup involving a center of gravity and equilibrium forces, leading to a system of equations that may allow for the computation of the unknown vector r if sufficient measurements are taken.

PREREQUISITES
  • Understanding of vector operations, specifically the cross product.
  • Familiarity with 3D vector mathematics and linear algebra.
  • Knowledge of Euler rotation matrices and their application in coordinate transformations.
  • Basic principles of mechanics, particularly moments and forces in equilibrium.
NEXT STEPS
  • Study the properties and applications of the cross product in vector calculus.
  • Learn about solving systems of equations involving vectors, particularly in 3D space.
  • Explore the use of Euler rotation matrices in transforming vector coordinates.
  • Investigate methods for determining unknown vectors in mechanics, particularly in equilibrium scenarios.
USEFUL FOR

This discussion is beneficial for physics students, mechanical engineers, and anyone involved in vector analysis and mechanics, particularly those working on problems related to forces and moments in three-dimensional space.

azizz
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Hey,

I have a problem that can be written in the following form:

u=v x w

where u, v, w are 3by1 vectors and x is the cross product.
now I want to write v in term of u and w, but I have no idea of how to get vector v out of the previous equation. Someone who can help me with this?

Thanks a lot
 
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azizz said:
Hey,

I have a problem that can be written in the following form:

u=v x w

where u, v, w are 3by1 vectors and x is the cross product.
now I want to write v in term of u and w, but I have no idea of how to get vector v out of the previous equation. Someone who can help me with this?

Thanks a lot

The cross product vector,u, points perpendicular to the plane spanned by w and v and its length is the area of the parallelogram that w and v span. Look at the simple case where w and v lie in the xy-plane. Is there a unique solution for v given w and u?
 
Last edited:
Sorry about that. Its more than obvious that it is impossible to solve the problem as I stated it before.

Let me give you some more information about my issue. And what I want to do. Perhaps that will make it more clear.

Suppose we have a wig with a centre of gravity x and a rotating point o.

-----------------------
| . . . . . . .x. . . . . . . .|
-----------0-----------
1. . . . . . . . . . . . . . . .2

(just ignore the points . . . . .)

Then we have three forces: Fg (gravity force working on x), F1 and F2 (forces to keep the wig in equilibrium, working on the points indicated by 1 and 2). The location of the cog x with respect to the rotating point o is denoted by r.
The moment due to the gravity working on the cog is computed as: M = r x F, with x being the cross product.
Now suppose I am able to measure M and F, and I want to compute the location of the cog, that is, I want to know r.
If I only do a measurement as indicated above, I can not determine r.
But if I now do two additional measurements by rotating the setup by +30 and -30 degrees I have three measurements:
M1 = R1 (r x F1)
M2 = R2 (r x F2)
M3 = R3 (r X F3)
where R is the euler rotation matrix to transform the coordinates in the rotated setup to the initial position as indicated in my figure. That means that for the first measurement R1=Identity.
So here comes my question again: is it now possible to compute r, if the only unknown variable in the three equations above is r?

Thanks again
 
Last edited:

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