Is the Vector Equation AxB=AxC Only True When A=0 or B=C?

In summary, The statement "If AxB = AxC, then either A=0 or B=C" is not entirely true. It can be true if A=0, or if the components of B and C normal to A are equal, or if A and B-C are parallel.
  • #1
cp255
54
0
True or False, if AxB = AxC then either A=0 or B=C.

A, B, and C are vectors and I thought this statement would be true. However the answer key says it is not. Why?
 
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  • #2
This can be explained by following example

1387076046331.jpg
 
  • #3
Ok that makes sense. Now if the the magnitude of the cross product was not zero then B would have to equal C Right?
 
  • #4
The magnitude of AxB is equal to the magnitude of A times the magnitude of B times the sine of the angle between the vectors A and B. If the magnitude of C is larger than the magnitude of B, but the sine of the angle between A and C is less, AxC can be equal to AxB.
 
  • #5
cp255 said:
Ok that makes sense. Now if the the magnitude of the cross product was not zero then B would have to equal C Right?
Not at all. All that AxB=AxC says is that either A=0 (in which case any two vectors B and C satisfy AxB=AxC), or that the components of B and C normal to A are equal.
 
  • #6
Or that since AxB=AxC is the same as AxB-AxC=Ax(B-C)=0 then A and B-C are parallel.
 
Last edited:

Related to Is the Vector Equation AxB=AxC Only True When A=0 or B=C?

1. What is the cross product of two vectors?

The cross product of two vectors is a mathematical operation that results in a third vector that is perpendicular to the two input vectors. It is also known as the vector product or outer product.

2. How is the cross product calculated?

The cross product is calculated by taking the determinant of a 3x3 matrix formed by the components of the two input vectors, with the first row being the unit vectors i, j, and k, the second row being the components of the first vector, and the third row being the components of the second vector.

3. What is the geometric interpretation of the cross product?

The cross product can be interpreted geometrically as the area of the parallelogram formed by the two input vectors, with the direction of the resulting vector being perpendicular to the plane of the parallelogram.

4. What is the relationship between the cross product and the dot product?

The cross product is related to the dot product by the identity a x b = |a||b|sinθ, where a and b are the input vectors and θ is the angle between them. This means that the cross product is zero when the two vectors are parallel, and reaches its maximum value when the two vectors are perpendicular.

5. In what applications is the cross product commonly used?

The cross product has many applications in physics, engineering, and computer graphics. It is used to calculate torque, magnetic fields, and angular momentum in physics, and is also used in 3D graphics to calculate lighting and shading effects.

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