Show this equality of limits - cross products

In other words, express \vec c and \vec h in Cartesian coordinates, and let \lambda\to0.)In summary, the given statement is not true for all cases. A counterexample is provided by letting c = i and h = tj, where t is a scalar. The limit is equal to 1, not 0. This shows that the limit is not always 0, even if h approaches 0.
  • #1
Ted123
446
0

Homework Statement


Show that

[itex]\displaystyle\lim_{\underline{h}\to 0} \frac{\underline{c} \times \underline{h}}{\|\underline{h}\|} =0[/itex]

where c and h are vectors and x denotes cross product.

Homework Equations





The Attempt at a Solution



No idea how to do this?
 
Physics news on Phys.org
  • #2


Ted123 said:

Homework Statement


Show that

[itex]\displaystyle\lim_{\underline{h}\to 0} \frac{\underline{c} \times \underline{h}}{\|\underline{h}\|} =0[/itex]

where c and h are vectors and x denotes cross product.

Homework Equations


The Attempt at a Solution



No idea how to do this?

I don't think it's true. Suppose the vectors are in R^3, with [itex]i,j,k[/itex] the usual coordinate vectors. Let [itex]c = i[/itex], [itex]h = tj[/itex]. (where [itex]t[/itex] is a scalar)

Then

[tex]\left|\frac{c \times h}{|h|}\right| = \left|\frac{tk}{|tj|}\right| = \frac{|tk|}{|tj|} = \frac{|t|}{|t|} = 1[/tex]

so the limit is certainly not zero as [itex]h[/itex] approaches zero along the [itex]i[/itex] coordinate axis. If the limit were zero, it would have to be zero no matter how [itex]h[/itex] approaches zero.
 
  • #3


What happens if you let

[tex]\vec c = \vec i,\, \vec h = \lambda \vec j[/tex]

where [itex]\vec i,\vec j[/itex] are the standard unit vectors?
 

FAQ: Show this equality of limits - cross products

What is the definition of cross product?

The cross product, also known as the vector product, is a mathematical operation that takes two vectors and returns a third vector that is perpendicular to both input vectors. It is denoted by the symbol "x" and is defined as A x B = (|A||B|sinθ)n, where A and B are the input vectors, θ is the angle between them, and n is the unit vector perpendicular to both A and B.

How is the cross product different from the dot product?

The cross product and the dot product are two different ways of multiplying vectors. The dot product, denoted by the symbol "·", is a scalar quantity that results in a single number, while the cross product, denoted by "x", is a vector quantity that results in a new vector. Additionally, the dot product measures the projection of one vector onto another, while the cross product measures the perpendicular component of one vector to another.

What are the properties of cross product?

The cross product has several important properties, including distributivity, commutativity, and anti-commutativity. It is also linear, meaning that it follows the rules of linearity, such as being able to factor out scalars and distribute over addition. Furthermore, it is not associative, which means that (A x B) x C is not equal to A x (B x C). Lastly, the cross product of two parallel vectors is equal to zero.

How is the cross product useful in physics and engineering?

The cross product is used extensively in physics and engineering to calculate torque, magnetic fields, and other physical quantities. It is also useful in calculating the direction of rotational motion, as well as determining the orientation of objects in three-dimensional space. It is a fundamental tool in vector calculus and is essential in understanding the relationships between different physical quantities.

How can the equality of limits be shown using cross products?

The equality of limits can be shown using cross products by using a proof technique called "squeeze theorem". This technique involves finding two other limits that squeeze the original limit between them and then using the properties of cross products to show that the limits on either side are equal. This demonstrates that the original limit is also equal. It is a common method used in mathematical proofs involving limits and is an important application of cross products in mathematics.

Similar threads

Replies
10
Views
6K
Replies
3
Views
2K
Replies
3
Views
1K
Replies
12
Views
3K
Replies
4
Views
2K
Replies
6
Views
1K
Replies
7
Views
2K
Replies
3
Views
1K
Replies
7
Views
868
Back
Top