Prove Vector Derivative (using Dot Product)

That is, (U• V)'= U'• V+ U• V'. That is the "product rule" for differentiation. In summary, the problem asks to prove that the derivative of the dot product of two differentiable 3-space vector-valued functions is equal to the dot product of the first function's derivative with the second function, plus the dot product of the first function with the second function's derivative. This can be shown using the component form of the dot product and the product rule for differentiation.
  • #1
karens
7
0

Homework Statement


n.

Let r1 and r2 be differentiable 3-space vector-valued functions.

Directly from the definition of dot product, and the definition of derivative of vector-
valued functions in terms of components, prove that
d/dt (r1(t) • r2(t)) = r′1(t) • r2(t) + r1(t) • r′2(t).

Homework Equations



Dot Product: U • V = u1v1 + u2v2 + u3v3

The Attempt at a Solution



I can just substitute things forever and get nowhere...
 
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  • #2
show us how?

in this example the dot product is a scalar valued function of t, use the component form you have shown & the product rule and it should follow straight forward
 
Last edited:
  • #3
karens said:

Homework Statement


n.

Let r1 and r2 be differentiable 3-space vector-valued functions.

Directly from the definition of dot product, and the definition of derivative of vector-
valued functions in terms of components, prove that
d/dt (r1(t) • r2(t)) = r′1(t) • r2(t) + r1(t) • r′2(t).

Homework Equations



Dot Product: U • V = u1v1 + u2v2 + u3v3

The Attempt at a Solution



I can just substitute things forever and get nowhere...
But have you differentiated?

Since, as you write, U• V= u1v1+ u2v2+ u3v3, (U• V)'= u1'v1+ u1v1'+ u2'v2+ u2v2'+ u3'v3+ u3v3'= (u1'v1+ u2'v2+ u3'v3)+ (u1v1'+ u2v2'+ u3v3').
 

What is a vector derivative?

A vector derivative is a mathematical operation that calculates the instantaneous rate of change of a vector with respect to a variable. It is similar to a regular derivative, but takes into account the direction of the vector as well as its magnitude.

Why is the dot product used in proving the vector derivative?

The dot product is used because it is a mathematical operation that calculates the projection of one vector onto another. This is essential in determining the rate of change of a vector, as it allows us to measure the change in the direction of the vector.

What are the steps to prove the vector derivative using dot product?

The steps to prove the vector derivative using dot product are as follows:
1. Write the vector as a function of the variable
2. Use the definition of the dot product to expand the expression
3. Apply the limit definition of the derivative
4. Simplify the expression and factor out the variable
5. Use the properties of the dot product to rearrange the terms
6. Take the limit as the variable approaches zero
7. The resulting expression is the vector derivative.

What is the relationship between the vector derivative and the gradient?

The gradient is a vector that contains the partial derivatives of a multivariable function. The vector derivative is also a vector that contains the derivatives of a vector function. They are related in that the gradient is the vector derivative of a scalar function, while the vector derivative is the gradient of a vector function.

How is the vector derivative used in real-world applications?

The vector derivative is used in physics, engineering, and other fields to model the motion and behavior of objects. It is used to calculate velocity, acceleration, and other important quantities that describe the movement of objects in space.

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