Chapter 12: Problem 75
Let \(\mathbf{u}(t)=\left\langle 1, t, t^{2}\right\rangle, \mathbf{v}(t)=\left\langle t^{2},-2 t, 1\right\rangle\) and \(g(t)=2 \sqrt{t} .\) Compute the derivatives of the following functions. $$\mathbf{v}(g(t))$$
Short Answer
Expert verified
The derivative of the given function $\mathbf{v}(g(t))$ is:
$$\frac{d}{dt}(\mathbf{v}(g(t))) = \left\langle 4, -2t^{-\frac{1}{2}}, 0\right\rangle$$
Step by step solution
01
Compute the derivative of \(\mathbf{v}(t)\)
Firstly, let's compute the derivative of the vector function \(\mathbf{v}(t)\). To do that, we need to find the derivative of its components with respect to \(t\). Recall that:
$$\mathbf{v}(t) = \left\langle t^{2}, -2t, 1\right\rangle$$
Let's find the derivatives of each component:
1. \(\frac{d}{dt}(t^2) = 2t\)
2. \(\frac{d}{dt}(-2t) = -2\)
3. \(\frac{d}{dt}(1) = 0\)
Now that we've found the derivatives of each component, we can find \(\mathbf{v}'(t)\) as follows:
$$\mathbf{v}'(t) = \left\langle 2t, -2, 0\right\rangle$$
02
Compute the derivative of \(g(t)\)
Now, let's find the derivative of the scalar function \(g(t)\). Recall that:
$$g(t) = 2\sqrt{t}$$
To find its derivative, we need to find the derivative of the function with respect to \(t\):
$$g'(t) = \frac{d}{dt}(2\sqrt{t}) = 2 \cdot \frac{d}{dt}(t^{\frac{1}{2}}) = 2 \cdot \frac{1}{2}t^{-\frac{1}{2}} = t^{-\frac{1}{2}}$$
03
Derivative of \(\mathbf{v}(g(t))\) using the chain rule
We have found the derivatives of both \(\mathbf{v}(t)\) and \(g(t)\). Now, we can apply the chain rule to find the derivative of the composite function \(\mathbf{v}(g(t))\). The chain rule states:
$$\frac{d}{dt}(\mathbf{v}(g(t))) = \mathbf{v}'(g(t)) \cdot g'(t)$$
Substituting the values of \(\mathbf{v}'(t)\) and \(g'(t)\) calculated above, we get:
$$\frac{d}{dt}(\mathbf{v}(g(t))) = \left\langle 2g(t), -2, 0\right\rangle \cdot t^{-\frac{1}{2}}$$
Now, substitute the expression of \(g(t)\) back into the formula:
$$\frac{d}{dt}(\mathbf{v}(g(t))) = \left\langle 2(2\sqrt{t}), -2, 0\right\rangle \cdot t^{-\frac{1}{2}}$$
Finally, we can simplify the equation by multiplying the components of the vector by \(t^{-\frac{1}{2}}\):
$$\frac{d}{dt}(\mathbf{v}(g(t))) = \left\langle 4\sqrt{t}t^{-\frac{1}{2}}, -2t^{-\frac{1}{2}}, 0\right\rangle = \left\langle 4, -2t^{-\frac{1}{2}}, 0\right\rangle$$
Thus, the derivative of the given function \(\mathbf{v}(g(t))\) is:
$$\frac{d}{dt}(\mathbf{v}(g(t))) = \left\langle 4, -2t^{-\frac{1}{2}}, 0\right\rangle$$
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Vector Calculus
Vector calculus is a branch of mathematics that deals with vector fields, which are essentially functions that assign a vector to every point in space. It's commonly used in physics and engineering to model phenomena such as fluid flow and electromagnetic fields. In this context, functions like \( \mathbf{u}(t) \) and \( \mathbf{v}(t) \) assign vectors to each time \( t \). These vector functions represent space curves, where each component of the function, such as \( t^2 \) or \( -2t \), changes with time. Understanding vector calculus involves:
- Grasping how these vectors can change or evolve, which requires understanding derivatives of vector functions.
- Learning how to perform operations like addition, subtraction, and differentiation on these vectors.
Differentiation
Differentiation refers to the process of finding a derivative, which tells us how a function changes as its input changes. When we differentiate vector functions, we individually find the derivatives of each component of the vector. For a vector function \( \mathbf{v}(t) = \langle t^2, -2t, 1 \rangle \), differentiation is performed component-wise:
- The derivative of \( t^2 \) with respect to \( t \) is \( 2t \).
- The derivative of \( -2t \) is \( -2 \).
- The derivative of the constant 1 is 0.
Composite Functions
Composite functions involve functions within functions. In this exercise, we have a composite function \( \mathbf{v}(g(t)) \), where \( g(t) \) is another function nested within \( \mathbf{v} \). Understanding these requires using the chain rule, a key differentiation technique. The chain rule for derivatives states:
- If you have a composite function \( f(g(x)) \), the derivative is \( f'(g(x)) \cdot g'(x) \).