Problem 53 - Thomas Calculus

Question

Differentiable vector functions are continuous Show that if \(\mathbf{r}(t)=f(t) \mathbf{i}+g(t) \mathbf{j}+h(t) \mathbf{k}\) is differentiable at \(t=t_{0},\) then it is continuous at \(t_{0}\) as well.

Step-by-Step Solution

Answer

If a vector function is differentiable at a point, it is also continuous at that point.

Step 1: Understand Differentiability

A vector function \( \mathbf{r}(t) \) is differentiable at \( t = t_0 \) if its derivative exists at that point. The derivative is given by \( \frac{d\mathbf{r}}{dt} = \frac{df}{dt} \mathbf{i} + \frac{dg}{dt} \mathbf{j} + \frac{dh}{dt} \mathbf{k} \).

Step 2: Analyze Component Functions

The vector function \( \mathbf{r}(t) = f(t) \mathbf{i} + g(t) \mathbf{j} + h(t) \mathbf{k} \) is composed of the scalar component functions \( f(t) \), \( g(t) \), and \( h(t) \) for \( \mathbf{i} \), \( \mathbf{j} \), and \( \mathbf{k} \), respectively. If \( \mathbf{r}(t) \) is differentiable at \( t_0 \), each of these functions must also be differentiable at \( t_0 \).

Step 3: Relate Differentiability to Continuity

Remember that for a function to be differentiable at a point \( t_0 \), it must also be continuous at that point. Specifically, if \( f(t) \), \( g(t) \), and \( h(t) \) are differentiable at \( t_0 \), then they are also continuous at \( t_0 \).

Step 4: Apply to Vector Function

Since each component function \( f(t) \), \( g(t) \), and \( h(t) \) of the vector function \( \mathbf{r}(t) \) is continuous at \( t_0 \), it follows that \( \mathbf{r}(t) \) itself is continuous at \( t_0 \).

Step 5: Final Conclusion

We conclude that if a vector function \( \mathbf{r}(t) \) is differentiable at a point \( t_0 \), then it must also be continuous at that point due to the continuity of its individual component functions.

Key Concepts

Vector Calculus

Vector calculus is an essential field in mathematics that deals with vector functions and the calculus operations you can perform on them. A vector function maps real numbers to vectors and is an extension of the familiar scalar-valued function from single-variable calculus.
\(\mathbf{r}(t)=f(t)\mathbf{i}+g(t)\mathbf{j}+h(t)\mathbf{k}\)represents a vector function with three component functions: \( f(t) \),\( g(t) \), and \( h(t) \). These components are scalar functions of \( t \), the real number input. Vectors have both direction and magnitude and are common in physical applications such as forces or velocities.

• Operations on Vector Functions: Differentiation and integration are the primary operations, similar to single-variable calculus. You differentiate each component function individually.
• Applications: Used in fields like physics and engineering to model phenomena such as electromagnetic fields and fluid flow.

A deep understanding of vector calculus aids in grasping complex systems and aiding the modeling of real-world applications.

Continuous Functions

A function is continuous if its graph is an unbroken path, with no gaps or jumps. In more formal mathematical terms, a function \( f \) is continuous at a point \( t_0 \) if the limit of \( f(t) \) as \( t \) approaches \( t_0 \) is equal to \( f(t_0) \).
In the context of vector functions:

• Each component function \( f(t) \), \( g(t) \), and \( h(t) \) must be continuous for the vector function \( \mathbf{r}(t) \) to be continuous.
• If all component functions are continuous at a point \( t_0 \), then the whole vector function is continuous at \( t_0 \).

Continuity is crucial because it ensures that there are no sudden jumps in the vector function's path, making it predictable and smooth over its domain. This property is particularly important in modeling realistic physical systems, where abrupt changes are often non-physical.

Differentiability and Continuity

Differentiability is a stronger condition than continuity for a function. If a function is differentiable at a point, then it is automatically continuous at that point, but not vice versa.
For a function to be differentiable at a point \( t_0 \), it means that its derivative exists at that point. This implies the function behaves in a linear way at that proximity, with no sharp turns.

• Relationship: Being differentiable implies being continuous. If \( \mathbf{r}(t) \) is differentiable at \( t_0 \), then each component \( f(t) \), \( g(t) \), and \( h(t) \) is continuous at \( t_0 \).
• Application in Vector Functions: Differentiability of vector functions implies a smooth change in direction and magnitude, important in physics to predict motion accurately.

Thus, differentiability assures the smoothness and predictability of the function's behavior right at the point of differentiation, connecting directly to the continuous nature of the function at that spot, gripping how changes accumulate over time without disruption.