# Tensor Calculus 2b: Two Geometric Gradient Examples (Torricelli's and Heron's Problems)

TLDRThis lecture explores Torricelli's and Perrins' problems using calculus, providing an alternative to the geometric solutions. It demonstrates how to find optimal points with minimal distances to vertices or along a riverbank without coordinates, by setting gradients to zero or orthogonal to constraints. The discussion highlights the elegance of combining calculus with geometry, offering intuitive solutions to classic problems and pondering the balance between geometric intuition and analytical methods.

###### Takeaways

- π The lecture discusses solving Torricelli's and Perrins' problems using calculus without the need for a coordinate system.
- π Torricelli's problem involves finding the point inside a triangle where the sum of distances to the three vertices is minimized, which is identified as the point equidistant from the vertices at 120-degree angles.
- π The solution to Torricelli's problem uses the concept of gradients in a geometric context, without coordinates, to find the point where the sum of gradients equals zero.
- π€ The lecture emphasizes the importance of understanding the gradient as the direction of the steepest increase in a function's value, which is key to solving these problems.
- π The concept of gradients is applied to the sum of three distances in Torricelli's problem, where the combined gradient points in the direction of increasing the sum of distances.
- π The minimum of the function (sum of distances) occurs where the combined gradient is zero, a principle borrowed from calculus optimization.
- π Perrins' problem introduces a constraint, requiring the point to be on a straight line, which changes the condition for the gradient to be perpendicular to the constraint.
- π The solution to Perrins' problem involves finding where the gradients of the distances to two points are orthogonal to the straight line constraint, leading to equal angles for the projections onto the line.
- π§© The script highlights the mix of calculus and geometry in solving these problems, emphasizing the visual and intuitive nature of the approach.
- π« The lecturer expresses concern about the temptation to avoid coordinate systems entirely, noting that while this method is appealing, it can be impractical.
- π The discussion concludes by acknowledging the value of tensor calculus, which allows for geometric understanding while incorporating the benefits of coordinate systems.

###### Q & A

### What are Torricelli's problem and Perrins' problem?

-Torricelli's problem involves finding a point within a triangle where the sum of distances to the three vertices is minimized. Perrins' problem is about finding a point along a straight river with two villages on either side, where the sum of distances to both villages is minimized, with the constraint that the point must be on the river.

### How does the solution to Torricelli's problem using calculus differ from the geometric solution?

-The calculus solution uses the concept of gradients to find the point where the sum of distances is minimized without introducing a coordinate system. The geometric solution, on the other hand, involves rotating the triangle and using geometric intuition, which is more complex and requires a deeper understanding of geometry.

### What is the significance of gradients in solving Torricelli's problem with calculus?

-Gradients are used to determine the direction in which the function (in this case, the sum of distances) increases the fastest. The point where the combined gradient of the function is zero is considered the optimal point, as it represents the minimum of the function.

### Why does the gradient need to be orthogonal to the constraint in Perrins' problem?

-The gradient being orthogonal to the constraint ensures that any movement along the constraint (the straight line in this case) would not decrease the function's value, indicating that the point found is indeed the minimum.

### How does the concept of gradients help in finding the solution to Perrins' problem?

-In Perrins' problem, the gradients of the distances to the two villages are combined, and the point where their sum is orthogonal to the river (the constraint) is found. This point is where the horizontal components of the gradients cancel each other out, leaving only the component orthogonal to the river.

### What does the transcript suggest about the relationship between calculus and geometry?

-The transcript suggests that calculus and geometry can be combined effectively to solve problems. While calculus provides the tools to find minima (like gradients), geometry provides the intuitive understanding of the problem's constraints and conditions.

### Why might someone be tempted to avoid using coordinate systems in solving geometric problems?

-Avoiding coordinate systems can lead to a more intuitive and visual approach to problem-solving, which might be appealing for its simplicity and directness. However, the transcript warns that this approach can be impractical and less effective in certain situations.

### What is the role of optimization in the context of the problems discussed in the transcript?

-Optimization plays a crucial role in finding the minimum values of the functions involved in Torricelli's and Perrins' problems. It provides the theoretical basis for using gradients to locate the points where the sum of distances is minimized.

### How does the transcript describe the process of finding the optimal point in Torricelli's problem?

-The transcript describes the process as finding the point where the combined gradient of the sum of distances to the vertices is zero. This is achieved by arranging the gradients such that their vector sum is zero, which occurs when the angles between them are 120 degrees.

### What insight does the transcript provide about the importance of understanding both calculus and geometry?

-The transcript highlights that while calculus provides powerful tools for optimization, a deep understanding of geometry is essential for interpreting the results and applying them to solve problems effectively.

### How does the transcript relate the concepts of gradients and optimization to the solution of geometric problems?

-The transcript demonstrates that gradients can be used to find the minimum of a function, which in the context of geometric problems, corresponds to finding the optimal point that minimizes a certain quantity, such as the sum of distances.

###### Outlines

##### π Torricelli's and Perrins' Problems with Calculus

The script begins with an introduction to Torricelli's and Perrins' problems, which are classic geometric problems that can be solved using calculus. The speaker explains that while geometric methods are possible, calculus provides a simpler approach without the need for coordinates. Torricelli's problem involves finding the point inside a triangle where the sum of distances to the three vertices is minimized. The solution is identified as Torricelli's point, where each angle from the point to the vertices is 120 degrees. The proof uses the concept of gradients, which are direction vectors that point in the direction of the steepest increase of a function. The speaker emphasizes that at the point of minimum, the combined gradient of the sum of distances must be zero. This is a fundamental principle from calculus, applied here in a geometric context without coordinates.

##### π Geometric Proof vs. Calculus: Perrins' Problem

The second paragraph delves into Perrins' problem, which is a constrained optimization problem involving a straight river and two villages. The goal is to find a point on the river where the sum of distances to the two villages is minimized. The script contrasts the geometric proof, which requires a high level of geometric intuition, with the calculus approach, which is more straightforward. The speaker discusses the concept of gradients in the context of constraints, stating that the gradient of the objective function must be perpendicular to the constraint for the point to be optimal. The explanation uses the idea of gradients to find the point where the horizontal components cancel out, leaving only the component orthogonal to the river. This approach recovers the geometric insight without complex geometric proofs, illustrating the power of calculus in solving optimization problems with constraints.

##### π€ The Balance Between Calculus and Geometry

In the final paragraph, the speaker reflects on the balance between using calculus and maintaining geometric intuition. They acknowledge the appeal of solving problems with calculus without introducing coordinate systems, but also express concern about the potential loss of geometric understanding that comes with avoiding coordinates. The speaker argues that while the calculus approach is intuitive and visual, it is impractical to avoid coordinates altogether. They conclude by highlighting the importance of tensor calculus, which allows for working with coordinate systems while still retaining geometric meaning, thus providing a practical solution to the dilemma of balancing calculus and geometry.

###### Mindmap

###### Keywords

##### π‘Torricelli's Problem

##### π‘Gradients

##### π‘Optimization

##### π‘Geometric Intuition

##### π‘Perrins' Problem

##### π‘Constraints

##### π‘Orthogonal

##### π‘Unit Vector

##### π‘120 Degrees

##### π‘Geometric Proof

##### π‘Tensor Calculus

###### Highlights

Introduction of Torricelli's problem and Perrins' problem using calculus without a coordinate system.

Explanation of Torricelli's point as the optimal point where each angle is 120 degrees from the vertices of the triangle.

Use of gradients in calculus to determine the direction for the fastest increase in segment length.

Concept that the gradient of a combined function is the sum of the gradients of individual functions.

The minimum of a function occurs at the point where the gradient is zero, a key concept from optimization.

Visual demonstration of adding three gradients to find the optimal point where their combined gradient is zero.

The geometric arrangement of three unit vectors at 120-degree angles for their sum to be zero.

Comparison of the simplicity of the calculus-based solution to the geometric proof of Torricelli's problem.

Discussion on the temptation to avoid coordinate systems due to the intuitive and visual appeal of the calculus method.

Introduction of Perrins' problem with a constraint that the point must be on a straight line.

Explanation of gradients in the context of constraints, where the gradient must be perpendicular to the constraint.

Intuitive reasoning for why the gradient must be orthogonal to the constraint to minimize the function.

Finding the optimal point where the combined gradient is orthogonal to the constraint line.

Geometric insight recovered through calculus by ensuring the angles are equal for the gradients to cancel out parallel components.

The beauty and effectiveness of using calculus to solve geometric problems without losing geometric intuition.

Warning against the impracticality of strictly avoiding coordinate systems despite the appeal of pure calculus methods.

Conclusion emphasizing the practicality of tensor calculus in combining geometric meaning with coordinate systems.

###### Transcripts

5.0 / 5 (0 votes)

Thanks for rating: