# Modeling Real-World Systems with Differential Equations Explore the power of differential equations in modeling complex phenomena. Learn to analyze population dynamics, thermal systems, and more. Gain skills to solve real-world problems in science and engineering.

Now Playing:Applications to differential equations – Example 0a
Intros
1. What sort of problems can be solved by first-order differential equations?
2. Modeling Population Dynamics
Examples
1. Modeling and Solving Differential Equations
The rate of change of the volume of a spherical balloon in terms of radius is equal to the surface area of the balloon itself. Find the general solution of the differential equation.
Order and solutions to differential equations
Notes
In this section, we will try to apply differential equations to real life situations. For each question we will look how to set up the differential equation. Afterwards, we will find the general solution and use the initial condition to find the particular solution. Depending on the question, we will even look at behaviours of the differential equation and see if it is applicable to real life situations. For example, one can notice that integrating the area of a sphere actually gives the volume of a sphere!
We will be learning how to create a differential equation out of the word problem, and then find the general and particular solutions. We will then take a look at the behaviour of the general solution to find results we need to answer the questions.

It may be convenient to use the following formula when modelling differential equations related to proportions:

$\frac{dy}{dt}=kM$

Where:
1. $\frac{dy}{dt}$ is the rate of change of $y$
2. $k$ is a constant
3. $M$ is the equation that models the problem

There are many applications to first-order differential equations. Some situations that can give rise to first order differential equations are:
• Population Dynamics (growth or decline)

Exponential Model:
$\frac{dP}{dt}=KP$
$P=Ce^{Kt}$

Logistic Model:
$\frac{dP}{dt}=KP(1-\frac{P}{M})$
$P= \frac{M}{1+Ce^{-kt}}$
$C= \frac{M-P_0}{P_0}$

• Newton's Law of Cooling
$\frac{dT}{dt}=-K(T-T_a)$
$|T-T_a |=Ce^{-kt}$
If $T \geq T_a$ (the object is more hot), then $T=Ce^{-kt}+T_a$
If $T$ < $T_a$ (the object is cooler), then $T=T_a-Ce^{-kt}$
Concept

## Introduction to Modeling with Differential Equations

Modeling with differential equations is a powerful tool for understanding complex real-world phenomena. The introduction video provides a crucial foundation, demonstrating how these mathematical concepts apply to various fields such as physics, engineering, and biology. Differential equations allow us to describe how quantities change over time or space, making them invaluable in predicting and analyzing dynamic systems. The process begins with setting up the appropriate differential equation based on observed relationships and known laws. Finding the general solution offers a broad understanding of the system's behavior, while particular solutions address specific initial conditions or constraints. Analyzing these solutions reveals important insights about the system's long-term behavior, stability, and response to changes. By mastering the art of modeling with differential equations, students gain a deeper appreciation for the mathematical underpinnings of natural and engineered systems, enhancing their problem-solving skills across various disciplines.

Example

What sort of problems can be solved by first-order differential equations?

#### Step 1: Introduction to Differential Equations

Differential equations are mathematical equations that relate some function with its derivatives. In the context of first-order differential equations, these equations involve the first derivative of the function. They are widely used in various fields to model real-world phenomena. This section will explore the types of problems that can be solved using first-order differential equations.

#### Step 2: Applications in Engineering

One of the primary fields where first-order differential equations are extensively used is engineering. Engineers use these equations to model and solve problems related to the design and analysis of structures, such as bridges. The equations help in understanding how different forces and stresses affect the structure, ensuring its stability and safety.

#### Step 3: Historical Context and Early Applications

Historically, first-order differential equations have been used to model various natural phenomena. For instance, Sir Isaac Newton used differential equations to describe the cooling process of objects. This method, known as Newton's Law of Cooling, is a classic example of how first-order differential equations can be applied to understand and predict the rate at which an object cools down over time.

#### Step 4: Modeling Radioactive Decay

Another significant application of first-order differential equations is in modeling radioactive decay. These equations can describe how the quantity of a radioactive substance decreases over time. By solving these equations, scientists can predict the half-life of the substance and understand the decay process in detail.

#### Step 5: Population Dynamics

One of the most impactful applications of first-order differential equations is in the study of population dynamics. These equations can model how populations grow or decline over time. For example, the exponential growth model, which is a first-order differential equation, explains why populations tend to grow exponentially under ideal conditions. This understanding is crucial for fields such as ecology, biology, and even economics.

#### Step 6: Deriving Equations for Real-World Problems

In this section, we will derive equations for specific real-world problems such as population dynamics and Newton's Law of Cooling. By understanding the derivation process, you can see how first-order differential equations are formulated and applied to solve practical problems. This step involves setting up the differential equation based on the problem's conditions and solving it to find the function that describes the system's behavior.

#### Step 7: Practical Examples and Exercises

To solidify your understanding, we will go through several practical examples and exercises. These examples will demonstrate how to apply first-order differential equations to solve various problems. By working through these exercises, you will gain hands-on experience and see the practical utility of these mathematical tools.

#### Step 8: Conclusion and Further Applications

In conclusion, first-order differential equations are powerful tools for modeling and solving a wide range of problems in engineering, natural sciences, and social sciences. From designing stable structures to understanding natural decay processes and predicting population growth, these equations provide valuable insights and solutions. As you continue to explore differential equations, you will discover even more applications and deepen your understanding of this essential mathematical concept.

FAQs
1. What is the importance of differential equations in real-world modeling?

Differential equations are crucial in real-world modeling because they allow us to describe how quantities change over time or space. They are used in various fields such as physics, engineering, biology, and economics to model complex systems and predict their behavior. For example, they can model population growth, heat transfer, electrical circuits, and financial markets, providing valuable insights for decision-making and problem-solving in these areas.

2. How do you find the general solution of a differential equation?

To find the general solution of a differential equation, you typically follow these steps: 1) Identify the type of differential equation (e.g., first-order, second-order, linear, nonlinear). 2) Choose an appropriate method based on the equation type (e.g., separation of variables, integrating factor, characteristic equation). 3) Solve the equation using the chosen method. 4) Express the solution with arbitrary constants to represent all possible solutions. For example, the general solution of dy/dx = 2x is y = x² + C, where C is an arbitrary constant.

3. What is the difference between general and particular solutions?

The general solution of a differential equation includes all possible solutions and contains arbitrary constants. It represents a family of functions that satisfy the equation. A particular solution, on the other hand, is a specific instance of the general solution that satisfies given initial or boundary conditions. To find a particular solution, you solve for the arbitrary constants using the provided conditions. For example, if the general solution is y = Ae^(kx) and we know y = 5 when x = 0, we can find that A = 5, giving the particular solution y = 5e^(kx).

4. How can you analyze the stability of equilibrium points in a differential equation?

To analyze the stability of equilibrium points, follow these steps: 1) Find the equilibrium points by setting the differential equation to zero and solving for the variable. 2) Linearize the system around each equilibrium point. 3) Determine the eigenvalues of the linearized system. 4) Analyze the eigenvalues: negative real parts indicate stability, positive real parts suggest instability, and purely imaginary eigenvalues require further analysis. You can also use graphical methods like direction fields or phase portraits to visualize stability. This analysis helps predict how the system behaves near equilibrium points and its long-term behavior.

5. What is Newton's Law of Cooling and how is it modeled using differential equations?

Newton's Law of Cooling states that the rate of temperature change of an object is proportional to the difference between its temperature and the ambient temperature. It is modeled using the differential equation dT/dt = -k(T - Ta), where T is the object's temperature, Ta is the ambient temperature, k is the cooling constant, and t is time. The solution to this equation is T = Ta + (T0 - Ta)e^(-kt), where T0 is the initial temperature. This model is used in various applications, including forensic science to estimate time of death, food industry for safe cooling processes, and in HVAC system design.

Prerequisites

Understanding the foundations of mathematical modeling is crucial when delving into the world of differential equations. To effectively model real-world phenomena using these powerful mathematical tools, it's essential to grasp several key prerequisite topics that form the building blocks of this advanced concept.

One of the fundamental prerequisites is continuous growth and decay. This concept is vital in modeling various natural and scientific processes, such as population dynamics, radioactive decay, or even the spread of information. By understanding how quantities change continuously over time, students can better comprehend the underlying principles of differential equations, especially when dealing with exponential temperature decay or similar phenomena.

Equally important is the concept of rate of change. This fundamental idea is at the heart of differential equations, as it describes how quickly a quantity is changing with respect to another variable, often time. Mastering this concept allows students to interpret and analyze the rate of temperature change in thermal systems or the velocity of objects in motion, which are common applications in differential equation modeling.

While it might seem less directly related, Newton's first law of motion plays a significant role in understanding the behavior of physical systems described by differential equations. This law, which states that an object will remain at rest or in uniform motion unless acted upon by an external force, provides a foundation for modeling many physical phenomena. It helps students grasp the concept of equilibrium and how systems respond to external influences, which is crucial when setting up and solving differential equations in physics and engineering contexts.

By thoroughly understanding these prerequisite topics, students can more easily grasp the complexities of modeling with differential equations. Continuous growth and decay provides insight into exponential behaviors, rate of change forms the basis for derivatives and their applications, and Newton's laws offer a physical context for many differential equation problems.

Moreover, these prerequisites help students develop the analytical thinking and problem-solving skills necessary for tackling more advanced mathematical modeling. They provide a conceptual framework that allows for a deeper understanding of how differential equations represent and predict real-world phenomena.

In conclusion, mastering these prerequisite topics is not just about fulfilling academic requirements; it's about building a solid foundation for understanding and applying differential equations in various fields. Whether you're studying population dynamics, heat transfer, or complex physical systems, a strong grasp of these fundamental concepts will significantly enhance your ability to model and solve real-world problems using differential equations.