# Preface

This tutorial was made solely for the purpose of education and it was designed for students taking Applied Math 0340. It is primarily for students who have some experience using Mathematica. If you have never used Mathematica before and would like to learn more of the basics for this computer algebra system, it is strongly recommended looking at the APMA 0330 tutorial. As a friendly reminder, don't forget to clear variables in use and/or the kernel. The Mathematica commands in this tutorial are all written in bold black font, while Mathematica output is in regular fonts.

Finally, you can copy and paste all commands into your Mathematica notebook, change the parameters, and run them because the tutorial is under the terms of the GNU General Public License (GPL). You, as the user, are free to use the scripts for your needs to learn the Mathematica program, and have the right to distribute this tutorial and refer to this tutorial as long as this tutorial is accredited appropriately. The tutorial accompanies the textbook Applied Differential Equations. The Primary Course by Vladimir Dobrushkin, CRC Press, 2015; http://www.crcpress.com/product/isbn/9781439851043

Introduction to Linear Algebra

# Euler Methods

A common example of a physics problem that requires the solution of a differential equation is the motion of a particle acted on by a force. For simplicity, we first discuss one-dimensional motion so that only a single vector component of position, velocity, and acceleration are needed. These variables are related using the language of differential calculus

$\frac{{\text d}y(t)}{{\text d}t} = v(t) , \qquad \frac{{\text d}v(t)}{{\text d}t} = a(t) ,$
where y is the displacement of a particle, v is its velocity, and a is the acceleration. These quantities are known as kinematical quantities, because they describe the motion without regard to its cause.

Why do we need the concept of acceleration in kinematics? The answer can be found only aposteriori. Thanks to Newton, we know that the net force acting on a particle determines its acceleration. Newton’s second law of motion tells us that

$F(y, v, t) = m\, a(t) .$
where F is the net force and m is the inertial mass. In general, the force depends on position, velocity, and time. Note that Newton’s law implies that the motion of a particle does not depend on $$\ddot{v} = {\text d}^2 v / {\text d}t^2$$ or on any higher derivative of the velocity. It is a property of nature, not of mathematics, that we can find simple explanations for motion.

The two ﬁrst-order equations can be combined to obtain the familiar second- order diﬀerential equation for the position:

$m\,\frac{{\text d}^2 y}{{\text d}t^2} = F(t,y,\dot{y}) .$
However, we will find it easier to describe the motion of a particle by two coupled first-order differential equations $$\dot{y} =v, \ \dot{v} = a .$$

Because many types of systems can be modeled by the above system of differential equations such, it is important to know how to solve such equations. In general, analytical solutions of differential equations, that is, solutions in terms of well-known functions, do not exist. We are therefore motivated to find numerical solutions of differential equations. However, analytical solutions are very important and often exist in special or limiting cases. We often use them to test our numerical solutions.

The power of mathematics when applied to physics comes in part from the fact that frequently seemingly unrelated problems can have the same mathematical formulation. Hence, if we can solve one problem, we can solve other problems that might appear to be unrelated.

We start demonstrating the Euler methods to Newton’s equations of motion. We write Newton’s second as a system of coupled first-order differential equations and apply the Euler algorithm to each variable. The variables of interest are

$\begin{split} t_{k+1} &= t_k + \Delta t , \\ y_{k+1} &= y_k + v_k \Delta t , \\ v_{k+1} &= v_k + a_k \Delta t . \end{split}$

In this section, we extend Euler methods to systems of differential equations. It is a custom to denote by t the independent variable that we associate with time. Then denoting the derivatives by dots, we illustrate the concepts by considering the initial value problem

$\begin{cases} \dot{x} &= f(t, x, y) , \\ \dot{y} &= g(t,x,y) , \end{cases} \qquad\mbox{subject to }\qquad \begin{cases} x(t_0 ) = x_0 , \\ y(t_0 ) = y_0 . \end{cases}$
Here f and g are given functions and t0, x0, and y0 are specified points. Using vectors, we can rewrite the initial value problem in compact form:
$\dot{\bf x} = {\bf f} (t,{\bf x}) , \qquad {\bf x} (t_0 ) = {\bf x}_0 ,$
where
${\bf x}(t) = \begin{bmatrix} x(t) \\ y(t) \end{bmatrix} , \qquad {\bf f}(t, {\bf x}) = \begin{bmatrix} f(t,x,y) \\ g(t,x,y) \end{bmatrix} , \qquad {\bf x}_0 = \begin{bmatrix} x_0 \\ y_0 \end{bmatrix} .$
A solution to the above problem is a pair of differentiable functions x(t) and y(t) with the property that when x(t) and y(t) are substituted in f(t,x,y) and g(t,x,y), the result is equal to the derivative $$\dot{x}$$ and $$\dot{y} ,$$ respectively.

A numerical solution to the given system over the interval $$a \le t \le b$$ is found by considering the differentials

${\text d}x = f(t,x,y)\,{\text d}t \qquad\mbox{and}\qquad {\text d}y = g(t,x,y)\,{\text d}t$
Euler's method for solving the system is obtained by substituting approximations to the differential dt = tk+1 - tk,    dx = xk+1 - xk,   , and   dy = yk+1 - yk into definition of the differential to get
$\begin{split} x_{k+1} - x_k &= f(t_k , x_k , y_k ) , \\ y_{k+1} - y_k &= g(t_k , x_k , y_k ) . \end{split}$
The interval is divided into m subintervals of width $$h = (b-a)/m ,$$ and the mesh points become $$t_{k+1} = t_k +h .$$ This is used to get the recursive formulas for Euler's rule:
$\begin{split} t_{k+1} &= t_k +h , \\ x_{k+1} &= x_k + h \,f(t_k , x_k , y_k ) , \\ y_{k+1} &= y_k + h \,g(t_k , x_k , y_k ) , \qquad \mbox{for}\quad k=0,1,2,\ldots , m-1. \end{split}$

Euler method for systems of differential equations

euler[F_, a_, Y0_, b_, Steps_] :=
Module[{t, Y, h = (b - a)/Steps//N, i},
t[0] = a; Y[0] = Y0;
Do[
t[i] = a + i h;
Y[i] = Y[i - 1] + h F[t[i - 1], Y[i - 1]],
{i, 1, n}
];
Table[{t[i], Y[i]}, {i, 0, n}]
]

Using for simplicity the initial point to be the origin. we present another way to implement the Euler rule:

h = .1;                (* step size *)
TimeMax = 3.5;
maxN = TimeMax/h;
plotmax = 5;
plotmin = -5;
fx[x_, y_] = y + x
Out[8]= x+y
fy[x_, y_] = -2 x + 3 y
Out[9]= -2 x + 3 y
xxo = -1.5;
yyo = -0.5;
xx[0] := xxo
yy[0] := yyo
xx[n_] := xx[n] = xx[n - 1] + h*fx[xx[n - 1], yy[n - 1]]
yy[n_] := yy[n] = yy[n - 1] + h*fy[xx[n - 1], yy[n - 1]]
MatrixForm[Table[{h*n, xx[n], yy[n]}, {n, 0, maxN}]]
Out[15]//MatrixForm=

We can make several different types of plots. Here we plot on the phase plane using dots and then using line segments

eulerdotplot =
ListPlot[Table[{xx[n], yy[n]}, {n, 0, maxN}],
PlotStyle -> {PointSize[Large], Red}]
eulerlineplot =
ListLinePlot[Table[{xx[n], yy[n]}, {n, 0, maxN}],
PlotStyle -> {Thick, Green}]

Here we create the associated streamplot

streamplot =
StreamPlot[{fx[x, y], fy[x, y]}, {x, plotmin, plotmax}, {y, plotmin,
plotmax}]

We plot x-component versus t

xPlot = ListLinePlot[Table[{h*n, xx[n]}, {n, 0, maxN}],
PlotStyle -> {Thick, Blue}]

We plot y-component versus t

yPlot = ListLinePlot[Table[{h*n, yy[n]}, {n, 0, maxN}],
PlotStyle -> {Thick, Green}]

Finally, we mash them all together

Show[streamplot, eulerlineplot]

We know from the previous section that Euler's rule is not recommended for practical usage. Recalling the backward Euler method, we arrive at the Euler--Cromer algorithm:

$\begin{split} t_{k+1} &= t_k + \Delta t , \\ v_{k+1} &= v_k + a_k \Delta t , \\ y_{k+1} &= y_k + v_{k+1} \Delta t . \end{split}$
It might occur to you that it would be better to compute the velocity at the middle of the interval rather than at the beginning or at the end of the interval. The Euler--Richardson algorithm is based on this idea. This algorithm is particularly useful for velocity-dependent forces, but does as well as other simple algorithms for forces that do not depend on the velocity. The algorithm consists of using the Euler algorithm to find the intermediate position ymid and velocity vmid at a time tmid = t +∆t/2. Then we compute the force, F(tmid, ymid, vmid) and the acceleration amid at tmid. The new position yk+1 and velocity vk+1 at time tk+1 is found using vmid and amid. We summarize the Euler--Richardson algorithm as follows:
$\begin{split} a_k &= F(t_k , y_k , v_k )/m , \\ v_{\small mid} &= v_k + \frac{1}[2}\, a_k \Delta t , \\ y_{\small mid} &= y_k + \frac{1}[2}\, v_k \Delta t , \\ y_{\small mid} &= y_k + \frac{1}[2}\, v_k \Delta t , \\ a_{\small mid} &= F \left( t + \frac{1}[2}\, \Delta t ,y_k, v_k \right) \\ \end{split}$
and
$\begin{split} t_{k+1} &= t_k + \Delta t , \\ v_{k+1} &= v_k + a_{\small mid} \Delta t , \\ y_{k+1} &= y_k + v_{\small mid} \Delta t \qquad\mbox{Euler--Richardson algorithm}. \end{split}$
Although we need to do twice as many computations per time step, the Euler--Richardson algorithm is much faster than the Euler algorithm because we can make the time step larger and still obtain better accuracy than with either the Euler or Euler--Cromer algorithms.

Example: Consider the initial https://www.researchgate.net/publication/237621801_Motion_in_One_Dimension ■

Example: Consider the initial ■

Stanley, R.W., Numerical metods in mechnaics, American Journal of Physics, 1984, Vol. 52, pp. 499--507.
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