Java Tech - Chapter 2 : Physics

For the first demonstration of the Euler method, we will look at an object falling in a constant gravitational acceleration field:

As mentioned earlier, we can convert this second order equation to two first order equations by substituting the velocity variable. The Euler formula for the velocity is then

The Falling Object demo program below shows the results of these formulas for four values of dt. It also compares the results to the analytical formulas. (We provide the code for both an applet and an application.)

FallingObject_Applet1.java
(Output goes to browser's Java console.)

public class FallingObject_Applet1 extends java.applet.Applet
{

  public void init()
  {
    // Put code between this line
    //--------------------------------------------------

     double t, y, vy;
     double total_t;
     int n;

     // Constants
     double y0 = 0.0;
     double v0 = 0.0;
     double g = -9.80;// meter per sec**2

     // Version 1
     double dt = 0.1;
     int n_steps = 10;
     y = y0;
     vy = v0;

     for(n=0; n < n_steps; n++)
     {
       y = y + vy * dt;
       vy = vy + g * dt;
     }

     total_t = n * dt;

     System.out.println("Version 1, dt=0.1 ");
     System.out.println("Time = " + total_t);
     System.out.println("y = " + y + ", vy = " + vy);
     System.out.println();

     // Version 2
     dt = 0.01;
     n_steps = 100;
     y = y0;
     vy = v0;

     for( n=0; n< n_steps; n++)
     {
       y = y + vy * dt;
       vy = vy + g * dt;
     }

     total_t = n * dt;

     System.out.println("Version 2, dt=0.01 ");
     System.out.println("Time = " + total_t);
     System.out.println("y = " + y + ", vy = " + vy);
     System.out.println();

     // Version 3
     dt = 0.001;
     n_steps = 1000;
     y = y0;
     vy = v0;

     for( n=0; n < n_steps; n++)
     {
       y = y + vy * dt;
       vy = vy + g * dt;
     }

     total_t = n * dt;

     System.out.println("Version 3, dt=0.001 ");
     System.out.println("Time = " + total_t);
     System.out.println("y = " + y + ", vy = " + vy);
     System.out.println();

     // Version 4
     dt = 0.0001;
     n_steps = 10000;
     y = y0;
     vy = v0;

     for( n=0; n< n_steps; n++)
     {
       y = y + vy * dt;
       vy = vy + g * dt;
     }

     total_t = n * dt;

     System.out.println("Version 4, dt=0.0001 ");
     System.out.println("Time = " + total_t);
     System.out.println("y = " + y + ", vy = " + vy);
     System.out.println();

     vy = v0 + g;
     y = y0 + 0.5 * g;
     System.out.println("Formulas for Total time = 1.0sec");
     System.out.println("y = " + y + ", vy = " + vy);
    //--------------------------------------------------
  // and this line.
  }

  // Paint message in Applet window.
  public void paint(java.awt.Graphics g)
  { g.drawString("FallingObject_app1",10,20); }
}

Application version:
FallingObject_App1.java

Note: The long fractions in the decimal values is rather annoying but we will wait till Chapter 5: Tech to discuss how to specify the format of numbers in the print output .

We see that the y drop distance varies according to the choice of dt. The finer the increment, the closer the distance matches the analytical result.

The discrepencies between the Euler method and the analytical formula come from three sources:

Exercise: Modify the above demo program to use float types instead of double. Note that you will need to change the floating point literals to float types as well by appending 'f', e.g. g = -9.80f.

Can you see changes in the results due to the lower precision float type? Try letting it fall further, e.g. larger nSteps, to see if the discrepancies grow.