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Our gravitational acceleration experiment simulation will follow at Model-View-Controls (MVC) design pattern. (A "design pattern" refers to a common generic type of structure for a program or program function.) Here the Model refers to the physics calculations that will generate the simulation behavior. The View refers to the display of the simulation graphics, the simulated detector results, and the buttons and parameter entry fields. The Controls refer to the code that provides the response to the buttons and entry fields, i.e. actually runs the program. (The Swing GUI system follows a MVC design.)

We will build an applet (which will include a main() method so it can also run as an application) to hold the elements of the simulation. It will also provide the control code for the program. We could have spun the controls into an separate class but for convenience here we will include it in the top level applet class.

  • Model:
    • DropModel instance generates the simulated data - position and velocity - of the falling mass
  • View:
    • DropTestApplet provides the JApplet subclass that holds the following graphic components:
      • DropPanel - this subclass of PlotPanel will display the animation of the falling mass.
      • HistPanel - displays the results of the experiment.
      • JButton components to start to dropping of the object, reset the histogram and other variables, and to exit the program
      • JTextField components to allow user input of the number of drops to do at one time and the animation rate factor.
  • Controls
    • DropTestApplet controls the program:
      • Builds the graphical interface, creates an instance of DropModel, initializes the setup.
      • It implements ActionListener interface and receives the action events from the buttons.
      • It creates timer objects that provided the periodic signals to do the incremental drop calculations with the model and then display the animation frame for the ball drop.

Figure Phy.9.1 shows the user interface for the simulation. Consists of 3 parts:
the controls, the display of the drop tower with falling ball, and the histogram.

The user interface will show a ball dropping after the user hits the "Drop" button. The histogram will record the time it takes to pass between two points along the vertical track. The following graphic shows the process the program follows after the user clicks on the drop button.

Figure Phy.9.2 When the drop button is clicked on the DropTest_JApplet11 progam creates a Timer instance that fires every dt=20ms. The run() method in the TimerTask subclass first invokes the step() method in DropModel to obtain the new y position for the ball. Next the DropPanel updates the position of the ball and redraws itself. The cycle repeats until the ball crosses a given finish line at the bottom. The cycle then repeats for a the next drop.

As one starts to code a physics simulation, various details start to emerge that were not necessarily apparent at the start (See also the Chapter 1: Physics : Learn by Coding discussion.). For example,:

  • Scaling, e.g. how many cm per pixel?

  • Time scale, e.g. will the animation frame rate reflect a true physical time or can it be speeded up or slowed down? Does the speed up affect the calculations or does it only affect the animation display?

  • Care must be taken in dealing with units, e.g. floating point values are required for many of the calculations of velocity, position and these may need transforming to integer pixel values. Are round-off errors significant?

Note that one of the primary benefits of a simulation is that an animation can show if the performance is realistic, thus giving a check on the calculations. In fact, that may be the reason for creating the simulation in the first place, especially if you are working on a theoretical description of a phenomena. A particular advantage of using Java for simulations is that, as we have seen, it has strong graphics and animation capabilities.

Issues with regard to the program's user interface involve:

  • We tried to follow a flexible, modular design that will allow addition of a detector section and other enhancements at later stages.
  • Controls:
    • The drop button initiates the drop and then changes to a stop button to allow the user the option of ending the drop(s) sequence.
    • Provides entry fields to set the number of drops and a speed up factor for the animation.

  • After using the program you might want to modify it to allow for more options:
    • You must decide how many options to allow and how many settings to hard code?
    • You must decide whether to use menus, lists, or other UI components to allow the user to modify settings?

Most recent update: Oct. 25, 2005

Histogram I/O
Hist I/O - Get/Set
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Hist I/O - Objects
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Filtering Data
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Physics Model
Simulation Design
Physics Simulator
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Experiment Design
Experiment Sim.
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Expt. + Analysis
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