With the program Interactive Physics 2.5, it is possible to simulate Newtonian mechanics problems by creating objects on the screen and specifying the interactions on or between them. Clicking the <Run> button animates the simulation, thereby producing a "movie" of the simulation.
Simulations are defined by how objects are placed relative to each other and by how they are connected (i.e. by physical contact, by ropes or pulleys, by springs, or by pin joints). Many of the physical properties of the objects, such as mass, friction, and initial velocity, can be controlled. Gravity, air resistance, electrostatics, and external force fields can be changed in magnitude or turned off completely.
While the simulation is running, many of the values of the physical quantities of the objects, such as velocity, acceleration, momentum, angular rotation, kinetic energy, frictional force, etc., can be displayed graphically as well as numerically.
All of the simulations in this site are in the Edit Mode so that you can investigate how they are constructed. However, most of the simulations employ every trick that I know, so they are not the best place for you to start learning how to create your own simulation. Start with Creating Objects and Moving and Re-scaling Objects.
The motion of the interacting objects is calculated using one of several numerical methods. At the heart of the numerical methods is the use of a small time step Dt. It operates something like this:
If Dt is small enough, then the motion of an object during this time interval can be found by assuming that its acceleration is nearly constant. The acceleration can be found at any one moment by applying Newton's Second Law to the forces acting on the object. The position of an object can then be calculated using the kinematics equations for constant acceleration. This process is repeated over and over every "Dt" of a second to generate the motion of the object.
The size of Dt can be changed. Its value is located in the <World> menu under <Accuracy...>. The default value is 1/100 of a second.
One way to speed up a simulation is to make Dt a larger number. Setting it to 1/10 second will make the simulation run 10 times faster. The price paid is that the simulation is less accurate. I could have made many of the problem simulations run much faster, except then the resulting values would be off from the predicted values calculated in the problem. Often the simulation-values do not match exactly for that reason.
If you would like to try some of the exercises in this tutorial, start up Interactive Physics and experiment with them as you go along. If Interactive Physics is already running, open a new simulation from within the program under the <File> menu and use it try the exercises.
Note that is possible to have both this tutorial and Interactive Physics open at the same time and to switch back and forth between them. How you do the switching is machine-dependent. Holding down the <Alt> key and pressing the <Tab> key repeatedly cycles through the active programs in Windows. Clicking in the window of one program or the other will make it active on any machine.
The Tool Bar can be turned on and off by going to the <View> menu and selecting <Workspace> and then <Tool Palette>. The Tool Bar palette is off in all of the simulations in PhysicsNet.