Friction
The model assumes that energy is lost only during the floor collision.  Modify the differential equation to include energy loss due to air friction.  This energy loss can be modeled by including a friction (fluid drag) force Ffriction that is proportional to the velocity:
Ffriction = - b v .
Because drag is a vector force it will affect the ball's motion in both the x and y directions.  Compare the effect of friction in this activity with the one-dimensional Free Fall activity.
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How i implement the model is

1. add variable k
2. add variable mass
3. dvy/dt = -g - k/mass*vy
4. dvx/dt = -  k/mass*vx[/color][/b]

[b][color=red]source code
download the *.jar for using the applet on standalone without internet connection.[/color][/b]

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