Pushing the DROP button will cause to ball to fall. Pushing the CALIBRATE button will bring the time elapsed in the stopwatch closer to time elapsed on the wall for your particular computer. (It should only need to be used when you first start and again if the load on your machine changes drastically.) Clicking and or dragging on the ruler you will get red and green markers (click closer to the one you want to move).  The markers magnify the numbers on the ruler and trigger the clock to start when the ball passes the green one and to stop when the ball passes the red one.  The {Earth, Moon, Mars} choice will change the "gravity" acting on the ball to approximate gravity on the surface of the specified planet.  The ball falls as it would in a vacuum or in "free fall". It is hoped that a person using this applet would become more familiar with basic measuring of displacement, and the concept of "average velocity". Average velocity is found by determining the displacement between the green and red markers and dividing that value by the time elapsed The resulting value is the average velocity between the green and red markers in meters per second. There exists some uncertainty in the measurement of time. (Thanks to Java's random thread activity and on Suns I have noticed  ummm, time waves)

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