When you move your mouse across its pad, the image on your monitor moves with it. Basically, the position of the mouse image on your screen corresponds to the position of your mouse on its pad.
Now, for a physics idea: instead of looking at the position of your mouse, this experiment looks at its velocity. The velocity of your mouse is the rate of change of its positionhow fast it is moving. As you move your mouse from side to side (moving it up and down won't make any change) a graph will be drawn in the window below. The graph is a graph of the velocity of your mouse. If you move it to the right, the graph will go up; if you move it to the left, the graph will go down.
I initially designed this experiment as an exercise to teach people about velocity, but I quickly noticed something: no matter how hard I tried, I couldn't use my mouse to make a nice, smooth graph. No matter how careful I was, the graph jerked around. I quickly discovered the reason: my mouse was pretty dirty inside; those little wheels were pretty darn caked up. So the wheels moved in a very jerky way, and so the mouse velocity was not steady. Now the mouse was still quite usable; the postion still tracked pretty well. But the velocity was all over the map. For folks who know a thing or two about acceleration: fuggedaboutit. This was even worse.
Use the little program below to try this out for yourself. Click "Start", and move your mouse steadily to one side or the other. How does the graph change? When you are tired of this game, click "Stop."
What do you observe? How good is your mouse? Can you keep a nice, steady velocity? Does the graph move as you expect when you change the speed? We are going to do some more with this whole concept at some point - so your feedback is quite welcome! What sort of exercises would you like to see?