Here is another version with two legs.
You can drag blue dots with mouse drag and drop. Try it!

translate strings in simulation to different language format before download
Full screen applet or Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list
Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop. This work is licensed under a Creative Commons Attribution 2.5 Taiwan License
Download EJS jar file(1035.8kB):double click downloaded file to run it. (3 times by 3 users) , Download EJS source View EJS source