A favorite physics demonstration at the University of Texas at Austin is a giant skateboard about 6 feet long, with about the same mass as a physics professor. Suppose the skateboard rolls with negligible friction on the level classroom floor. The professor is standing at rest on the skateboard, of length L, and the end of the board opposite to the professor is a distance d from the wall. d L If the professor and board have the same mass, and if the professor slowly walks towards the wall, how far is he from the wall when he stops at the opposite end of the skateboard from where he started? (Note his initial distance from the wall is d + L.)