Choose a flat surface like a table. Take a normal piece of printer or lined paper. Holding one end of the paper, pull it across the surface of the table. There is some friction, but it is very little. Here is your puzzle: Without changing the table in any way, nor changing the paper in any way, how can you increase the friction between the two when you pull the paper across the table?

Asheet of steel 1.5 mm thick has nitrogen atmospheres on both sides at 1200oc and is permitted to achieve a steady-state diffusion condition. the diffusion coefficient for nitrogen in steel at this temperature is 6 x 10- 11 m2 /s, and the diffusion flux is found to be 1.2 x 10-7 kg/m2 -s. also, it is known that the concentration of nitrogen in the steel at the high-pressure surface is 4 kg/m3 . how far into the sheet from this high-pressure side will the concentration be 2.0 kg/m3 ? assume a linear concentration profile.