Yes, it takes some time for the static friction to establish itself.
I still have the students determine static and kinetic friction coefficients in the lab. One reason for doing it in the lab (as opposed to a demo or simulation) is so they can get a feel for the difficulty of carrying out what would seem to be a straight forward determination.
A wood block is placed on the smooth Formica surface of a level table. A string is attached to the block, then the string runs over a pulley to a weight hanger. To find the static coefficient the students add weight in small increments until the block breaks free of the static friction. Then they repeat the experiment by adding weight, then tapping the block sideways to dislodge it, and they adjust the weight until the block appears to move with constant velocity. This, of course, leads to the kinetic coefficient.
Extra masses are added to the block, to increase the normal force, and the weights to pull the block increase proportionately, and the coefficients of friction still come out the same... in theory.
Determining the kinetic coefficient is a snap. It works well, and different students get good agreement for the kinetic coefficient. The static trials are more difficult. The longer the block sits, the more force it takes to dislodge it. Even though the table surface looks uniform and clean, if the block is started in a slightly different position, the force can change. When placing the block at the starting position, if they press the block into the table momentarily, it will increase the force required to dislodge it. When they add the weights to the hanger to build up to the weight needed to dislodge the block, they need to add the weight very gently in order to get the maximum value.
I like this simple lab because it really helps them understand the difference between kinetic and static friction. I also like it because they get very frustrated with it. Some level of student frustration is good, especially when it occurs with an experiment that seems like it ought to be easy.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu