Static versus Sliding Friction

Many of us never take the time to consider the physical laws that affect our lives. A case in point involves the difference between static versus sliding friction, which certainly becomes a factor any time that we enter a car. We all know that jamming the brakes is not the best way to stop. We also know that spinning the wheels is not the best way to extricate ourselves from a snow bank. Nevertheless, we all do these things, practically as a matter of instinct.

If we were to tie a string to a block of wood resting on a table, we would probably have to pull on the string with a force roughly half the weight of the block to get it to move. In the vernacular, this means that the coefficient of static friction between the block and the table is about 0.5. However, once the block is moving, we would not have to exert as much force to keep it in motion--probably only about one-fifth of the block's weight. This is equivalent to saying that the coefficient of sliding friction between the block and the table is only about 0.2. In other words, once something has begun to slide, the friction between the two surfaces is not nearly so great as it was when it was at rest.

This relates to driving a car in a very meaningful way. The simple lesson is that on slippery roads it is important to maintain gentle braking pressure so as not to slide and break the grip on the road. And when trying to escape from a ditch in which you are stuck, do not spin the wheels. You're only damaging your cause. In fact, in the latter case, spinning the wheels adds a second factor to even further complicate matters--the heat produced by the spinning tire can further melt the snow or ice on which it rests, making the footing even more slippery.