A number of things determine the strength of static friction. Some friction takes place on the electromagnetic level, in the form of charged particles which are attracted to or repelled by each other. The texture of the objects also plays a role. For example, a silk sheet has a very smooth texture which creates less resistance, while a dry asphalt road is coarse, generating more resistance.
Many people are familiar with static friction, since they interact with it on an almost daily basis. For example, when someone slides a book across a table, static friction is at work. Initially, a small level of force needs to be exerted to get the book to move. Once it is moving, however, kinetic friction comes into play, and less effort will be required to move the book. The amount of force required can vary. For example, if a book has a library cover on it and it got damp, the wet book will require more force to move, while a brand new paperback book might slide very easily across a dry wooden table with a varnished surface.
Sometimes, static friction may be such a negligible force that people don't even realize that it is present. In other cases, it can be quite significant. For example, a car has a tremendous amount of static friction which makes it very difficult to push a stopped car into motion. Some friction is actually specifically desired with cars, because it allows the driver more control; when a car loses friction, it can slide out of control, as seen on icy roads in the winter.
A number of formulas can be used to determine the coefficient of friction of various objects. Calculating friction can get complicated, because it is subject to many variables, including gravity, which can make it difficult to plug values into an equation and quickly generate a coefficient. Coefficients of static friction have been calculated for some well-known objects for convenience: for example, two pieces of Teflon®, a very slippery substance, have a coefficient of friction of .04.
