Friction arises when two surfaces are in contact with each other and are pushed in a way that makes the surfaces want to slide relative to each other. Note they don't have to be sliding. Air resistance is also a form of friction.
This concept trailer from OpenStax talks about drag.
Read the BoxSand Introduction and watch the pre-lecture videos before doing the pre-lecture homework or attending class. If you have time, or would like more preparation, please read the OpenStax textbook and/or try the fundamental examples provided below.
Friction is a force that arises between two objects that have surfaces in contact with one another. Friction opposes the relative motion between two objects, if they are slipping, and opposes the direction they would slip if the friction was not present, for those that are not slipping. Friction comes in two types, static friction and kinetic friction. Kinetic friction acts when the surfaces of the objects are slipping. It is equal in magnitude to the normal force at the surface times the coefficient of kinetic friction. Static friction acts when there is no relative motion between the objects. The force of static friction is as large as it needs to be to prevent the objects from slipping. The maximum force of static friction is equal to the normal force at the surface times the coefficient of static friction.
This is an image of a wooden box on top of a table with some friction. There is an arrow to the right that shows the direction of motion or attempted motion and arrow to the right that shows friction opposing the motion. A zoomed view between the crate and the table shows that there are rough edges rubbing against each other which is where the frictional force between two objects derive from.
Now, take a look at the pre-lecture reading and videos below.
