When an object is in motion either on a surface or in a viscous medium such as air or water, there is resistance to the motion because the object interacts with its surroundings. We call such resistance a force of friction. Forces of friction are very important in our everyday lives. They allow us to walk or run and are necessary for the motion of wheeled vehicles.

Imagine that you are working in your garden and have filled a trash can with yard clippings. You then try to drag the trash can across the surface of your concrete patio, as in Figure 26. This is a real surface, not an idealized, frictionless surface. If we apply an external horizontal force F to the trash can, acting to the right, the trash can  emains stationary if F is small. The force that counteracts F and keeps the trash can from moving acts to the left and is called the force of static friction f_s. As long as the trash can is not moving, f_s = F. Thus, if F is increased, f_s also increases. Likewise, if F decreases, f_s also decreases.