Have you ever been outside and your hands get cold; so you quickly rub them together to get them warm? What you are doing is using the friction caused by two surfaces sliding over each other. The amount of friction depends on the surfaces that are in contact with one another. The more irregular the surface is, even microscopically, the more friction will be produced by that surface sliding past another. It's actually the electromagnetic forces involved with atoms clinging together, but we'll save that for a later discussion. Just know that the "rougher" the surface is, the more friction there will be in that interaction.
We use the concept of friction often in our daily lives—sometimes we want more friction (as in lighting a match or warming up your hands) and sometimes we want to reduce it (as in putting oil in our car or sledding down a hill of snow). The presence of friction effects just about all types of motion. Though we have up to this point often ignored the presence of friction, we can take it into consideration as we look at the forces involved with the motion of objects. Most of our force diagrams have had some sort of force opposing the motion of the object, and in many cases, it has been labeled as the force of friction. In this lesson, you will learn how to quantify this force and use it in problem solving.
Question
You push on a box that sits on the floor with 70 N of force to the right, but the box does not move. How much friction force opposes the motion? How do you know?
Since there is no change in the box's motion, the sum of the forces must equal zero. The friction force that opposes the motion must be equal to 70 N in the opposite direction (left).
