What are the properties of friction?

Force and Newton’s Laws of Motion Read more from
Chapter Motion and Its Causes

Your toy car or ball almost certainly slowed down, even when you didn’t tap it. Do moving things, by their nature, slow down, or is there some force causing them to slow down? You might say friction slows things down. Perhaps you think that friction is just there all the time, or is something that has no direction. But Newton would say that it must be a force that acts in the direction opposite motion.

If you push a couch across a room, it will feel like someone is on the other end pushing back. If you pull on the couch, it will feel like someone on the other end is also pulling. If the couch is heavier, the opposing force will be larger. The amount of force opposing you will depend on the surface the couch is on. A carpet will have a stronger opposing force than a smooth surface, like wood or tile. These observations summarize the properties of friction between two objects that are in contact. Friction is always in the direction opposite the motion. Friction is greater if the moving object is heavier. Friction is greater if the surface is rougher. But, friction does not increase if the speed of motion increases.


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