Why does the force that opposes motion is friction?

Answer 1

Because gravity drags something down, and then friction slows something down because the thing traveling is hitting into other particles. It is lower in air because air particles are more spaced out, and therefore not as many to hit. On a solid, it is much higher due to the higher consentration of particles. In liquids, because they are a mix between a solid and gas, it can effect it less than a gas, or more than a solid.

Answer 2

Friction is the force that opposes motion because it results from the interactions between surfaces in contact. When two surfaces slide against each other, microscopic irregularities cause resistance, generating heat and slowing down the movement. This resistance is what we perceive as friction, impeding smooth motion.