How friction produces heat?
Answers
Fire is one of the most important forces in human history.
You're probably familiar with friction, the force that resists motion between two surfaces. In everyday life we tend to think of friction as a bad thing. After all, it's what makes it so hard to drag a couch across a room. Even outside the realm of physics, the word has a negative connotation (in terms of interpersonal relationships, it means "disagreement").
However, if you're ever lost in the wilderness and in need of fire for survival, you'll thank your lucky stars for friction. You may have heard that rubbing two sticks together can start a fire, but we're going to investigate the physics of how that really happens.
Laws of Attraction
To understand friction, we have to zoom into the microscopic level of what's going on between surfaces. No surface -- not even slippery ice -- is perfectly smooth. All surfaces have irregularities on the molecular level, and when two touching surfaces move relative to each other, they get caught in each other's little hills and valleys. Not only that, but the molecules from one surface actually start attracting molecules from the other surface, and they form chemical bonds with each other. Breaking these bonds and pushing past the hills and valleys takes considerable work.
Warming Up
When you push a couch across the floor, much of your energy is wasted pushing the surface of the couch along the surface of the floor, but that energy doesn't just disappear. The law of conservation of energy states that it can't be created or destroyed. Friction converts useful kinetic energy (also called ordered energy) into thermal energy (disordered energy), or heat.
Friction gets a bad rap for turning useful energy into useless energy. It's true that a lot of kinetic energy in the world is lost into the atmosphere as heat. But heat isn't always useless. For instance, whenever our hands get cold, we can rub them together and feel the heat from the friction almost immediately. But more importantly, heat is one of the three pillars required for starting a fire.