How does friction start a fire?

posted: 04/11/12
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Fire is one of the most important forces in human history.
Jeremy Hoare/Life File/Getty Images

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.

C'mon Baby, Light My Fire

The ingredients needed to start a fire are traditionally illustrated in the fire triangle. The three legs of the triangle are fuel, oxygen and heat. When enough of these three factors is present, a chemical reaction occurs that results in fire. This is why you can start a fire with two sticks: The fuel is the sticks, the oxygen comes from the air and fiction provides the necessary heat.

But this is far from the only circumstance in which friction starts a fire. Even lighting a match relies on the heat that comes from the friction of striking. And friction can feed on more than just wood -- fuel can take the form of any flammable substance. You might be surprised by what kinds of materials are flammable. For instance, metals, plastics and rubber can all be set afire under the right conditions and temperatures. Such materials are especially prone to catching on fire - and fast - when they are in the form of powdery dust.

So to paraphrase the advice your mom gave you? Don't go playing with friction.

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