eli5: How can friction between two sticks create fire

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Friction creates heat, you can easily test this by rubbing your hand against some cloth or a rug.

When you rub two sticks together really fast, you can actually get the sticks really hot, hot enough to start a fire. When wood reaches a high enough temperature (few hundred degrees), it ignites itself and you’ve got a fire.

Friction generates heat.

1 wood has good friction. Wood has an average coefficient of friction, meaning it’s not that slippery, most of the force you apply to slide it onto another wood piece becomes friction, therefore heat.

2 wood gets hot easily. Wood has a relatively small heat capacity meaning you need little heat to make it full of “hotness”

3 wood doesn’t lose heat. Wood has a low heat conductivity, meaning the hot part of the wood will not lose much heat toward the cold part of the stick. This keeps the heat trapped in the hot part of the wood.

4 wood wants to burn. Wood (and paper and hay) do have a very low self ignition point, around 300 degrees Celsius. Petrol self ignites at 280*C.

It’s true that petrol is dangerous because of its flammable vapors, and you can set fire to petrol at very low temperatures. But we are talking about the temperature at which it ignites by itself with no ignition sources, no sparks, no flames around.

So to recap, you have a material that needs little energy to get hot, does not lose the heat, it does generate an average heat by friction, and has a relatively low self ignition point.

You rub the wood, it gets quickly to 100*C, you se water vapor coming out, soon after it starts to get black, it’s already oxidizing. You need to add a lot more force and friction but you just need a fraction of the stick, just a narrow spot, to get to 300 and there it goes. You have ignition, it start burning and you see a red amber, this red amber is all you need because this little combustion is enough. It’s glowing red so it is past 500C, you now add some thin hay or wood fragments, they too ignite and get red and soon after you have a fire. You need to blow gently some air onto it to remove the smoke that wound starve the combustion otherwise. As long as there is oxygen and something red, it will keep ignite other wood around it.

Fire requires 3 things, fuel, oxygen, and heat. Oxygen is in the air and the stick itself it fuel, so if we add heat, we should be able to make fire.

Friction creates heat, so if we have enough friction, it should create enough heat to start a fire. Starting a fire with sticks is actually very difficult to just do on the spot, but when you do it, you usually use that heat to burn some fibers that easily catch and use that to start a fire.