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The overcoming of friction releases energy. That energy is heat. Heat burns the rubber. Tires are black because they contain carbon black. Natural rubber is actually whitish. Also, the temperatures created do not get hot enough to completely burn the rubber. That is why the smoke is white, but the marks are black. Look at a stick that has been charred in a fire. It's smoke is white, but it is blackened. Burn it in a hot enough fire, and it turns to white ash.
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