Fire is actually a chemical reaction. It's an oxidation reaction to be specific. When wood gets hot enough (the part of the wood that is burning) the large hydrocarbons break down to charred solids and a gas. The gas is what reacts with oxygen in the atmosphere to produced light, CO2 and H2O.
Lord Kelvin, were he alive today, would be considered a Thermochemist. Thermochemistry is interested in the role of heat in chemical reactions. This includes the role of heat both as a biproduct of chemical reactions and a facilitator.
Kelvin's description of absolute zero is an important concept in thermochemistry. At absolute zero, there is no movement of molecules, and no energy available facilitate chemical reactions.
The formula of mechanical advantage in this situation is:
MA = Input Force ÷ Output Force
The input force is the 30N applied to a screwdriver while the output force is the 75N force to the lid.
So,
MA = 30N/75N
MA = 0.40
Hence the mechanical advantage of the screwdriver is 0.40.
