Propane, a colourless, easily liquefied, gaseous hydrocarbon (compound of carbon and hydrogen), the third member of the paraffin series following methane and ethane. The chemical formula for propane is C3H8. It is separated in large quantities from natural gas, light crude oil, and oil-refinery gases and is commercially available as liquefied propane or as a major constituent of liquefied petroleum gas (LPG).
As with ethane and other paraffin hydrocarbons, propane is an important raw material for the ethylene petrochemical industry. The decomposition of propane in hot tubes to form ethylene also yields another important product, propylene. From propylene such organic chemicals as acetone and propylene glycol are derived. The oxidation of propane to such compounds of carbon, hydrogen, and oxygen as acetaldehyde is also of commercial interest.
Although a gas at ordinary atmospheric pressure, propane has a boiling point of -42.1° C (−43.8° F) and thus is readily liquefied under elevated pressures. It therefore is transported and handled as a liquid in cylinders and tanks. In this form, alone or mixed with liquid butane, it has great importance as a fuel for domestic and industrial uses and for internal-combustion engines.
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Both of these work together to form a simple machine. You can't have one without the other.
If you try to turn just the axle itself, then you'll find it takes a lot of work. This is because the inertia of the axle wants to keep the object at rest. Also, you won't have a lot of torque due to the small radius compared to what a doorknob can offer.
Using a doorknob is like putting a (steering) wheel on an axle. This increases the radius and therefore increases the torque. You put in less work into the system and get more out of it.
In a movable pulley system, where does the other effort come from? is carried by this end of the rope. This means that the structure (ceiling) does half the effort.
