The big question is, how is human power used to turn the wheels of the bicycle to cause it to move? Well, when riding a bike, human power is used to turn the pedals. The pedals turn the front cog (the larger one), which has teeth that are pulling a chain. The chain is what causes the rear cog to turn, which turns the back wheel of the bicycle.
The physics is involved in the pedaling itself. The lever arm is the bar that connects the pedal to the cog. When a person places his foot on the pedal, the force he puts on it is perpendicular to the lever arm. This produces torque.
