The reason we need to pedal a bicycle to move forward is due to the basic physics and mechanics involved in the operation of a bicycle.

When we pedal a bicycle, the rotational motion of the pedals is transferred to the rear wheel through the bicycle’s drivetrain, which typically consists of the chainring, chain, and rear cassette. As the rear wheel rotates, it applies a forward force to the ground through friction between the tire and the surface. This forward force, known as traction, propels the bicycle and the rider in the desired direction.

The act of pedaling generates the necessary torque and rotational speed of the rear wheel to overcome the forces that oppose the bicycle’s motion, such as air resistance, rolling resistance, and gravity (if going uphill). Without pedaling, the bicycle would simply remain stationary or gradually slow down due to these opposing forces.

The gearing system on the bicycle allows the rider to adjust the amount of force required to turn the rear wheel, providing more torque for climbing hills or accelerating, or less torque for maintaining a steady speed on flat or downhill terrain.

In summary, pedaling a bicycle is necessary to generate the forward propulsive force that overcomes the resistive forces and allows the bicycle to move forward efficiently.