(a) Why do you push down harder on the pedals of a bicycle when first starting out than when moving at a constant speed? (b) Why do you need to pedal at all when the cycle is at a constant speed?

By Newton's first law of motion, every particle remains in a situation of rest or uniform motion except when constrained by an external force.

(a)

The bicycle is initially at rest. The inertia of the bicycle and the person must be overcome to start on the bicycle. Also, the static frictional force must be overcome. So, a strong force is required to overcome the inertia and frictional forces.

Therefore, a person needs to push harder on the pedals while starting on a bicycle.

(b)

The net force acting on an object moving with a constant speed is zero. Hence, the net force on the bicycle when it is moving at a constant speed is zero. However, frictional force and air resistance are present. The bicycle slows down and eventually comes to rest due to these forces if it is not pedaled.

Therefore, the bicycle needs to be pedaled even when it is moving at a constant speed.