Final answer:
The forward force on a bicycle needed to accelerate as one starts pedaling is due to friction between the bicycle's tires and the road surface, which provides traction and propels the bicycle forward.
Step-by-step explanation:
As you begin pedaling your bicycle and it accelerates forward, the forward force exerted on the bicycle needed to accelerate comes from friction. Specifically, this friction acts between the tires of the bicycle and the road's surface. When you pedal, you are applying force to turn the gears, which in turn rotates the chain and subsequently the rear wheel. However, it is the frictional force at the point where the rubber of the tires meets the road that actually propels the bicycle forward. Without this friction, the tires could spin freely without gaining traction and the bicycle would not move forward. Additionally, this friction helps with stability and control, especially when making turns, as it provides the necessary centripetal force to keep the bicycle on a curved path.