Final answer:
While riding a bike, gravity applies a downward force and the ground provides an upward normal force that are balanced. During a turn, forces must align through the center of gravity, and while accelerating, an unbalanced forward force is exerted.
Step-by-step explanation:
When riding a bike, there are several forces acting in different directions. The force of gravity acts downward on the bike and rider, while the ground exerts an upward normal force that balances the weight of the system. While riding on a flat surface at constant velocity, these forces are generally balanced, maintaining vertical stability. However, when turning, riders must lean to maintain balance and ensure the force from the ground is aligned through the center of gravity. Additionally, there is a frictional force parallel to the road that provides the necessary centripetal force for turning. When accelerating forward, the force exerted by the rider through pedaling is unbalanced, resulting in forward motion of the bike. Conversely, if the bike is moving at a constant velocity, the net horizontal force is zero because the frictional force is balanced by the forward force exerted by the rider.
The correct answer to the question is: C) Riding a Bike: Forces acting upward and downward, balanced. However, it's essential to note that additional unbalanced forces may come into play when accelerating or decelerating.