Frictional force
Explanation
According to Newton's First Law of Motion (Law of Inertia), an object in motion will continue in its state of uniform motion along a straight line unless acted upon by an external unbalanced force. The deceleration of the bicycle indicates the presence of such an external force acting opposite to the direction of motion.Detailed Analysis:
- Gravitational force is incorrect: Gravitational force acts vertically downwards. On a level road, this force is perpendicular to the direction of motion and is balanced by the normal reaction force from the ground. It does not directly cause horizontal deceleration.
- Magnetic force is incorrect: Magnetic force arises due to the interaction of magnetic fields or moving charges. It is not relevant to the mechanical interaction between a bicycle and a road.
- Frictional force is correct: Frictional force is the resistive force that opposes the relative motion between surfaces in contact. As the bicycle moves, rolling friction acts between the tires and the road, and air resistance (drag) acts on the rider and the frame. These forces act in the direction opposite to the velocity, doing negative work and reducing the kinetic energy of the bicycle, causing it to stop.
- Electrostatic force is incorrect: Electrostatic force acts between static electric charges. It plays no significant role in the dynamics of a moving bicycle.
Key Takeaway: Friction is a contact force that always opposes relative motion. Without a continuous driving force (pedaling) to overcome friction, any object moving on a rough surface will eventually come to rest.