Final answer:
The frictional force is responsible for changing the momentum of a car moving along a horizontal road. It acts between the tires and the road surface, enabling acceleration, deceleration, or direction changes.
Step-by-step explanation:
The external force responsible for changing the momentum of a car moving along a horizontal road is most commonly the frictional force. This force acts between the tires of the car and the surface of the road, allowing the car to accelerate, decelerate, or change direction. When analyzing such scenarios in physics, we often consider friction as the key external force, especially when the motion involved is horizontal and on a level surface.
According to Newton's First Law of Motion, an object in motion remains in uniform motion unless acted upon by a net external force. Friction is an external force that can change the momentum of a car by opposing its motion or enabling it to grip the road when turning. Gravitational force, on the other hand, acts vertically and does not directly affect horizontal momentum unless the car is on an inclined plane.
In the case of a car accelerating, the frictional force is greater than the resistance forces, leading to a net force that causes acceleration. However, in the case of a car moving at a constant velocity, the net horizontal force is zero, meaning the frictional force is balanced with other forces such as air resistance.