Final answer:
A car moving at a constant speed in a straight line has multiple forces acting on it, but the total net force is zero. the opposing forces such as friction and air resistance balance the driving force which maintains the car's constant velocity.
Step-by-step explanation:
The question relates to the forces that may be acting on a car moving with a constant speed in a straight line. According to Newton's first law of motion, an object will continue to move at a constant velocity unless acted upon by a net external force. Therefore, if a car is moving at a constant speed, there can be multiple forces acting on it, but the net force must be zero. This means that the forward driving force provided by the car's engine is exactly balanced by the opposing forces, such as friction and air resistance (drag).
When a car accelerates, the driving force exceeds the opposing forces, resulting in a nonzero net force. However, for a car maintaining a constant velocity, the driving and opposing forces are equal in magnitude and opposite in direction, leading to no change in speed. Thus, while individual forces can indeed act on the car, they do not alter its velocity because of the balance between them. The car continues in a state of uniform motion, which implies not only constant speed but also no change in direction.