Final answer:
To make a car follow a curve instead of continuing in a straight line, Newton's first law of motion, the law of inertia, is the key principle that must be put into effect. Friction provides the necessary force to change the car's direction.
Step-by-step explanation:
To cause a car traveling down a straight road to follow a curve, Newton's first law must be put into effect. This law, also known as the law of inertia, tells us that an object will continue to move in a straight line at a constant speed unless it is compelled to change by an outside force. When a car approaches a curve, there must be a force applied to cause the car to change direction and follow the curve. This force is typically provided by the friction between the car's tires and the road, which allows the car to turn rather than slide off the road.
Newton's second law of motion and Newton's third law of motion also play roles in the process of a car turning. The second law explains the relationship between the force applied, the mass of the car, and the car's acceleration, while the third law will apply to every action of the car's tires pushing against the road having an equal and opposite reaction.
However, when considering the direct cause of the change in the car's straight-line motion, it is Newton's first law which most directly applies to the situation.