a car needs to generate 75.0 hp in order to maintain a constant velocity of 27.3 m/s on a flat road

The car, as well as the driver, is actually accelerating to the right. The force to the left sensed by car passengers is an inertial force having no physical origin (it is due purely to the inertia of the passenger, not to some physical cause such as tension, friction, or gravitation). The car is a noninertial frame of reference because it is accelerated to the side. In such a frame of reference, Newton’s laws of motion take the form given in Newton’s Laws of Motion. The physicist might make this choice because Earth is nearly an inertial frame of reference, in which all forces have an identifiable physical origin. Passengers instinctively use the car as a frame of reference, whereas a physicist might use Earth. We can reconcile these points of view by examining the frames of reference used. Only the normal force has a horizontal component, so this must equal the centripetal force, that is,

Because this is the crucial force and it is horizontal, we use a coordinate system with vertical and horizontal axes.

A car needs to generate 75.0 hp in order to maintain a constant velocity of 27.3 m/s on a flat road