Which principle explains why a body in motion will continue in the same direction at a constant speed?

The principle that explains why a body in motion will continue in the same direction at a constant speed is inertia. Inertia is a fundamental concept from Newton's First Law of Motion, which states that an object will remain at rest or move in a straight line at constant speed unless acted upon by a net external force. This means that if no forces (or a balanced set of forces) are influencing the object, it will maintain its state of motion.

Inertia specifically refers to the tendency of an object to resist changes in its state of motion. A larger mass indicates greater inertia, which means a heavier object will require a larger force to alter its motion compared to a lighter object. This principle is crucial for understanding motion in physics and helps explain phenomena such as why passengers in a car lurch forward when the car suddenly stops—it's the inertia of their bodies that wants to continue moving forward.

Gravity, friction, and pressure do not account for the inertia of an object. While gravity can influence motion, friction acts to oppose motion, and pressure relates to force per area but does not directly explain constant motion in the absence of external forces. Thus, inertia is the appropriate concept to understand the behavior of a body in motion in the absence of unbalanced forces