What property causes a body in motion to remain in motion?

The property that causes a body in motion to remain in motion is inertia. Inertia is a fundamental concept in physics, defined as the tendency of an object to resist changes in its state of motion. This principle was famously articulated by Sir Isaac Newton in his first law of motion, which states that an object will remain at rest or continue to move in a straight line at constant speed unless acted upon by a net external force.

This means that if no external forces, such as friction or gravity, are acting on it, a moving object will maintain its motion indefinitely. This characteristic of inertia explains why spacecraft can travel vast distances in the vacuum of space without the need for continuous propulsion, as there is minimal resistance to overcome.

While momentum is related to the motion of an object (as it is the product of mass and velocity), it is inertia specifically that describes the property that enables an object to maintain its motion in the absence of external forces. Friction acts as a force that tends to slow down or stop motion, and gravity, while it influences motion, does not account for the tendency of an object to keep moving.