Answer:
inertia
Step-by-step explanation:
The principal of inertia explains why objects do not voluntarily change their speed or direction without the influence of an external force. Inertia is not unaffected by other forces in planetary systems such as Earth and is, as such, not really observable on planetary surfaces. This is due to the resistance forces of gravity and friction from both the air and the ground. The drag slows objects down and makes it appear as if a constant force were necessary to keep them in motion.