Answer: unbalanced, accelerates, equilibrium, no, velocity, gravitational, mass, square, inverse square, one-fourth
Explanation: A net force, aka an unbalanced force, causes an acceleration on an object. Objects in equilibrium experience no net force, therefore no acceleration, and therefore a constant velocity (zero is a constant velocity.)
Newton's law of universal gravitation: Force due to gravity = (mass one x mass two)/(distance between them)^2
This explains the 1/4th ratio