Final answer:
An object that is in free-fall is generally acted upon only by gravity, a concept that is true in the absence of atmospheric resistance. However, on Earth, free-falling objects experience some air resistance. The necessity of an external force to move a stationary object in space is true, as it would continue in a straight line until another force acts upon it, demonstrating Newton's first law.
Step-by-step explanation:
The statement that an object in free-fall is acted upon only by the force of gravity is generally true. However, it's important to clarify that in practical terms, on Earth, objects in free-fall are also subjected to air resistance which opposes the force of gravity, albeit to a small degree. In a theoretical situation where an object is free-falling without any atmospheric influences, such as in a vacuum, gravity would indeed be the only force acting on the object.
A related concept is that a stationary object in outer space, away from significant gravitational influences and without atmospheric friction, would require an external force to set it in motion; once moving, it would continue to travel in a straight line until another force acts upon it, according to Newton's first law of motion, often referred to as the law of inertia. The true or false question concerning whether an external force is needed to set such an object in motion is true.
As for the net external force on a free-falling object on Earth, if we neglect the effect of air (which is usually done in basic physics problems for simplicity), the force would be directed downward with a magnitude equal to mg, where m is the mass of the object and g is the acceleration due to gravity. This is represented in a free-body diagram by a single vector pointing downward from the object.