Final answer:
If a constant force is applied to an object and then stops on a zero-friction surface, the object's acceleration becomes zero. A nonzero constant force results in a constant acceleration and linearly increasing velocity. In free-fall without air resistance, the constant acceleration is approximately 9.80 m/s² due to gravity.
Step-by-step explanation:
When a constant force is applied to an object and then the force stops, the object's acceleration becomes zero (Option a). It will continue to move at a constant speed in the same direction due to Newton's first law of inertia, assuming there is zero friction as stated. The object's speed remains constant, not zero, and it does not accelerate further or in the opposite direction.
Considering a parachutist, if the gravitational force is 539 N and, after 2 seconds, the air resistance is 615 N, we cannot determine the exact speed without additional information such as mass or initial velocity. However, with a net upward force, the parachutist will start decelerating.
If a nonzero constant force is applied to an object, the object will have a constant acceleration, and its velocity will increase linearly over time in the direction of the force, according to Newton's second law of motion.
On Earth, objects in free-fall without air resistance experience a constant acceleration due to gravity, which is approximately 9.80 m/s² (g).