Final answer:
The acceleration of the sack will be zero when air resistance equals 200 N, which is equal to its weight, because the forces are balanced.
Step-by-step explanation:
When the air resistance equals the weight of the falling sack of potatoes, which is 200 N, then the net force acting on the sack is zero. With a net force of zero, according to Newton's second law, the acceleration of the sack will also be zero. This is because the downward force of gravity is perfectly balanced by the upward force of air resistance, resulting in no change in velocity; the sack continues to fall at a constant speed, which is known as the terminal velocity.