Final answer:
When the air resistance on a falling sack equals its weight, the acceleration of the sack is zero due to balanced forces resulting in no net force.
Step-by-step explanation:
When the air resistance equals the weight of the falling sack (10 pounds in this case), the acceleration of the sack will be zero. This is because the forces on the sack are balanced (the force of gravity downwards is equal to the force of air resistance upwards), resulting in no net force on the object. Without a net force, according to Newton's second law of motion, an object will not accelerate.