11. How great is the air resistance that acts on a 126 N sack that falls at constant velocity?

126 N

Explanation:

Without air resistance, the sack would have a downward acceleration of 9.8 m/s².

But since, the sack is falling at a constant velocity, the air resistance must counteract the gravitational force so that the net force is zero, which would be 126 N.

t = 1/400 h   ⇔ 0.0025 h ⇔ 9s ⇔ 0.15 m

any of the answers are correct.