일반물리학 1 — Force and Motion-II: Problem 33

33 A 1000 kg boat is traveling at 100 km/h when its engine is shut off. The magnitude of the frictional force \( f_k \) between boat and water is proportional to the speed \( v \) of the boat: \( f_k = 70v \), where \( v \) is in meters per second and \( f_k \) is in newtons. Find the time required for the boat to slow to 45 km/h.