# How Lighthouses Work 5, Exercise 3: weight-driven clockwork
G = 9.81

def weight_kg(power_w, hours, drop_m):
    return power_w * hours * 3600 / (G * drop_m)

def hours_run(mass_kg, drop_m, power_w):
    return mass_kg * G * drop_m / (power_w * 3600)

POWER_W = 5.0       # assumed, mercury-floated lens
DRY = 4.0           # assumed factor for a dry bearing
# Part 1: 8 m drop, winding every 2 h and every 4 h
for hours in (2, 4):
    print(hours, "h:", round(weight_kg(POWER_W, hours, 8)), "kg floated,", round(weight_kg(POWER_W * DRY, hours, 8)), "kg dry")
# Part 2: how long a 300 kg weight with a 10 m drop runs
print("floated:", round(hours_run(300, 10, POWER_W), 2), "h; dry:", round(hours_run(300, 10, POWER_W * DRY), 2), "h")
# Part 3: windings per 12 h night and the energy the keeper lifts each time
for hours in (2, 4):
    print("windings in 12 h at", hours, "h:", 12 / hours, "; energy:", POWER_W * hours * 3600, "J")

# Output:
# 2 h: 459 kg floated, 1835 kg dry
# 4 h: 917 kg floated, 3670 kg dry
# floated: 1.64 h; dry: 0.41 h
# windings in 12 h at 2 h: 6.0 ; energy: 36000.0 J
# windings in 12 h at 4 h: 3.0 ; energy: 72000.0 J
#
# With the assumed 5 W to turn a mercury-floated lens, a 2 hour winding at an 8 m drop needs a weight of about
# 459 kg and a 4 hour winding about 917 kg. A dry bearing that needs four times the power needs four times the
# weight (1,835 and 3,670 kg), which shows why floating the lens in mercury matters: the lighthouse article
# says the lens assembly sometimes floated in liquid mercury to reduce friction. A 300 kg weight falling 10 m runs
# 1.64 hours floated and 0.41 hours dry. A 12 hour night needs six windings at a 2 hour interval and three at a 4
# hour interval. The 5 W and the factor of 4 are assumptions for the exercise, not measurements; only the 2 hour
# winding interval comes from the source.