When the truck's weight is added to the boat, the boat sinks 5 cm deeper,
and displaces additional water whose weight is equal to the weight of the
truck.
The volume of the additional displaced water is
(3.9 m) x (6.3 m) x (5.0 cm)
= (3.9 m) x (6.3 m) x (0.05 m) = 1.2285 m³ .
The weight of that much water is the weight of the truck.
Mass of 1 liter of water = 1 kilogram
1.2285 m³ = 1,228.5 liters = 1,228.5 kg of water.
Weight = (mass) x (gravity)
= (1,228.5 kg) x (9.8 m/s²) = 12,039 Newtons.
(about 2,708 pounds)
