In a Joule experiment, a $12\ \text{kg}$ weight falls $2.5\ \text{m}$ twenty times, turning paddles in $1.5\ \text{kg}$ of water in an insulated tank. By how much does the water warm? ($g = 9.8\ \text{m/s}^2$, $c = 4186\ \text{J kg}^{-1}\text{K}^{-1}$.)