In winter, people play hockey on Alta Lake in Whistler. However, it is not considered safe to play hockey on ice until the ice is 20 cm thick. On a winter day with a temperature -10?C, it is observed that the lake has a 1cm thick layer of ice over the entire surface. Assuming the temperature stays constant, ?nd how long it will take for the ice to be safe:
(a) If the air temperature remains at -10?C, ?nd the rate in cm per hour at which ice is added to the layer. The thermal conductivity of ice is 2.18W/m·K.
(b) Find the time that the ice layer reaches a thickness of 20 cm.