A solid plate ,with a thickness of 15 cm and a thermal conductivity of 80 W/m*K,is being cooled at the upper surface by air. The air temperature is 10c ,while the temperature at the upper and lower surfaces of the plate are 50 and 60 c ,respectively.Determine the convection heat transfers coefficient of air at the upper surface and discuss whether the value is reasonable or not for force convection of air.