A steel drill making 180 rpm is used to drill a hole in a block of steel. The mass of the steel block and the drill is 180 gm. If the entire mechanical work is used up in producing heat and the rate of raise in temperature of the block and the drill is `0.5 °C/s`. Find the couple required to drive the drill. Specific heat of steel = `0.1 (cal g^-1)^@^-1`