552 MACHINES, WORK, AND ENERGY 



into work without loss, we should be able to produce 3080 

 ft.-lb. of work for every calorie of heat energy in the food or 

 fuel. We have seen that it is impossible to do this. The 

 horse is able to convert but about 20 per cent, of the energy 

 in his food into work. Therefore we can not hope to secure 

 more than about 616 ft.-lb. of work for each calorie (1 Cal.) 

 of energy in the food the horse eats. 



665. Calories of Energy Needed to do 1 Horse-power- 

 hour of Work. We have seen that 1 horse-power is the ability 

 of a motor to do 33,000 ft.-lb. of work per minute (Art. 639). 

 A horse- power-hour of work, then, is 60 X 83,000 ft.-lb. or, 

 1,980,000 ft.-lb. Since 1 greater calorie of heat equals 3080 

 ft.-lb., we see that 640 Cal. of energy are required to do 1 

 horse- power-hour of work. A motor having an efficiency of 

 100 per cent, would, then, consume 640 Cal. of food or fuel 

 while doing 1 horse-power-hour of work. 



640 greater calories of heat = 1 horse-power-hour of work 



656. Cost of 1 Horse-power-day of Work by the Steam 

 Engine. One Ib. of coal when burned yields from 3000 to 

 3600 Cal. of heat (Art. 122). Since the average stationary 

 steam engine has an efficiency of from 5 to 8 per cent., it utilizes 

 only from 150 to 275 Cal. to the pound of coal. Even then 

 it requires only from 2.5 Ib. to 4.5 Ib. of coal to do 1 horse- 

 power-hour of work. In practice, a steam engine is considered 

 as being in fair condition if it does 1 horse- power-hour of work 

 on 4 Ib. of coal. A steam engine, having the usual efficiency, 

 will probably require from 25 to 35 Ib. of coal per horse-power- 

 day of 8 hours. The cost of this coal at $3.00 per ton would 

 be from 3% cts. to 5J4 cts. per horse-power-day. (The 

 student should verify these calculations in every case.) 



657. How Much Work a Horse Can Do. King, in his 

 Physics of Agriculture, says that it is commonly agreed that 

 for steady and continuous work 10 hours per day, walking 

 at the rate of 2J^ miles per hour, a horse should not be asked 

 to pull (exert a force of) more than about J-f or % of its own 



