MASS, WEIGHT, FORCE, WORK, AND POWER 541 



when we lift the 1-lb. mass 1 ft. against the pull of gravity; 

 we also do 1 ft.-lb. of work when we support it so as to prevent 

 it from falling while we lower it 1 ft. We do work when we 

 climb a flight of stairs; we also do the same amount of work 

 when we descend the same flight of stairs. 



The most common metric unit of work is the KILOGRAM- 

 METER. It is the amount of work done when a force of one kilo- 

 gram acts through a distance of one meter. 



PROBLEMS 



1. How many foot-pounds of work does a 150-lb. man do in climb- 

 ing a flight of stairs 10 ft. in height? How much does he do in 

 descending the same flight? 



2. If a horse exerts an average pull of 100 Ib. while plowing, how 

 much work does he do while plowing a furrow 1 mile in length? If 

 he walks at the rate of 3 miles per hour, how many foot-pounds of 

 work does he do per hour? 



3. A boy carries a skuttle of coal weighing 10 Kg. up a flight of 

 stairs 3 meters in height. How many kilogram-meters of work 

 does he do? 



638. Time is not a Factor in Work. Time is not considered 

 in determining amount of work. The amount of work done 

 by a man in shoveling a ton of coal into a wagon is independent 

 of the time required to do it. It requires neither more nor less 

 work to plow an acre of land if the plowing be done in an hour 

 or in a day. We all recognize this in everyday life. We are 

 willing to pay no more for the shoveling of the coal or the plow- 

 ing of the ground because the man who does the work requires a 

 longer time in which to do it. In fact, we are often willing to 

 pay a little extra if the work be done in the shorter time. 



639. Power, Activity, or Rate of Work. The unit in which 

 POWER, ACTIVITY, or RATE OF WORK is measured is the HORSE- 

 POWER. A machine is said to be a ONE-HORSE-POWER MACHINE 

 when it is capable of doing 33,000 ft.-lb. of work per minute, 

 or 550 ft.-lb. of work per second. This unit of power was 

 chosen and named by James Watt (see Art. 680) ; he supposed 

 that an average horse could work at about this rate. In 



