120 ELEMENTARY GENERAL SCIENCE CHAP. 



It will have been noticed that the question of time does not 

 enter into an estimation of the amount of work done. It is 

 manifest that the same quantity of work is accompli shed whether 

 a day is spent in raising a weight to a given height from the 

 ground or only a minute. If we introduce the time taken to 

 perform the work we begin to consider what is called the power 

 of the agent. We should measure this power by the quantity of 

 work the agent can perform in a given time ; or power is the rate 

 of doing work and is measured by the work done in a second. 

 Thus, engineers use the expression horse-power, by which they 

 mean the rate at which a good horse works. James Watt 

 estimated this at 33,000 foot-pounds per minute, or 550 foot- 

 pounds a second. 



Generally, then, to find the amount of work performed by 

 any force, we multiply the value of the force (expressed in 

 suitable units) by the space through which it acts (using the 

 corresponding unit in measuring this quantity also). The follow- 

 ing simple application of the rule will familiarise the student 

 with the method for employing it : 



How much work is done when an engine weighing 12 tons 

 moves a mile on a horizontal road, if the total resistance is equal 

 to a retarding force of 10 Ibs. weight per ton ? 



The total resistance equals 12 x 10 = 120 Ibs. weight, the dis- 

 tance traversed is 5,280 feet. 



.-. Work done = (120x5, 280) foot-pounds. 



Energy. By the energy of a body we mean its power of 

 overcoming resistance or doing work. All moving bodies 

 possess energy. Moving air or wind drives round the sails of a 

 windmill and so works the machinery to which the sails are 

 attached ; it drives along a ship, thus overcoming the resistance 

 of the water. The running stream works the mill-wheel and 

 the energy it possessed is expended in grinding corn. The 

 bullet fired from a rifle can pierce a sheet of metal by over- 

 coming the cohesion between its particles. 



EXPT. 112. Stretch a piece of tissue-paper over the top of 

 an empty jam-pot. Carefully place a bullet on the paper and 

 notice the paper will support it. Now lift the bullet and 

 allow it to drop on to the paper. It is seen that the bullet 

 pierces the paper. 



EXPT. 113. Support a weight by a thin thread. Show that 



