Energy and Heat 119 



the direction of the line of action of the force is the 

 distance the barrow travels. If this distance be 10 yards, 

 a certain amount of work is done by the man's muscles 

 on the barrow, while wheeling the barrow over twice 

 that distance involves doing twice as much work. 

 Again, suppose that a weight of 14 Ib. be raised from 

 the floor to the top of a table. A definite amount of 

 work must be done in order to effect this. Raising 

 a weight of 28 Ib. through the same height will cause 

 twice as much work to be performed. Further, if the 

 14 Ib. weight be raised from the floor to a shelf at a 

 height twice that of the table, twice as much work 

 must be done as was required to raise it to the table-top. 



74. Since work is a measurable quantity, units have 

 been chosen in which to express the values of different 

 quantities of work. The common British unit is " the 

 work performed in raising a weight of 1 Ib. against 

 the resistance due to gravity through a vertical distance 

 of 1 foot". This quantity of work is called a " foot- 

 pound". In lifting a 14 Ib. weight from the floor to 

 the top of a table 3 feet high, the amount of work 

 done is (14 x 3) = 42 foot-pounds. A similar unit 

 derived from the Metric System is the kilogram-metre, 

 i.e. the work done in raising a weight of 1 kilogram 

 through a vertical height of 1 metre. 



We have defined the work done on the body by the 

 force above, but it does not always happen that the 

 point of application of the force moves in the direction 

 of the line of action of the force. Thus a man may 

 pull at the end of a rope, attached to a moving motor- 

 car, in the opposite direction to that in which the car 

 is moving, but may find that this does not stop the 

 car, and in this case the man is dragged along by the 

 car. The point of application of the man's muscular 



