MECHANICAL WORK. 73 



orizontal table requires very little work, because the 

 resistance is small. To move the same ball upon a very 

 rough surface requires more work, because the resistance 

 of friction is greater. To raise the ball from the table 

 requires still more work, for in this case the resistance 

 of gravity has to be overcome. The work done in 

 moving a body through a certain space is jointly pro- 

 portional to this space and to the force or resistance 

 overcome ; or, in other words, it is the product of these 

 two quantities. The work required to lift a body which 

 weighs 5 kgr through 3 m is 3 x 5 = 15 times as great 

 as the work required to lift l kgr through l m . The 

 work necessary to overcome the resistance of l kgr through 

 l m is the unit used for computing work, and is called 

 the Kilogrammetre, or Metre-kilogramme. The work 

 which we perform when we raise a body is not lost ; it 

 reappears when we leave the body to itself. It is again 

 drawn downwards by gravity and acquires a certain 

 velocity. The work now done by gravity consists in 

 overcoming the inertia of the body ; for the body which 

 was at rest begins to move with increasing velocity. In 

 the moving body the work done by gravity is accumu- 

 lated ; it acquires, by virtue of its velocity, the capacity 

 itself doing work, of overcoming any other resist- 

 A stone raised and allowed to fall upon a peg or 

 stake on the ground, drives the peg or stake some 

 distance into the soil, and thus overcomes the resistance 

 which the soil offers to these bodies. 



We know from the preceding article that the velocity 

 acquired by a body in falling from a certain height is 

 equal to the velocity with which a body must be thrown 

 upwards to make it rise to the same height. With 





