ON THE CONSERVATION OF FORCE 195 



The driving force depends here on the velocity of moving 

 masses. 



A bullet resting in the hand is the most harmless thing in 

 the world; by its gravity it can exert no great effect; but 

 when fired and endowed with great velocity it drives through 

 all obstacles with the most tremendous force. 



If I lay the head of a hammer gently on a nail, neither 

 its small weight nor the pressure of my arm is quite sufficient 

 to drive the nail into the wood; but if I swing the hammer 

 and allow it to fall with great velocity, it acquires a new 

 force, which can overcome far greater hindrances. 



These examples teach us that the velocity of a moving 

 mass can act as motive force. In mechanics, velocity in so 

 far as it is motive force, and can produce work, is called 

 ins viva. The name is not well chosen; it is too apt to 

 suggest to us the force of living beings. Also in this case 

 you will see, from the instances of the hammer and of the 

 bullet, that velocity is lost, as such, when it produces work- 

 ing power. In the case of the water-mill, or of the wind- 

 mill, a more careful investigation of the moving masses of 

 water and air is necessary to prove that part of their 

 velocity has been lost by the work which they have per- 

 formed. 



The relation of velocity to working power is most simply 

 and clearly seen in a simple pendulum, such as can be con- 

 structed by any weight which we suspend to a cord. Let M, 

 FIG. 95, be such a weight, of a spherical form ; A B, a hori- 

 zontal line drawn through the centre of the sphere; P thp 

 point at which the cord is fastened. If now I draw tht 

 weight M on one side towards A, it moves in the arc M a, 

 the end of which, a, is somewhat higher than the point A 

 in the horizontal line. The weight is thereby raised to the 

 height A a. Hence my arm must exert a certain force to 

 bring the weight to a. Gravity resists this motion, and 

 endeavours to bring back the weight to M, the lowest point 

 which it can reach. 



Now, if after I have brought the weight to a I let it go, it 

 obeys this force of gravity and returns to M, arrives there 

 with a certain velocity, and no longer remains quietly hang- 

 ing at M as it did before, but swings beyond M towards b t 



