CHAP, in Energy ', the Power of Nature 31 



train moving at 60 miles an hour on a smooth level railway 

 only requires the engine to give out enough energy to over- 

 come the resistance of the air and the rails ; when that is done 

 the train, however great its mass, continues to move with 

 undiminished speed. When it has to be stopped quickly, 

 shutting off steam from the engine is not enough ; great 

 resistance has to be introduced by means of brakes which 

 convert the energy of motion rapidly into heat. The energy 

 expended in setting a mass in motion is preserved in the 

 moving mass when there is no external resistance, and 

 returned unaltered in quantity when the motion is stopped. 

 The amount of motion in a moving body is called its 

 momentum, and is measured by the mass and the velocity 

 .together. A mass of I Ib. moving with a velocity of 1000 

 ft. per second has the same momentum as a mass of I ooo 

 Ibs. moving at i ft. per second. 



5 1 . The Gyroscope illustrates the first law of motion. 

 It consists of a heavy leaden wheel turning on an axle in a 

 brass ring. The inertia of the fly-wheel requires to be 

 overcome by imparting a considerable amount of energy to 

 it by means of a cord and a strong pull of the arm ; once 

 set in motion it would never stop but for the friction of its 

 axle and of the air. A gyroscope in rotation behaves 

 differently from one at rest. When the experimenter takes 

 it by the stand and attempts to change the direction of its 

 axis of rotation it seems to have a will of its own ; it strongly 

 resists any change of position, although when the fly-wheel 

 is at rest its axis may be easily turned in any direction. In 

 the fly-wheel itself there is a struggle going on ; the particles 

 tend to move in straight lines, and it is only the attraction 

 of cohesion that compels them to move in a circle. In 

 factories grindstones are sometimes made to rotate so fast 

 that they burst ; the tendency of the parts to move in straight 

 lines is too great for the cohesion of the stone to counter- 

 balance. The tendency for bodies to move in a straight 

 line, unless compelled by some power to follow a curve, is 

 often called centrifugal force. 



52. Work against Gravity. In employing energy to 

 overcome weight there seems at first sight to be a real loss 



