THE FIRST LAW 27 



Consider, for instance, a railway truck standing at rest on a level line 

 of rails. If a second truck runs into it the first truck will start into motion, 

 so that force has been applied. 



The first law, however, tells us more than this. It tells us that 

 if a body is kept free from the action of forces, it will remain in 

 its state of rest or of uniform motion in a straight line. Thus the 

 normal state for a body to be in is one of rest or of uniform motion 

 in a straight line, i.e. motion with uniform velocity; it is only 

 the presence of force which can alter this normal state. 



Consider again the case of the railway truck. Let us suppose it has 

 been set in motion by collision, and that it starts off with a velocity of, 

 say, 10 miles an hour. The first law tells us that unless forces act on the 

 truck, it will continue its motion with an unaltered velocity of 10 miles 

 an hour in the same straight line in which it started. When a truck is 

 actually started into motion by collision, it may be taken for granted that 

 it will not continue in uniform motion in a straight line, but will sooner 

 or later be brought to rest. Thus forces must be at work. Let us consider 

 the nature of these forces. 



, In the first place there is a force known as the resistance of the air. 

 The air in front of the truck presses against it in such a way as to retard 

 its motion. The air therefore exerts force on the truck just as a man might 

 exert force on the truck by pressing against it with his hand. This force 

 alone would stop the truck in time. 



Let us suppose that the brakes are applied, and that the wheels are 

 gripped so firmly as to be at rest relatively to the truck, so that they slide 

 on the rails. There is then a large force applied to the truck by the rails, 

 and this again tends to stop the motion of the truck. Even if the brakes 

 are not applied, so that the wheels are left free to turn, there will still be a 

 force applied by the rails, although this force will be smaller than before. 



Suppose that the track is curved instead of straight. We can imagine 

 the motion continuing for some time, but it will be motion along the 

 curve, and not motion in a straight line, such as we are told by the first 

 law would take place if it were not for the action of force. Force has 

 therefore been applied, the force being that of the rails on the flanges of 

 the wheels, tending to turn the truck round the curve. If the flanges were 

 not present, this force could not act, so that the motion would continue in 

 a straight line the truck would run off the rails. 



As another illustration of the meaning of the first law, let us examine 

 the motion of a bullet fired from a gun. Here the forces which start the 

 motion are supplied by the pressure of the powder. After the bullet has 

 left the gun, the forces which act are small compared with those which 



