88 MECHANICAL RECREATIONS [CH. V 



The idea of force is difficult to grasp. How many people, 

 for instance, could predict correctly what would happen in a 

 question as simple as the following? A rope (whose weight 

 may be neglected) hangs over a smooth pulley ; it has one end 

 fastened to a weight of 10 stone, and the other end to a sailor 

 of weight 10 stone, the sailor and the weight hanging in the 

 air. The sailor begins steadily to climb up the rope ; will the 

 weight move at all ; and, if so, will it rise or fall ? In fact, it 

 will rise. 



It will be noted that in the first law of motion it is asserted 

 that, unless acted on by an external force, a body in motion 

 continues to move (i) with uniform velocity, and (ii) in a 

 straight line. 



The tendency of a body to continue in its state of rest 

 or of uniform motion is called its inertia. This tendency 

 may be used to explain various common phenomena and 

 experiments. Thus, if a number of dominoes or draughts are 

 arranged in a vertical pile, a sharp horizontal blow on one of 

 those near the bottom will send it out of the pile, and those 

 above will merely drop down to take its place — in fact they 

 have not time to change their relative positions before there 

 is sufficient space for them to drop vertically as if they were 

 a solid body. On this principle depends the successful per- 

 formance of numerous mechanical tricks and puzzles. 



The statement about inertia in the first law may be taken 

 to imply that a body set in rotation about a principal axis 

 passing through its centre of mass will continue to move with 

 a uniform angular velocity and to keep its axis of rotation fixed 

 in direction. The former of these statements is the assumption 

 on which our measurement of time is based as mentioned below 

 in chapter xx. The latter assists us to explain the motion of 

 a projectile in a resisting fluid. It affords the explanation of 

 why the barrel of a rifle is grooved ; and why, similarly, anyone 

 who has to throw a flat body of irregular shape (such as a card) 

 in a given direction usually gives it a rapid rotatory motion 

 about a principal axis. Elegant illustrations of the fact just 

 mentioned are afforded by a good many of the tricks of acrobats, 



