v MATTER IN RELATION TO MOTION 67 



represented by / which pulls the pendulum in the direction BF, 

 and the other represented by BE which has no part in moving 

 the pendulum, and merely causes tension in the string BC. 



CHIEF POINTS OF CHAPTER V. 



Equal Masses. If two bodies, moving with equal velocities in 

 opposite directions, stop dead after colliding, we can conclude that 

 their masses are equal. If the combined bodies moved after the 

 collision their masses could evidently not have been equal. 



Two masses are equal if when they are made to impinge on one 

 another in opposite directions with equal speeds, and stick together, 

 they come to rest. 



The momentum of a body is the quantity of motion it has and is 

 equal to the product of its mass and its velocity. 



The unit of momentum is the quantity of motion in a mass of one 

 pound moving with a velocity of one foot per second. 



The total momentum of several moving masses remains unaltered 

 by impact. This can be expressed by an equation (p. 57). 



(m -f m') V= mv + m'v' 



Force. When a gradual change of momentum is either produced 

 or tends to be produced in a body, that body is acted on by force. 

 Unit of Force. The unit of force can be defined in several ways : 



1. A unit force acting for the unit of time is able to produce a unit 

 of velocity in a unit of mass. 



2. A unit of force produces a unit of acceleration in a unit of 

 mass. 



3. A unit of force gives rise to a unit of momentum in a unit of 

 time. 



A poundal is a force which produces an acceleration of one foot 

 per second per second in the mass of a pound, and a dyne is the 

 force which, acting upon a mass of one gram, produces an accelera- 

 tion of one centimetre per second per second. 



Equal Forces produce equal momenta in equal times. 



The number of units of force in any force is equal to the product 

 of the number of units of mass in any body on which it may act, 

 and the number of units of acceleration produced in that mass by 

 the force in question. 



If F represents the number of units of force in a given force, m 

 the number of units of mass on which it acts producing a units of 

 acceleration, then we can write 



Newton's First Law of Motion. Every body will continue in its 

 state of rest or of uniform motion in a straight line, except in so far 

 as it is compelled by impressed force to change that state. 



F 2 



