in TIME, VELOCITY AND ACCELERATION 37 



or the number of units of length it moves over in a unit of time. 

 Velocity may be either uniform or variable. In the case of our 

 boy, if during every second through which he moves he travels 

 over a line of five yards in length, AVC should say he had a 

 uniform velocity of live yards a second. But suppose this boy 

 does not move regularly over five yards in every second ; he 

 sometimes dawdles, sometimes stops to look at a shop, at other 

 times he puts on a spurt to make up for lost time. How should 

 we describe his motion now ? His rate varies from time to time, 

 or his velocity is variable, and to describe such a variable velocity 

 it is usual to speak of the velocity at any instant as being a 

 certain number of yards per second. Say the boy of our 

 example, moving with a variable velocity, has at a given instant 

 a velocity of eight yards per second. We should mean that he 

 would, if he continued to move at the same rate as he had at the 

 given instant, travel over eight yards in the succeeding second. 



Average Velocity. But it is sometimes better to find the 

 average 'velocity of the moving body. Returning to our boy, 

 suppose lie travelled 800 yards in 400 seconds ; if we divide the 

 first number by the second we obtain the boy's average rate, 

 namely, two yards in a second ; this, then, is the rate with 

 which he would have had to travel, if he moved uniformly, in 

 order to complete his journey in the same time. 



The unit of velocity is the velocity of a point which passes 

 over the unit of length in a unit of time ; it is generally taken 

 as being a velocity of one foot per second. Thus a velocity of 

 six means a velocity of six feet per second. 



Measurement of Uniform Linear Velocity. It is a very 

 simple matter to calculate the velocity of a body moving 

 uniformly in a straight line when we know the distance it has 

 travelled, measured in units of length, and the time it has taken 

 to perform the journey, measured in units of time. Thus if we 

 represent the space it has passed over, measured in units of 

 length, by the letter s, and the time taken, measured in units of 

 time, by the letter t, all that we have to do in order to find its 

 uniform velocity is to divide the number of units of length 

 passed over by the number of units of time taken to complete 

 the distance, or 



uniform velocity (v) = na of miits of length passed over = s_ 

 no. of units of time taken t 



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