LAWS OF MOTION 59 



length of time the speed is constant. On the other hand, 

 when the speed changes and equal distances are not traversed 

 or covered in the same length of time, the motion is said to 

 be variable. The rate of change of velocity is called the 

 acceleration. It is said to be positive when it increases, 

 and negative when it decreases. 



A body moving from one place to another in a straight 

 line is said to undergo translation or rectilinear motion. When 

 a body moves around a fixed point or axis it is said to rotate. 

 The particles of the body make concentric circles, as a pulley 

 rotates on a shaft; such motion is said to be curvilinear. 



60. Cams. In a great many machines, such as looms, sew- 

 ing machines, printing presses, punch presses, automobile 

 engines, etc., it is often necessary to give to each machine a 

 motion peculiar to itself. In one machine it may be necessary 

 to change circular or rotary motion into back-and-forth or 

 reciprocating motion at definite times during the working 

 of the machine. In another machine the opposite effect 

 may be desired. A curved plate or groove, called a cam, 

 is used for producing such irregular motion. Cams are 

 constructed in various shapes and dimensions. They may 

 consist of simply a wheel, a projecting part of a wheel, 

 or a revolving piece. The nature of the motion given 

 by the cam to a machine is determined by the shape of 

 the cam. 



61. Centrifugal Force. Rotating bodies like grindstones, 

 fly-wheels, etc., are built to run at a certain maximum speed. 

 If this speed is exceeded the body may fly to pieces, as there 

 is a tendency for particles of a rotating body to fly off in 

 straight lines. The force that causes this movement away 



