CHAPTER IX 



WORK AND ENERGY 



Work. Newton's first law of motion teaches us that a body 

 at rest is only set in motion by the action of a force upon it, and 

 also that a moving body only changes the direction of its motion, 

 or its speed, as the result of the action of a force. When a body 

 moves from rest the continued action of the force upon it causes 

 an acceleration in the body. 



In the case of a body already moving, though we have every 

 right to argue that a change of direction or a change of velocity 

 is the result of an external force, we cannot apply the converse 

 statement and say that an external force acting upon a moving 

 body causes a change of direction or of velocity, for in some 

 instances the force may be entirely occupied in maintaining 

 such motion in opposition to other forces acting upon it. Thus, 

 when a ship is sailing with a uniform speed, the force of the 

 wind is exhausted in maintaining this velocity by overcoming the 

 resistance of the water. 



When a force acts in either of these ways it is said to do work, 

 that is, work is done by a force in setting a body at rest into motion 

 and giving it a regularly increasing velocity, or by maintaining a 

 uniform motion in opposition to the action of other forces. We 

 may class all these forces acting in opposition to the force which 

 is being considered under the inclusive name of resistance. 



We shall thus obtain for our definition of work the following 

 statement : Work is done by a force, either when it acts 

 upon a body producing an acceleration in its velocity, or 

 when it maintains a uniform velocity in a body in opposition 

 to resistance. Or, more briefly, Work is done when the 

 point of application of a force moves, A little consideration 



