MAS*, WEIGHT, FORCE, WORK, AND POWER 539 



the lower end of the spring is so mounted that it hangs just 

 in front of the face of the case, upon which is stamped the 

 so ale. When a mass is suspended on the hook, the coiled 

 spring is stretched and the index indicates the weight. 



Spring balances are very convenient and easy to handle but 

 usually they are not very accurate. Even though a spring 

 balance may be carefully made and fairly accurate 

 when new, it is likely to wear with use and give 

 false readings later. 



633. Force. By FORCE we mean a push or a 

 pull. It is force which tends to produce motion in 

 a body or to change the direction or speed of a mov- 

 ing body. All forces are pushes or pulls. Solids 

 may be either pushed or pulled; liquids and gases, 

 however, must be moved by being pushed. A rail- 

 road train may be either pushed or pulled; but 

 water and air can be moved only by being pushed. 

 Explain why this is so (see suction, Art. 374). 



634. The Units of Force. The names given to 

 the units of force are the same as those given to the 

 units of mass. We speak of a POUND OF FORCE and 

 a POUND OF MASS; of a GRAM OF FORCE and a GRAM 

 OF MASS; of a KILOGRAM OF FORCE and a KILOGRAM 

 OF MASS. This use of the same names for units of 



mass and units of force is unfortunate and confusing. Many 

 people do not clearly see the difference between a pound of 

 force and a pound of mass. We must never forget that a pound 

 of mass is a certain quantity of matter while a pound of force is a 

 certain amount of push or pull. We can eat a pound-mass of 

 beefsteak but we exert a pound force of effort when we lift the 

 steak against the pull of gravity. 



635. Comparison of Force and Weight. A pound of force 

 is equal to the weight of a 1-lb. mass; the gram-force is equal 

 to the weight of a 1-gram mass; the kilogram-force is equal 

 to the weight of a 1-kg. mass. It must be remembered, how- 

 ever, that while weight always acts in a vertical line, i.e. } 



FIG. 338. 

 Spring 

 balances. 



