LAWS OF MOTION 55 



attempts to unscrew a nut, the pull or force he applies to 

 the nut is called action, and the resistance is called reaction. 

 The reaction consists of friction and of the tendency of the 

 nut to remain stationary. The relation between action and 

 reaction is such that every action is resisted by an opposite 

 and equal reaction. This is the third law of motion. 



54. Momentum of Bodies. The momentum of a body is 

 the quantity of motion in the body, and is the product of the 

 mass and the speed. 



As an example: To find the momentum of a body 9 Ibs. in weight, 

 moving with a velocity of 75 ft. per second, the rule is : 



Mass x Velocity = Unit of momentum 

 9 X 75 = 675 units of momentum 



We may abbreviate this rule by substituting letters for quantities. 

 Let the mass be represented by M and velocity by V. Then 



Momentum = M X V 



The multiplication sign is usually left out between letters; there- 

 fore the quantity is written MV. Momentum may be expressed as 

 a product of pounds by feet per second and tons by feet per second. 

 In the metric standard it may be expressed as a product of grams 

 by centimeters per second, or kilograms by centimeters per second. 



55. Gravitation and Center of Gravity. If we take a 

 thin bar of iron and place it on a table, it will remain there. 

 Remove the support, and the bar will fall to the ground. 

 All bodies act in the same way. The earth attracts them, 

 and this force is called gravitation. If a bar of iron is laid 

 across a support, one particular point will be found at which 

 it will balance, and remain at rest; that point is called the 

 center of gravity of the iron bar, because it is the point at which 

 the entire weight of the body may be considered as centered; 



