MECHANICAL FORCES 27 



forts to penetrate the generic nature of the "force field" — to develop a uni- 

 fied theory — received much impetus, without much success, during the life 

 of Albert Einstein, but one notices now that efforts at unification are falling 

 off as theorists drift into other problems. Hence the question most funda- 

 mental to all science, biophysics included, viz: "What is force?", seems 

 destined to remain unanswered for a long time yet. It is a more fundamental 

 question even than "What is life?", for life is only one manifestation of force! 



MECHANICAL FORCES 



Newton's Three Laws of Motion 



These three laws are the basic description of mechanical systems. From 

 the simple statements can be inferred many properties of mass and inertia. 



First Law: A body at rest tends to stay at rest, and a body in motion tends 

 to continue moving in a straight line unless the body is acted upon by some 

 unbalanced force (F). The property of the body by virtue of which this is 

 true is given the name inertia. The measure of amount of inertia is called 

 the mass (m). 



Second Law: A body acted on by an unbalanced force will accelerate in the 

 direction of the force; the acceleration (a) is directly proportional to the un- 

 balanced force and inversely proportional to the mass of the body. 



This second law describes the familiar experimentally derived relationship 

 F a ma, or F = kma. If the dimensions of F are suitably defined, this be- 

 comes F = ma. The need to choose the dimensions in this manner results 

 from the fact, discussed earlier, that we really do not know what the nature 

 of force is, but rather do we know only its effects. This is certainly true of 

 the common forces of gravitation, electrostatics, and magnetism. Yet fric- 

 tional force we are able to relate to physical interference of microrough- 

 nesses and physical attraction of two surfaces — and thus have some idea of 

 what this force is. The force exerted by the finger to push the pencil, or the 

 force exerted by the thumb on a hypodermic needle drive home to us a 

 meaning of mechanical force based on its effects. 



Third Law: For every physical action there exists an equal and opposite 

 reaction. The recoil of a rifle as the bullet is ejected, and the swinging arms 

 which help man to maintain his balance while walking briskly, are examples. 



Careful consideration of the statements themselves will enable the reader 

 to appreciate the far-reaching consequences of these laws, consequences 

 which range from suspension bridges to the molecular interactions of bio- 

 chemistry, from the effects of high centrifugal forces on the pilot of a high- 

 speed aircraft to the simple levers of which the human body in motion is a 

 remarkably complex, though well coordinated, example. 



