30 SOME PHYSICAL FORCES EXEMPLIFIED IN MAN 



at its slowest, when no work is being done, basal metabolism amounts to 

 about 0.1 hp. The human machine needs a minimum of 0.1 hp to keep it 

 alive, and can put out continuously a maximum of about 0.01 hp of useful 

 mechanical work, with occasional surges to several horsepower. 



The football player's momentum just before collision was (200/32) x 

 (300/12) = 154 lb sec. If this were transferred in 0.1 sec during collision, 

 the impressed force, defined as rate of change of momentum, dM/dt, was 

 154/0.1 = 1540 lbs. This can be expressed as a "shock" (force per unit 

 mass) of about 7.7 g, where g is the acceleration of all bodies due to gravita- 

 tional attraction to the earth (32 ft/sec 2 , or 980 cm/sec 2 ). The value 7.7 g 

 is obtained directly from the second law, viz 



J7I 154 ° 77 



a = t m = = 7.7 g 



200/g 



By contrast, and as further illustration, the passengers on a modern com- 

 mercial jet line experience about 2 g during take-off. The jet pilots for 

 fighter aircraft and the astronauts have been tested up to 18 g. The famous 

 right hand of boxer Joe Louis was said to impart up to 40 g to a stationary 

 and nonelastic target. A laboratory centrifuge will provide a centrifugal 

 acceleration of some thousands ofg; and the ultracentrifuge used in sedi- 

 mentation experiments in which molecular weights of large molecules are 

 obtained, develops up to 100,000 g. Centrifugal motion is convenient for 

 varying at will the inertial mass of a body: e.g., in the human centrifuges in 

 space-research laboratories. 



As a machine, man is very versatile. However, he is quite inefficient be- 

 cause of the continuous power being expended to keep him alive when he is 

 not "in use." His highest purely physical role is that of a computer. 



Two forces will now be considered: a mechanical force as applied to a 

 lever, and the mechanical force of a compressed gas. 



The Lever 



A lever is one of a great number of machines — devices for doing work. This 

 particular device permits mechanical energy to be factored into such values 

 of force and distance that some desired mechanical result can be ac- 

 complished. The lever does not create energy, of course, but simply makes 

 the energy more available to do the particular job at hand. The familiar 

 example of the crowbar to dislodge a large stone, using a log as a pry, is an 

 example. In this case a relatively small force applied over a relatively large 

 distance at the hands is transformed into a relatively large force applied over 

 a relatively small distance at the stone. The mechanical advantage is the 

 ratio of the two forces; it is inversely proportional to the ratio of the two dis- 

 tances since F i d ] must equal F 2 d 2 . 



