28 SOME PHYSICAL FORCES EXEMPLIFIED IN MAN 



Units and Dimensions 



It is useful now to introduce definitions of certain quantities in mechanics. 

 By the first law, a force is defined as anything which changes the state of rest 

 or of motion in matter. The basic unit of force, in the centimeter-gram- 

 second system, is called the dyne. This is the force which will produce an ac- 

 celeration of 1 cm per sec each sec (1 cm sec -2 ) on a mass of 1 gram (1 g). 

 All other forces (electrical, etc.) can be related by suitable experiments to 

 this fundamental quantity of motion. 



Force gives to mass an energy, a capability of doing work. In the system 

 of mechanics, the amount of energy acquired by a mass under the influence 

 of a force depends upon how long or over what distance the force acts. The 

 energy imparted to 1 g of mass by a force sufficient to give the mass an ac- 

 celeration of 1 cm sec -2 within the distance 1 cm, is called 1 erg. One 

 erg = 1 dyne cm. This is an inconveniently small unit of energy, and a 

 quantity often million (10 7 ) ergs has been defined as 1 joule (1 jou). 



By contrast with this definition of energy units in the mechanical system, 

 the unit of heat energy, the small calorie, has been defined as the amount of 

 energy which it takes to raise the temperature of 1 g of water 1° C, between 

 4.5 and 5.5° C, where water is the most dense.* (As the temperature is 

 lowered, water molecules begin to line up in "anticipation" of freezing, and 

 the volume increases; as the temperature is raised, increased thermal energy 

 tends to drive the molecules apart, and the volume also increases). Experi- 

 mentally, by transformation of mechanical motion into heat in a water 

 calorimeter, 1 cal has been found to equal 4.18 jou. One thousand cal, or 

 1 kilocalorie (1 kcal), has been defined 1 Cal, or large calorie. This is the 

 unit used to describe the energy available from different foods. 



Power is the rate at which energy is expended; that is, energy expended per 

 unit time. The basic unit of power is the joule per second, called the watt 

 (w). One-thousand watts is 1 kilowatt (1 kw). One horsepower (1 hp) is 

 equivalent to 746 w or 3/4 kw. 



Entergy exists in two general forms, kinetic and potential. Kinetic energy 

 is that possessed by mass in motion. In mechanics potential energy is that 

 possessed by a mass because of its position. In other disciplines potential 

 energy assumes different forms: the energy stored in chemicals, or that 

 stored in extended muscle, or in an electrostatic charge separation across a 

 cell membrane, could be released to do useful work or provide heat. 



Heat energy is all kinetic energy. It is the total energy of motion of all 

 the molecules in the body under consideration. Temperature is an indicator 

 of the amount of heat in a body, and can be considered to be the "force-like" 



*The amount of heat required to raise 1 g of a substance 1°C is called the specific heat, c. It 

 can be measured under constant pressure (c p ) or under constant volume (cy ). 



