488 PHYSIOLOGICAL PHYSICS. [Chap. XL. 



on it, and the time during which it acted, but also on 

 the mass of the body. The velocity of a body multi- 

 plied by its mass gives what is called the momentum of 

 the body, A body at rest tends to remain at rest, and 

 a body in motion tends to remain in motion. To 

 change its state of rest or motion, the application of 

 a force is necessary. This is due to the inertia of the 

 body. 



The measurement of force. Suppose two 

 forces of unknown amount to act upon two bodies of 

 equal mass and free to move under the same condi- 

 tions. It is evident that the forces could be esti- 

 mated by the velocities imparted to the bodies. If 

 the velocities were equal the forces would be equal. 

 If the velocity of one body were half that of the 

 other, the force acting on the body must have been 

 the half of that acting on the other. So that a force 

 can be measured by the velocity imparted to a body 

 of unit mass after acting upon it for a second (unit of 

 time). This is called the absolute measurement of 

 force. Forces are also estimated by the gravitation 

 method. A standard pound weight is attracted 

 towards the earth with a definite force. A weight of 

 2 pounds is attracted with twice the force, a weight 

 of 3 pounds with thrice the force, and so on. If the 

 weight is to be prevented from falling, the force of 

 the earth's attraction must be counterbalanced by an 

 equal force in an opposite direction. The force with 

 which a pound weight is attracted towards the earth 

 can, therefore, be used as a measurer of force ; and 

 we can speak of a force of 10 pounds, of a pressure of 

 50 pounds, and so on. 



Now in London, a weight of 1 pound, if allowed 

 to fall freely, would fall a distance of 32-1889 feet 

 in a second of time. That is to say, the force of 

 gravity at that place acting on the pound weight 

 (the unit of mass) for one second would produce a 



