THE LAWS OP MOTION. 



LESSON 18. 



The Laws of Motion. 



Momen'tum, (pi. momenta) the force acquired by different masses 

 of matter moving with different velocities. A body, twice the 

 weight of another, moving with equal velocity, will strike with 

 twice the momentum, — with twice the velocity, with /owr times 

 the momentum, — with three times the velocity, with six times 

 the momentum, and so on. 



A BODY is in motion whenever it is changing its situation 

 with regard to a fixed point, and the cause which produces mo- 

 tion is called/orc6. The causes of motion, or the motive powers 

 are either muscular, as the action of men and other animals, 

 or mechanical, as the force of wind, water, gravity, the pres- 

 sure of the atmosphere or any elastic medium, and steam. 

 The motion of a body acted upon by a single force is always 

 in a straight line, in the direction in which it received the 

 impulse; and the degree of quickness with which it moves, 

 or the velocity, must be proportional to the force by which 

 it is impelled. If a given force, therefore, will produce a 

 given motion, a double force will produce the double of that 

 motion. If a new force be impressed upon a body in motion, 

 its motion will be increased proportionably to the new force 

 impressed. The velocity with which a body moves is mea- 

 sured by the space passed over, divided by the time which it 

 employs in that motion ; for if you travel one hundred miles 

 in twenty hours, your velocity is five miles in each hour. 

 You may reverse this rule and say, that the time is equal to 

 the space divided by the velocity, for one hundred divided 

 by five gives twenty hours for the time ; and you may say 

 also that the space is equal to the velocity multiplied by the 

 time, for twenty multiplied by five gives one hundred miles 

 for the space. 



Motion is uniform, accelerated, or retarded. Uniform 

 motion is regular, and at an equal rate throughout. The 

 hand of a watch is an example of uniform motion, for it 

 passes over equal spaces in equal times. If neither gravity 

 nor any other force opposed its motion, a ball thrown by the 

 hand would proceed onwards in a right line, and with a uni- 

 form velocity for ever. Perpetual motion, however, cannot 

 be produced by art, for gravity ultimately destroys all motion 



