Lesson XI.] 



NATURAL PHILOSOPHY. 



P. It is the motion of one body con- 

 sidered in relation to that of another body ; 

 thin : suppose I roll two balls, one of wood 

 and the other of lead, along a table, and 

 >th move in the same direction, the 

 ice between their motions is the 

 relative motion. 



136. T. If I let a bullet fall at the same 

 moment as cork, but of the same s 

 there be any great difference in the descent 

 of tlv 



/'. Yes ; because it will depend upon 

 the momentum. 



T. What do you mean by the 

 momentum ? 



P. It is the raotal force of a body. All 

 . whether light or heavy, may move 

 with the same speed, but the momentum 

 of a body being proportionate to its in iss 

 and velocity, it is very evident that the 

 difference will be relative. 



138. T. How can you estimate the 

 momentum of a b< 



/'. If you multiply the weight of a body 

 by the number of feet it passes through or 

 over in a second, the produce will be the 



momentum. If the velocity of a cannon 

 ball, weighing 30 Ibs., be 1,800 

 second, its momentum will be 54,000. 



139. T. Suppose that I let a bullet, 

 weighing an ounce, fall at the same mo- 

 mentwiih a thin sheet of lead, weighing an 

 ounce, which will reach the ground sooner ? 



P. The bullet ; because the resistance 

 of the atmosphere impedes the descent of 

 the sheet 



GENERAL QUESTIONS ON LESSON* X. 



1. What is meant by motion and rest ? 



2. Is rest in accordance with the esta- 

 blished laws of Nature f 



3. What is meant by the terms absolute 

 and relative rest? 



4. Is it more natural for bodies to be in 

 a state of rest or motion ? 



5. What are the causes of motion? 



6. Is motion constant or not ? 



7. What is the difference between the 

 velocity and momentum of a body .' 



8. How can you estimate the momentum 

 of a body .' 



LESSON XL 



rnt LATION, &c. HALLAW found that the velocity of every filling body is uni- 

 formly increased as it approaches the earth, no matter from what height it falls; th.it 

 i<, without taking into account atmospheric resistance. Experiment determined his 

 observations, for he found that the motion of falling bodies is increased in regular arith- 

 metical progression, and, by computation, he was enabled to ascertain the s;>nce follrn 

 through in a given time. Thus he found that a dense or compact body passed through 

 a space 1 inch during the first second of time that it was descending towards the 



earth; and, in order to ascertain the time a body occupied in falling, he multiplied the 

 square of that number by 16, and the result was, the number of feet the body hid fallen* 

 For example, he observed that a stone occupied 6 seconds in falling from a certain height, 

 and he took the square (the number multiplied by itself) of 6, which IB . 

 multiplying 36 by 16, the result was 576, or the number of feet fallen. In making these 

 calculations, he omitted the odd inch, because it is near enough for all practical purposes : 

 and he knew that even the best calculation would only be an approximation of the actual 

 distance, in consequence of the gravity of bodies, a fact he had learned by experi- 

 ment [Sec Experiment 14]. 



QUI . &c. 



ll'>. T '-'' ' do you mean by the the influence of gravitation in, as it were, 



con and upon which a bod. 



P. It is that point in all bodies at which rest if it be freely suspended ; it is 



