58 INTRODUCTION TO GENERAL SCIENCE 



c. 1806 : 92-93. Center of Gravity. 



d. 1807 : 60-61. Center of Gravity. 



e. 1808 : 63-64. Center of Gravity. 



/. 1809 : 85-86. Center of Gravity and Methods of Locat- 

 ing It. 

 g. 1810 : 13-14. Center of Gravity and Method of Finding It. 



Experiment 21. Center of Gravity. 



Apparatus : Bottle, two cork stoppers, two knives or table 

 forks, pin, needle. 



a. Place one stopper in the bottle and stick the pin into it. 

 Into one end of the other cork push the needle, head first, and 

 stick the knives or forks into the opposite sides of the cork so that 

 they hang in a slanting direction. If the point of the needle 

 is placed on the head of the pin, the whole system will balance. 

 Devise some method of balancing a half dollar, on its edge, on 

 the rim of a table glass. If the center of gravity of any sys- 

 tem is low, the whole system is stable and will balance. 



41. THE EFFECT OF Two FORCES ACTING AT RIGHT 

 ANGLES TO EACH OTHER 



The second law of motion states that each force acts inde- 

 pendent of any other force. See Section 38. If the forces 

 act at right angles to each other, a curved motion is produced 

 which may be a closed curve, like the circle or the ellipse, or 

 an open curve, of which the parabola and the hyperbola are 

 examples. 



The earth is held in its path around the sun by means of 

 the attraction which the earth and sun have for each other, 

 gravitation, while its forward motion continues on account 

 of its inertia. Refer to Section 37. A more complete dis- 

 cussion of the earth's motions is given in Section * 50, The 

 Motions of the Earth. 



