IN POPULAR PHYSICS. 21 



44. If a body weighing 1 Ib. on the earth were carried 

 to the sun, it would weigh 27 Ibs. How much would it at- 

 tract the sun ? Ans. 27 Ibs. 



75 45. Why does watery vapor float and rain fall ? 



Perfect vapor, which is quite invisible, is lighter than air, 

 and is diffused through it. When condensed into minute drop- 

 lets forming clouds, these are prevented from falling fast be- 

 cause of the great amount of surface, in proportion to their 

 weight, exposed to the resistance of the air. 



46. If a body weighs 1O Jcilos. on the surface of the 

 earth, ^v1lat ivill it ^veigh 1,OOO km. above ? 



x : 10 kilos. : : (6,366 *) 2 : (7.36G) 2 

 x = 7.5 kilograms. 



47. A body is thrown vertically upward with a velocity 

 of 1OO meters. Hoiv long before it ivill return to its origi- 

 nal position ? Ans. 2O.4 seconds. 



48. How much time will be required for a body to fall 

 a distance of 2,OOO meters ? 



Equation (6) s = $gl*. 2,000 = ?LpP. 

 .-. t = 20.2 seconds. 



49. IVJiat would be the time of vibration of a pendulum 

 39.1 indies long at the surface of the moon, where the ac- 

 celeration of gravity is only 4.8 ft. ? 



t : f : : -- : -4= (see 3d Law of Pendulum, p. 69). 



Vo vV 



t' = 2.58 seconds. 



5O. What would be the time of vibration for the 

 same pendulum at the surface of the sun, where the accel- 



* The radius, or semi-diameter of the earth, is given by French as- 

 tronomers at 6,366 km. 



