IN PHYSICS. 205 



53. Why is a gun firing blank cartridges more quickly 

 heated than one firing balls ? 



In the one case, the energy of the burning powder is 

 changed to heat ; in the other, largely to the motion of 

 the ball. 



54. When an animal is jumping or falling, can any 

 exertion made in mid-air change the motion of its centre of 

 gravity ? 



The centre of gravity falls steadily 16.08 feet (see 

 P- 54)> whatever other force may act on the body. 



(See Second Law of Motion.) 



55-60. (See Answers under Questions i to 7, on pages 

 5 and 6 of this Key.) 



61. Why is a " running jump " longer than a " standing 

 jump "? 



This is generally spoken of as an illustration of inertia. 

 It is really an example under the first law of motion. 

 The momentum of the person when running (m x v) is 

 added to the force with which he finally springs from the 

 ground for the jump. 



62. Why, after the sails of a vessel are furled, does it 

 still continue to move ? and why, after the sails are spread, 

 does it require some time to get it under full headway ? 



This illustrates the tendency of matter to continue in 

 its present state, whether of rest or of motion, i. e., its 

 inertia. For the former part of the question, apply the 

 first law of motion, and for the latter, the first paragraph 

 on p. 28 of the Physics. If, on starting with a heavy 

 load, the horses leap suddenly forward, they will break 



