10 AJSWJBRS TO PRACTICAL QUESTIONS 



54. Why is a " running jump " longer than a " stand- 

 ing jump " ? 



This is an example under the first law of motion. The mo- 

 mentum of the person when running (mxv) is added to the 

 force with which he finally springs from the ground for the 

 jump. 



55. 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 tiine to get it under full headway? 



This illustrates the tendency of matter to continue in its 

 present state, whether of rest or of moticn. For the former 

 part of the question, apply the first law of motion, and for the 

 latter, the second paragraph on p. 21 cf the Physics. If, on 

 starting with a heavy load, the horses leap suddenly forward, 

 they will break the harness ; but, by a steady, constantly-in- 

 creased draught, they will communicate motion to the mass. 



56. Why can a tallow candle be fired through a board? 



Because of its high velocity. Motion can not be communi- 

 cated at once to the entire mass of the board, hence this yields 

 at the place where pressure is suddenly applied. 



COHESION. 



48 ! Why can we not weld apiece of copper to one of 

 iron ? 



Cohesion acts most readily between molecules of the same 

 kind. 



2. Why is a bar of iron stronger than one of wood ? 



All we can say is that there is more cohesion between its 

 molecules. The wood, moreover, is perforated with minute hol- 

 low tubes, so that its molecules can not be so compactly massed 

 together as those of the iron. 



3. Why may a piece of iron, when perfectly ivelded, be 

 stronger than before it was broken ? 



By the hammering, more particles are brought within the 

 range of cohesion. 



