ANSWERS ;; 



TO THE 



PKACTICAL QUESTIONS AND PROBLEMS 



IN THE 



POPULAR PHYSICS. 



36 ! A rifle-ball thrown against a board standing 

 on its end will knock it down ; the same bullet fired at the 

 board will pass through it without disturbing its position. 

 Why is this? 



The ball which is thrown has time to impart its motion to 

 the board; the one fired has not. 



2. Why can a boy skate safely over a piece of thin ice, 

 when 9 if he should pause, it would break under him di- 

 rectly ? 



In the latter case there is time for the weight of his body 

 to be communicated to the ice; in the former, there is not. 



3. WJiy can a cannon-ball be fired through a door 

 standing ajar, without moving it on its hinges ? 



Because the cannon-ball is moving so quickly that its mo- 

 tion is not imparted perceptibly to the door. 



4. Wliy can ^ve drive on the head of a hammer by sim- 

 ply striking the end of the handle ? 



This can only be done by a quick, sharp blow, which will 

 drive the wooden handle through the socket before the motion 

 has time to overcome the inertia of the iron head. A slow, 

 steady blow will be imparted to the head, and so fail of the 

 desired effect. 



