Energy and Heat 135 



2. Classify the following bodies as possessing (a) kinetic energy ; 

 (b) potential energy ; (c) both kinetic and potential energy : 



(1) A sack of flour on a man's back. 



(2) The arms of a windmill in motion. 



(3) A barrel of gunpowder on the top of a wall. 



(4) A kite suspended in the air at the end of a string. 



(5) A kettle full of boiling water resting on the top of a kitchen 



range. 



(6) A rifle-bullet just leaving the muzzle of the gun. 



3. Why do the bars of a fire-grate usually fit loosely in their 

 sockets ? 



4. Why does the thread of mercury in a thermometer move up 

 and down ? 



5. Straight lengths of railway line, if laid in the winter with 

 the end of each length pressed closely against those of its neighbours, 

 become bent during the hot weather of summer. What is the reason 

 of this ? 



6. What kind of energy is possessed by the fire in a range ? 

 Give the reason for your opinion as fully and clearly as you can. 



CHAPTER IX 



TEMPERATURE. THERMOMETERS 



88. In everyday life we speak of one body being 

 hotter or colder than another, and our opinion is in 

 general based upon information afforded by our bodily 

 sensations. No difficulty is experienced in judging by 

 this means whether a red-hot lump of coal is hotter 

 than a kettle-lid, or a piece of ice colder than a cup of 

 tea. It is far from easy, however, to determine by 

 touch whether one glass of cold water is exactly as 

 cold as another. Further, it is possible to obtain 

 contradictory evidence on this point with reference to 

 whether a body is hot or cold by asking the opinions 

 of different people, and the same person will not always 



