172 MECHANICS 



side up, but in a pinhole camera upside down? 



2. What are the sources of heat and light? Can you 

 obtain heat without light, or light without heat? 



3. What is a porous body? Can it be changed so 

 as not to be porous? Is there any advantage in having 

 bodies porous? Name one very important porous body. 



4. Name all the uses of water. 



5. What should we do if we wish to keep in good 

 health? 



6. If a clock is going too slowly, what should you 

 do to regulate it? Explain. 



7. What is a force? What can forces do? 



8. Name two ways by which you can exert a large 

 force by using a small force. 



9. What effect have forces upon elastic bodies? 

 What use is made of this effect? 



10. What are machines? What are the uses of 

 machines? 



