52 QUESTIONS AND PROBLEMS. CHAPTER III. 



1. How are live loads determined? Dead loads? Wind 

 loads? Reactions? Stresses? 



2. In applying the laws of equilibrium, are external and in- 

 ternal forces treated alike? 



3. Illustrate the things which fully determine a force. 

 Must it always be assumed that there is a rigid connection 

 between a force and the body upon which it acts? 



4. Must concurrent forces have the same point of appli- 

 cation? 



5. When there is actual rotation produced by a force, where 

 is the center of moments taken? 



6. If the resultant of the two forces of a couple he regarded 

 as an infinitely small force, what must be its lever arm to make 

 its moment equal to that of the couple? Why is the moment 

 of a couple the same for any center in its plane? 



7. What are the simplest systems of forces that are equiv- 

 alent to any system of co-planar forces? 



8. If in Fig. 20, P= 10000 Ibs., a = 8.5 ft., and 6 = 5 ft., 

 what must be the weight of the car to just balance the wind load? 

 What will the vertical reactions be if the car weighs this amount? 



Ans. 34000 Ibs. ; and 34000 Ibs. 



9. Into how many sets of two components at right angles 

 to each other may a given force be resolved? 



10. If the dead load stress in a certain member of a truss 

 is 15000 Ibs. tension, what will be the stress in it if the live load 

 produces 24000 Ibs. compression? 



11. Give three very fundamental statements in graphic 

 statics which are derived from the parallelogram of forces. 



12. If three forces are in equilibrium, prove by means of 

 the force triangle which represents them, that the algebraic sums 

 of the horizontal and of the vertical components are zero. Draw 

 a rectangle which will circumscribe the triangle. 



13. Resolve a force into two components by drawing a force 

 triangle, so that both components shall be greater than the given 

 force. 



14. Show how to find the magnitude of the equilibrant of 

 any two forces (not parallel) and the location of its line of action. 



15. In Fig. 24 mark the order in which the magnitudes of 

 the forces are determined when P 2 is the known force. 



