Chapter 4. Translational Equilibrium and Friction.

Note: For all of the problems at the end of this chapter, the rigid booms or struts are considered to be of negligible weight. All forces are considered to be concurrent forces.

Free-body Diagrams

4-1. Draw a free-body diagram for the arrangements shown in Fig. 3-18. Isolate a point where the important forces are acting, and represent each force as a vector. Determine the reference angle and label components.

(a) Free-body Diagram (b) Free-body with rotation of axes to simplify work.
4-2. Study each force acting at the end of the light strut in Fig. 3-19. Draw the appropriate free-body diagram. There is no particular advantage to rotating axes. …ver más…

The ends of three 8-ft studs are nailed together forming a tripod with an apex that is 6ft above the ground. What is the compression in each of these studs if a 100-lb weight is hung from the apex? Three upward components Fy hold up the 100 lb weight: 3 Fy = 100 lb; Fy = 33.3 lb sin φ = (6/8); φ = 48.90 F sin 48.90 = 33.3 lb; [pic] F = 44.4 lb, compression
*4-14. A 20-N picture is hung from a nail as in Fig. 4-20, so that the supporting cords make an angle of 600. What is the tension of each cord segment? According to Newton’s third law, the force of frame on nail (20 N) is the same as the force of the nail on the rope (20 N , up). (Fy = 0; 20 N = Ty + Ty; 2Ty = 20 N; Ty = 10 N Ty = T sin 600; So T sin 600 = 10 N, and T = 11.5 N.
Friction

4-15. A horizontal force of 40 N will just start an empty 600-N sled moving across packed snow. After motion is begun, only 10 N is needed to keep motion at constant speed. Find the coefficients of static and kinetic friction. [pic] (s = 0.0667; (k = 0.016
4-16. Suppose 200-N of supplies are added the sled in Problem 4-13. What new force is needed to drag the sled at constant speed? N= 200 N + 600 N = 800 N; Fk = (kN = (0.0167)(800 N); Fk = 13.3 N
4-17. Assume surfaces where (s = 0.7 and (k = 0.4. What horizontal force is needed to just start a 50-N block moving along a wooden floor.