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solution mecanica vetorial para engenheiro cap 5, Exercícios de Mecânica dos sólidos

solution mecanica vetorial para engenheiro cap 5

Tipologia: Exercícios

2020

Compartilhado em 13/08/2020

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Baixe solution mecanica vetorial para engenheiro cap 5 e outras Exercícios em PDF para Mecânica dos sólidos, somente na Docsity! PROBLEM 5.1 Locate the centroid of the plane area shown. SOLUTION 2, inA , in.x , in.y 3, inxA 3, inyA 1 8 6 48× = 4− 9 192− 432 2 16 12 192× = 8 6 1536 1152 Σ 240 1344 1584 Then 3 2 1344 in 240 in xAX A Σ = = Σ or 5.60 in.X = and 3 2 1584 in 240 in yAY A Σ = = Σ or 6.60 in.Y = PROBLEM 5.2 Locate the centroid of the plane area shown. SOLUTION 2, mmA , mmx , mmy 3, mmxA 3, mmyA 1 1 60 75 2250 2 × × = 40 25 90 000 56 250 2 105 75 7875× = 112.5 37.5 885 900 295 300 Σ 10 125 975 900 351 600 Then 3 2 975 900 mm 10 125 mm xAX A Σ = = Σ or 96.4 mmX = and 3 2 351 600 mm 10 125 mm yAY A Σ = = Σ or 34.7 mmY = PROBLEM 5.5 Locate the centroid of the plane area shown. SOLUTION 2, mmA , mmx , mmy 3, mmxA 3, mmyA 1 120 200 24 000× = 60 120 1 440 000 2 880 000 2 ( )260 5654.9 2 π − = − 94.5 120 534 600− 678 600− Σ 18 345 905 400 2 201 400 Then 3 2 905 400 mm 18 345 mm xAX A Σ = = Σ or 49.4 mmX = and 3 2 2 201 400 mm 18 345 mm yAY A Σ = = Σ or 93.8 mmY = PROBLEM 5.6 Locate the centroid of the plane area shown. SOLUTION 2, inA , in.x , in.y 3, inxA 3, inyA 1 ( )29 63.617 4 π = ( ) ( ) 4 9 3.8917 3π − = − 3.8917 243− 243 2 ( )( ) 1 15 9 67.5 2 = 5 3 337.5 202.5 Σ 131.1 94.5 445.5 Then 3 2 94.5 in 131.1 in xAX A Σ = = Σ or 0.721 in.X = and 3 2 445.5 in 131.1 in yAY A Σ = = Σ or 3.40 in.Y = PROBLEM 5.7 Locate the centroid of the plane area shown. SOLUTION First note that symmetry implies X Y= 2, mmA , mmx 3, mmxA 1 40 40 1600× = 20 32 000 2 2(40) 1257 4 π − = − 16.98 21 330− Σ 343 10 667 Then 3 2 10 667 mm 343 mm xAX A Σ = = Σ or 31.1 mmX = and 31.1 mmY X= = PROBLEM 5.9 CONTINUED Then 2(16 9 ) (16 9 ) 4(16)(16 9 ) 2(16) p π π π− − ± − − − = or 0.5726 1.3397p p= − = Taking the positive root 2 1 1.340r r = PROBLEM 5.10 Show that as 1r approaches 2,r the location of the centroid approaches that of a circular arc of radius ( )1 2 / 2.r r+ SOLUTION First, determine the location of the centroid. From Fig. 5.8A: ( ) ( ) ( ) 2 2 2 2 2 22 2 sin2 3 y r A r π π π α α α − = = − − ( )2 2 2 cos 3 r π α α = − Similarly ( ) ( ) 2 1 1 1 12 2 2 cos 3 y r A rπ π α α α = = − − ( ) ( ) ( ) ( ) ( ) 2 2 2 2 1 12 2 2 2 3 3 2 1 2 cos 2 cosThen 3 3 2 cos 3 yA r r r r r r π π π π α αα α α α α    Σ = − − −   − − = − ( ) 2 2 2 1 2 2 2 1 and 2 2 2 A r r r r π πα α π α    Σ = − − −         = − −    ( ) ( )2 2 3 32 1 2 1 3 3 2 1 2 2 2 1 2 Now 2 cos 2 3 2 cos 3 Y A yA Y r r r r r rY r r π π α α α α Σ = Σ   − − = −      − = −− PROBLEM 5.10 CONTINUED Using Figure 5.8B,Y of an arc of radius ( )1 2 1 is 2 r r+ ( ) ( )( ) 2 1 2 2 sin1 2 Y r r π π α α − = + − ( )1 2 2 1 cos( ) 2 r r π α α = + − (1) ( )( ) ( )( ) 2 23 3 2 1 2 1 2 12 1 2 2 2 1 2 12 1 2 2 2 1 2 1 2 1 Now r r r r r rr r r r r rr r r r r r r r − + +− = − +− + + = + 2 1 Let r r r r = + ∆ = − ∆ Then ( )1 2 1 2 r r r= + ( ) ( )( ) ( ) ( ) ( ) 2 23 3 2 1 2 2 2 1 2 2 and 3 2 r r r rr r r rr r r r + ∆ + + ∆ − ∆ + − ∆− = + ∆ + − ∆− + ∆ = 1 2In the limit as 0 (i.e., ), thenr r∆ → = 3 3 2 1 2 2 2 1 1 2 3 2 3 1 ( ) 2 2 r r r r r r r − = − = × + so that ( )1 2 2 2 3 cos 3 4 Y r r π α α = × + − or ( )1 2 2 1 cos 2 Y r r π α α = + − Which agrees with Eq. (1). PROBLEM 5.13 Locate the centroid of the plane area shown. SOLUTION 2, mmA , mmx , mmy 3, mmxA 3, mmyA 1 ( ) ( ) 2 40 80 2133 3 = 48 15 102 400 32 000 2 ( ) ( ) 1 40 80 1600 2 − = − 53.33 13.333 85 330− 21 330− Σ 533.3 17 067 10 667 Then X A XAΣ = Σ ( )2 3533.3 mm 17 067 mmX = or 32.0 mmX = and Y A yAΣ = Σ ( )2 3533.3 mm 10 667 mmY = or 20.0 mmY = PROBLEM 5.14 Locate the centroid of the plane area shown. SOLUTION 2, mmA , mmx , mmy 3, mmxA 3, mmyA 1 ( )( ) 2 150 240 24 000 3 = 56.25 96 1 350 000 2 304 000 2 ( ) ( ) 1 150 120 9000 2 − = − 50 40 450 000− 360 000− Σ 15 000 900 000 1 944 000 Then X A xAΣ = Σ ( )2 315 000 mm 900 000 mmX = or 60.0 mmX = and Y A yAΣ = Σ ( )215 000 mm 1 944 000Y = or 129.6 mmY = PROBLEM 5.15 Locate the centroid of the plane area shown. SOLUTION 2, inA , in.x , in.y 3, inxA 3, inyA 1 ( ) ( ) 1 10 15 50 3 = 4.5 7.5 225 375 2 ( ) 215 176.71 4 π = 6.366 16.366 1125 2892 Σ 226.71 1350 3267 Then X A x AΣ = Σ ( )2 3226.71 in 1350 inX = or 5.95 in.X = and Y A y AΣ = Σ ( )2 3226.71 in 3267 inY = or 14.41 in.Y = PROBLEM 5.18 The horizontal x axis is drawn through the centroid C of the area shown and divides the area into two component areas A1 and A2. Determine the first moment of each component area with respect to the x axis, and explain the results obtained. SOLUTION First, locate the position y of the figure. 2, mmA , mmy 3, mmyA 1 160 300 48 000× = 150 7 200 000 2 150 80 16 000− × = − 160 2 560 000− Σ 32 000 4 640 000 Then Y A y AΣ = Σ ( )2 332 000 mm 4 640 000 mmY = or 145.0 mmY = PROBLEM 5.18 CONTINUED ( ) I I 6 3 : 155 115 (160 155) 80 115 2 2 1.393 10 mm A Q yA= Σ  = × + − ×  = × ( ) ( ) II II 6 3 : 145 85 160 145 80 85 2 2 1.393 10 mm A Q yA= Σ  = − × − − ×   = − × ( )area I II Q 0x Q Q∴ = + = Which is expected since xQ yA yA= Σ = and 0y = , since x is a centroidal axis. PROBLEM 5.19 The first moment of the shaded area with respect to the x axis is denoted by .xQ (a) Express xQ in terms of r and .θ (b) For what value of θ is xQ maximum, and what is the maximum value? SOLUTION ( ) With and using Fig. 5.8 A,xa Q yA= Σ ( ) ( ) ( ) ( ) 2 3 2 2 2 2 3 2 3 2 sin 1sin 2 cos sin 2 2 cos cos sin 3 x r Q r r r r r π π π θ θ θ θ θ θ θ θ θ  −    = − − × ×  −     = − or 3 32 cos 3x Q r θ= ( )b By observation, is maximum when xQ 0θ = and then 32 3x Q r= PROBLEM 5.22 A thin, homogeneous wire is bent to form the perimeter of the figure indicated. Locate the center of gravity of the wire figure thus formed. SOLUTION First note that because the wire is homogeneous, its center of gravity will coincide with the centroid of the corresponding line. , mmL , mmx , mmy 2, mmxL 2, mmyL 1 165 82.5 0 13 612 0 2 75 165 37.5 12 375 2812 3 105 112.5 75 11 812 7875 4 2 260 75 96.05+ = 30 37.5 2881 3602 Σ 441.05 40 680 14 289 Then X L x LΣ = Σ 2(441.05 mm) 40 680 mmX = or 92.2 mmX = and Y L y LΣ = Σ 2(441.05 mm) 14 289 mmY = 32.4 mmY = PROBLEM 5.23 A thin, homogeneous wire is bent to form the perimeter of the figure indicated. Locate the center of gravity of the wire figure thus formed. SOLUTION First note that because the wire is homogeneous, its center of gravity will coincide with the centroid of the corresponding line. , mmL , mmx , mmy 2, mmxL 2, mmyL 1 2 212 6 13.416+ = 6 3 80.50 40.25 2 16 12 14 192 224 3 21 1.5 22 31.50 462 4 16 9− 14 144− 224 5 2 26 9 10.817+ = 4.5− 3 48.67− 32.45 Σ 77.233 111.32 982.7 Then X L x LΣ = Σ 2(77.233 mm) 111.32 mmX = or 1.441 mmX = and Y L y LΣ = Σ 2(77.233 mm) 982.7 mmY = or 12.72 mmY = PROBLEM 5.24 A thin, homogeneous wire is bent to form the perimeter of the figure indicated. Locate the center of gravity of the wire figure thus formed. SOLUTION First note that because the wire is homogeneous, its center of gravity will coincide with the centroid of the corresponding line. By symmetry 0X = , in.L , in.y 2, inyL 1 2 0 0 2 ( )6π ( )2 6 3.820 π = 72 3 2 0 0 4 ( )4π ( )2 4 2.546 π = 32 Σ 35.416 104 Then Y L y LΣ = Σ 2(35.416 in.) 104 inY = or 2.94 in.Y = PROBLEM 5.27 The homogeneous wire ABCD is bent as shown and is supported by a pin at B. Knowing that 30 ,θ = ° determine the length l for which portion CD of the wire is horizontal. SOLUTION First note that for equilibrium, the center of gravity of the wire must lie on a vertical line through B. Further, because the wire is homogeneous, its center of gravity will coincide with the centroid of the corresponding line. Thus 0, which implies that 0BM xΣ = = or 0i ix LΣ = Hence ( ) ( ) ( )2 6 in. cos30 6 in. sin 30 6 in.π π   − ° + ° ×    ( ) ( ) in. cos30 in. 2 l l   + °    ( ) ( )6 in.in. cos30 6 in. 0 2 l + ° − =   or 2 12.0 316.16 0l l+ − = 1with roots 12.77 and 24.77.l = − Taking the positive root 12.77 in.l = PROBLEM 5.28 The homogeneous wire ABCD is bent as shown and is attached to a hinge at C. Determine the length L for which the portion BCD of the wire is horizontal. SOLUTION First note that for equilibrium, the center of gravity of the wire must lie on a vertical line through C. Further, because the wire is homogeneous, its center of gravity will coincide with the centroid of the corresponding line. Thus 0, which implies that 0CM xΣ = = or 0i ix LΣ = ( ) ( ) ( ) ( ) ( )Hence 4 in. 8 in. 4 in. 10 in. 0 2 L L + − + − = or 2 2144 inL = or 12.00 in.L = PROBLEM 5.29 Determine the distance h so that the centroid of the shaded area is as close to line BB′ as possible when (a) 0.2,k = (b) 0.6.k = SOLUTION Then yAy A Σ = Σ ( ) ( ) ( ) ( ) 2 2 or a ha ab kb a h y ba kb a h  +  − −     = − − ( ) 2 211 2 (1 ) a k kh a k kh − + = − + Let 1 and hc k a ζ= − = Then 2 2 a c ky c k ζ ζ + = + (1) Now find a value of ζ (or h) for which y is minimum: ( ) ( ) ( ) 2 2 2 0 2 k c k k c kdy a d c k ζ ζ ζ ζ ζ + − + = = + or ( ) ( )22 0c k c kζ ζ ζ+ − + = (2) PROBLEM 5.31 Determine by direct integration the centroid of the area shown. Express your answer in terms of a and h. SOLUTION For the element of area (EL) shown hy x a = ( )and 1 dA h y dx xh dx a = −  = −    ( ) Then 1 2 1 2 EL EL x x y h y h x a = = +  = +   2 0 0 1Then area 1 2 2 a a x xA dA h dx h x ah a a   = = − = − =       ∫ ∫ 2 3 2 0 0 2 2 20 0 2 3 2 2 0 1and 1 2 3 6 1 1 1 2 2 1 2 33 a a EL a a EL a x x xx dA x h dx h a h a a h x x h xy dA h dx dx a a a h xx ah a    = − = − =               = + − = −                = − =     ∫ ∫ ∫ ∫ ∫ 2 Hence 1 1 2 6 ELxA x dA x ah a h =   =    ∫ 1 3 x a= 21 1 2 3 ELyA y dA y ah ah =   =    ∫ 2 3 y h= PROBLEM 5.32 Determine by direct integration the centroid of the area shown. Express your answer in terms of a and h. SOLUTION For the element (EL) shown At 3, :x a y h h ka= = = or 3 hk a = Then 1/31/3 ax y h = 1/3 1/3 Now dA xdy a y dy h = = 1/3 1/ 3 1 1 , 2 2EL EL ax x y y y h = = = Then ( )1/3 4/31/3 1/30 0 3 3 4 4 h h a aA dA y dy y ah h h = = = =∫ ∫ 1/3 1/3 5/3 2 1/3 1/3 2/30 0 1 1 3 3and 2 2 5 10 h h EL a a ax dA y y dy y a h h h h    = = =        ∫ ∫ 1/3 7/3 21/3 1/30 0 3 3 7 7 h h EL a ay dA y y dy y ah h h    = = =        ∫ ∫ Hence 23 3: 4 10EL xA x dA x ah a h = =    ∫ 2 5 x a= 23 3: 4 7EL yA y dA y ah ah = =    ∫ 4 7 y h= PROBLEM 5.33 Determine by direct integration the centroid of the area shown. Express your answer in terms of a and h. SOLUTION For the element (EL) shown At 31, :x a y h h k a= = = or 1 3 hk a = 32a k h= or 2 3 ak h = Hence, on line 1 3 3 hy x a = and on line 2 1/3 1/3 hy x a = Then 1/3 3 1/3 3 1/3 3 1/3 3 1 and 2EL h h h hdA x x dx y x x a a a a    = − = +       1/3 3 4/3 4 1/3 3 1/3 30 0 3 1 1 24 4 a a h hA dA x x dx h x x ah a a a a    ∴ = = − = − =        ∫ ∫ 1/3 3 7/3 5 2 1/3 3 1/3 30 0 3 1 8 357 5 a a EL h hx dA x x x dx h x x a h a a a a    = − = − =        ∫ ∫ 1/3 3 1/3 31/3 3 1/3 30 1 2 a EL h h h hy dA x x x x dx a a a a   = + −      ∫ ∫ 2 2/3 6 2 5/3 6 2 2/3 6 5/3 60 0 3 1 8 2 2 5 7 35 a ah x x h x xdx ah a a a a     = − = − =           ∫ From 28: 2 35EL ahxA x dA x a h = =    ∫ or 16 35 x a= and 28: 2 35EL ahyA y dA y ah = =    ∫ or 16 35 y h= PROBLEM 5.36 Determine by direct integration the centroid of the area shown. SOLUTION For the element (EL) shown 2 2by a x a = − ( ) ( )2 2 and dA b y dx b a a x dx a = − = − − ( ) ( )2 21; 2 2EL EL bx x y y b a a x a = = + = + − Then ( )2 20a bA dA a a x dxa= = − −∫ ∫ To integrate, let 2 2sin : a cos , cosx a x a dx a dθ θ θ θ= − = = ( )( )/20 /2 2 2 0 Then cos cos 2sin sin 1 2 4 4 bA a a a d a b a a ab a π π θ θ θ θ θ πθ = −     = − + = −          ∫ ( ) ( ) /2 3/22 2 2 2 2 0 0 3 1and 2 3 1 6 a EL b b ax dA x a a x dx x a x a a a b π     = − − = + −        = ∫ ∫ ( ) ( ) ( ) 2 2 2 2 0 2 2 3 2 2 2 20 0 2 1 3 62 2 a EL a a b by dA a a x a a x dx a a b b xx dx ab a a  = + − − −     = = =     ∫ ∫ ∫ 21: 1 4 6EL xA x dA x ab a bπ  = − =      ∫ or ( ) 2 3 4 ax π = − 21: 1 4 6EL yA y dA y ab abπ  = − =      ∫ or ( ) 2 3 4 by π = − PROBLEM 5.37 Determine by direct integration the centroid of the area shown. Express your answer in terms of a and b. SOLUTION For the element (EL) shown on line 1 at 22, x a b k a= = or 2 2 bk a = 22 by x a ∴ = On line 2 at 31, 2x a b k a= − = or 2 3 2bk a − = 3 3 2 by x a − ∴ = 2 3 2 3 2b bdA x x dx a a  = +   3 3 4 2 2 20 0 2 2Then 3 4 1 1 5 3 2 6 a a b x b x xA dA x dx x aa a ab ab     = = + = +        = + =   ∫ ∫ 4 5 2 3 2 2 3 20 0 2 2 2 1 2and 4 5 4 5 13 20 a a EL b b b x xx dA x x x dx a b aa a a a b     = + = + = +          = ∫ ∫ 2 3 2 3 2 3 2 30 2 2 2 5 2 3 7 2 3 4 20 0 2 5 2 1 2 2 2 1 2 2 2 52 7 1 2 13 10 7 70 a EL a a b b b by dA x x x x dx a a a a b b b xx x dx x a a a a b a ab     = − +                = − = −                 = − = −    ∫ ∫ ∫ Then 25 13 : 6 20EL xA x dA x ab a b = =    ∫ or 39 50 x a= 25 13: 6 70EL yA y dA y ab ab = −    ∫ or 39 175 y b= − PROBLEM 5.38 Determine by direct integration the centroid of the area shown. Express your answer in terms of a and b. SOLUTION At 0,x y b= = ( )20b k a= − or 2 bk a = Then ( )22 by x a a = − Now ( )22, 2 2EL EL y bx x y x a a = = = − and ( )22 bdA ydx x a dx a = = − ( ) ( )2 32 20 0 1Then 33 aa b bA dA x a dx x a ab a a  = = − = − =  ∫ ∫ ( ) ( ) ( ) ( ) ( ) 2 3 2 2 2 20 0 4 2 3 2 2 2 2 2 2 5 2 2 40 0 2 and 2 2 1 4 3 2 12 1 52 2 1 10 a a EL a a EL b bx dA x x a dx x ax a x dx a a b x aax x a b a b b by dA x a x a dx x a a a a ab  = − = − +     = − + =        = − − = −       = ∫ ∫ ∫ ∫ ∫ Hence 21 1: 3 12EL xA x dA x ab a b = =    ∫ 1 4 x a= 21 1: 3 10EL yA y dA y ab ab = =    ∫ 3 10 y b= PROBLEM 5.40 CONTINUED 22 2 2 20 0 232 5 2 4 0 2 2 2 2 2 2 2 5 2 3 17 30 a a EL aa a b b b x xy dA x x dx b dx a aa a b x b a x aa ab       = + − −                = + − −           = ∫ ∫ ∫ Hence 27 7: 6 6EL xA x dA x ab a b = =    ∫ x a= 27 17: 6 30EL yA y dA y ab ab = =    ∫ 17 35 y b= PROBLEM 5.41 Determine by direct integration the centroid of the area shown. Express your answer in terms of a and b. SOLUTION 2For y at , :x a y b= = 2a kb= or 2 ak b = Then 1/22 by x a = 1/2 1/2 2 2 Now and for 0 : , 2 2 2 EL EL x x a y b x xx y dA y dx b dx a a = ≤ ≤ = = = = For ( ) 1/2 1 2 1 1: 2 2 2 2EL a b x xx a y y y a a   ≤ ≤ = + = − +     ( ) 1/2 2 1 1 2 x xdA y y dx b dx aa   = − = − +   ( ) ( ) ( ) 1/2 1/2 /2 0 /2 /2 3/2 2 3/2 0 /2 3/2 3/2 3/2 2 2 1Then 2 2 2 1 3 3 2 2 2 3 2 2 1 1 2 2 2 2 a a a aa a x x xA dA b dx b dx aa a b x xx b x aa a b a aa a a ab a a a   = = + − +       = + − +          = + −                 + − − + −            ∫ ∫ ∫ 13 24 ab        = PROBLEM 5.41 CONTINUED ( ) ( ) ( ) 1/2 1/2 /2 0 /2 /2 5/2 3 4 5/2 0 /2 5/2 5/2 5/2 3 3 2 1and 2 2 2 5 5 3 4 2 5 2 2 1 1 3 2 4 a a EL a aa a x x xx dA x b dx x b dx aa a b x x xx b aa a b a aa a ab a a a      = + − +               = + − +          = + −             + − − + −       ∫ ∫ ∫ 2 2 1/2 1/2 /2 0 1/2 1/2 /2 /2 32 2 2 2 0 /2 2 71 240 2 1 1 2 2 2 1 1 1 2 2 2 2 3 2 4 a EL a a aa a a a b b x xy dA b dx a a b x x x xb dx a aa a b b x xx a a a a b               =   =          + − + − +                    = + − −           = ∫ ∫ ∫ ( ) 2 2 32 2 2 1 2 2 6 2 2 11 48 a a b aa a a ab       + − − −               = Hence 213 71: 24 240EL xA x dA x ab a b = =    ∫ 17 0.546 130 x a a= = 213 11: 24 48EL yA y dA y ab ab = =    ∫ 11 0.423 26 y b b= = PROBLEM 5.44 A homogeneous wire is bent into the shape shown. Determine by direct integration the x coordinate of its centroid. SOLUTION First note that because the wire is homogeneous, its center of gravity coincides with the centroid of the corresponding line Now 3cosELx a θ= and 2 2dL dx dy= + Where 3 2cos : 3 cos sinx a dx a dθ θ θ θ= = − 3 2sin : 3 sin cosy a dy a dθ θ θ θ= = ( ) ( ) ( ) 1/22 22 2 1/22 2 / 2 0 Then 3 cos sin 3 sin cos 3 cos sin cos sin 3 cos sin 1 3 cos sin 3 sin 2 dL a d a d a d a d L dL a d aπ θ θ θ θ θ θ θ θ θ θ θ θ θ θ θ θ θ θ  = − +   = + =  ∴ = = =    ∫ / 2 2 0 3 2 a π = ∫ ( )/ 2 30 /2 2 5 2 0 and cos 3 cos sin 1 33 cos 5 5 ELx dL a a d a a π π θ θ θ θ θ =  = − =   ∫ ∫ Hence 23 3 : 2 5EL xL x dL x a a = =    ∫ 2 5 x a= PROBLEM 5.44 CONTINUED Alternative solution 2/3 3 2 2/3 3 2 cos cos sin sin xx a a yy a a θ θ θ θ  = ⇒ =     = ⇒ =    ( ) ( ) ( ) 2/3 2/3 3/22/3 2/3 1/22/3 2/3 1/3 1 or Then x y y a x a a dy a x x dx −    ∴ + = = −       = − − ( ) ( ) 1/22 21/22/3 2/3 1/3 Now and 1 1 ELx x dydL dx a x x dx dx − =      = + = + − −          1/3 1/3 2/3 1/ 30 0 3 3Then 2 2 a a aL dL dx a x a x  = = = =   ∫ ∫ and 1/3 1/3 5/3 2 1/30 0 3 3 5 5 a a EL ax dL x dx a x a x    = = =       ∫ ∫ Hence 23 3 : 2 5EL xL x dL x a a = =    ∫ 2 5 x a= PROBLEM 5.45 Determine by direct integration the centroid of the area shown. SOLUTION 2 2Have cos cos 3 3 2 2sin sin 3 3 EL EL x r ae y r ae θ θ θ θ θ θ = = = = ( )( ) 2 21 1and 2 2 dA r rd a e dθθ θ= = Then ( )2 2 2 2 2 2 20 0 1 1 1 1 1 133.623 2 2 2 4 A dA a e d a e a e a π π θ θ πθ  = = = = − =   ∫ ∫ and 2 2 3 30 0 2 1 1cos cos 3 2 3EL x dA ae a e d a e dπ πθ θ θθ θ θ θ = =    ∫ ∫ ∫ To proceed, use integration by parts, with 3 3 and 3 cos and sin u e du e d dv d v θ θ θ θ θ θ = = = = Then ( )3 3 3cos sin sin 3e d e e dθ θ θθ θ θ θ θ= −∫ ∫ Now let 3 3 then 3u e du e dθ θ θ= = sin , then cosdv d vθ θ θ= = − Then ( )( )3 3 3 3sin sin 3 cos cos 3e d e e e dθ θ θ θθ θ θ θ θ θ− = − − − − ∫ ∫ So that ( ) 3 3 cos sin 3cos 10 ee d θ θ θ θ θ θ= +∫ ( ) ( ) 3 3 3 3 3 0 1 sin 3cos 3 3 1239.26 3 10 30EL e ax dA a e a πθ πθ θ   ∴ = + = − − = −    ∫ Also 2 2 3 30 0 2 1 1sin sin 3 2 3EL y dA ae a e d a e dπ πθ θ θθ θ θ θ = =    ∫ ∫ ∫ PROBLEM 5.46 CONTINUED Hence 2 3 2 2 3 1:EL L zxA x dA x L π π π    = = −        ∫ or 0.363x L= 2 3 2 2 2 3 1 2: 96EL L LyA y dA y π π π π    = = −        ∫ or 0.1653y L= PROBLEM 5.47 Determine the volume and the surface area of the solid obtained by rotating the area of Problem. 5.2 about (a) the x axis, (b) the line 165x = mm. SOLUTION From the solution to Problem 5.2: ( )2 area area10 125 mm , 96.4 mm, 34.7 mm AreaA X Y= = = From the solution to Problem 5.22: ( )line line441.05 mm 92.2 mm, 32.4 mm LineL X Y= = = Applying the theorems of Pappus-Guldinus, we have (a) Rotation about the x axis: ( )( ) 3 2lineArea 2 2 32.4 mm 441.05 mm 89.786 10 mmY Lπ π= = = × 3 289.8 10 mmA = × ( )( ) 6 3areaVolume 2 2 34.7 mm 10125 mm 2.2075 10 mmY Aπ π= = = × 6 32.21 10 mmV = × (b) Rotation about 165 mm:x = ( ) ( ) ( ) 5 2lineArea 2 165 2 165 92.2 mm 441.05 mm 2.01774 10 mmX Lπ π  = − = − = ×  6 20.202 10 mmA = × ( ) ( ) ( ) 6 3areaVolume 2 165 2 165 96.4 mm 10 125 mm 4.3641 10 mmX Aπ π  = − = − = ×  6 34.36 10 mmV = × PROBLEM 5.48 Determine the volume and the surface area of the solid obtained by rotating the area of Problem 5.4 about (a) the line 22y = mm, (b) the line 12x = mm. SOLUTION From the solution to Problem 5.4: 2 area area399 mm , 1.421 mm, 12.42 mm (Area)A X Y= = = From the solution to Problem 5.23: line line77.233 mm, 1.441 mm, 12.72 mm (Line)L X Y= = = Applying the theorems of Pappus-Guldinus, we have (a) Rotation about the line 22 mm:y = ( ) ( ) ( ) 2lineArea 2 22 2 22 12.72 mm 77.233 mm 4503 mmY Lπ π  = − = − =  3 24.50 10 mmA = × ( ) ( ) ( )2 3areaVolume 2 22 2 22 12.42 mm 399 mm 24 016.97 mmY Aπ π  = − = − =  3 324.0 10 mmV = × (b) Rotation about line 12 mm:x = ( ) ( ) ( ) 2lineArea 2 12 2 12 1.441 mm 77.233 mm 5124.45 mmX Lπ π  = − = − =  3 25.12 10 mmA = × ( ) ( ) ( )2 3Volume 2 12 1.421 2 12 1.421 mm 399 mm 26 521.46 mmAπ π  = − = − =  3 326.5 10 mmV = × PROBLEM 5.51 Determine the volume and the surface area of the chain link shown, which is made from a 2-in.-diameter bar, if 3R = in. and 10L = in. SOLUTION First note that the area A and the circumference C of the cross section of the bar are 2 and 4 A d C dπ π= = Observe that the semicircular ends of the link can be obtained by rotating the cross section through a horizontal semicircular arc of radius R. Then, applying the theorems of Pappus-Guldinus, we have ( ) ( ) ( ) ( ) ( )side endVolume 2 2 2 2 2V V AL RA L R Aπ π= + = + = + ( ) ( )2 3 2 10 in. 3 in. 2 in. 4 122.049 in ππ   = +     = 3122.0 inV = ( ) ( ) ( ) ( ) ( )side endArea 2 2 2 2 2A A CL RC L R Cπ π= + = + = + ( ) ( )2 10 in. 3 in. 4 in.π π   = +    2488.198 in= 2488 inA = PROBLEM 5.52 Verify that the expressions for the volumes of the first four shapes in Figure 5.21 on page 261 are correct. SOLUTION Following the second theorem of Pappus-Guldinus, in each case a specific generating area A will be rotated about the x axis to produce the given shape. Values of y are from Fig. 5.8A. (1) Hemisphere: the generating area is a quarter circle Have 242 2 3 4 aV yA aππ π π   = =       32or 3 V aπ= (2) Semiellipsoid of revolution: the generating area is a quarter ellipse Have 42 2 3 4 aV yA haππ π π   = =       22or 3 V a hπ= (3) Paraboloid of revolution: the generating area is a quarter parabola Have 3 22 2 8 3 V yA a ahπ π   = =       21or 2 V a hπ= (4) Cone: the generating area is a triangle Have 12 2 3 2 aV yA haπ π   = =       21or 3 V a hπ= PROBLEM 5.53 A 15-mm-diameter hole is drilled in a piece of 20-mm-thick steel; the hole is then countersunk as shown. Determine the volume of steel removed during the countersinking process. SOLUTION The required volume can be generated by rotating the area shown about the y axis. Applying the second theorem of Pappus-Guldinus, we have 5 12 2 7.5 mm 5 mm 5 mm 3 2 V xAπ π     = = + × × ×         3or 720 mmV = PROBLEM 5.56 The aluminum shade for a small high-intensity lamp has a uniform thickness of 3/32 in. Knowing that the specific weight of aluminum is 30.101 lb/in , determine the weight of the shade. SOLUTION The weight of the lamp shade is given by W V Atγ γ= = where A is the surface area of the shade. This area can be generated by rotating the line shown about the x axis. Applying the first theorem of Pappus-Guldinus, we have ( )1 1 2 2 3 3 4 42 2 2A yL yL y L y L y L y Lπ π π= = Σ = + + + ( ) ( ) ( ) 2 20.6 mm 0.60 0.752 0.6 mm mm 0.15 mm 1.5 mm 2 2 π  + = + × +    ( ) ( ) 2 20.75 1.25 mm 0.50 mm 0.40 mm 2 + + × +    ( ) ( )2 2 2 1.25 1.5 mm 0.25 mm 1.25 mm 2 22.5607 in + + × +      = Then 3 2 3lb/in0.101 22.5607 in in. 0.21362 lb 32 W ×= × = 0.214 lbW = PROBLEM 5.57 The top of a round wooden table has the edge profile shown. Knowing that the diameter of the top is 1100 mm before shaping and that the density of the wood is 3690 kg/m , determine the weight of the waste wood resulting from the production of 5000 tops. SOLUTION All dimensions are in mm ( ) ( ) waste blank top 2 6 3 blank top 1 2 3 4 Have 550 mm 30 mm 9.075 10 mm V V V V V V V V V π π = − = × = × = + + + Applying the second theorem of Pappus-Guldinus to parts 3 and 4 ( ) ( ) ( ) ( ) ( ) ( ) ( ) 2 2 top 2 2 6 3 6 3 529 mm 18 mm 535 mm 12 mm 4 122 535 mm 12 mm 3 4 4 182 529 mm 18 mm 3 4 5.0371 3.347 0.1222 0.2731 10 mm 8.8671 10 mm V π π ππ π ππ π π π    = × + ×         ×  + + ×           ×  + + ×         = + + + × = × ( ) 6 3waste 3 3 9.0750 8.8671 10 mm 0.2079 10 m V π π − ∴ = − × = × ( ) ( ) waste wood waste tops 3 3 3 2 Finally 690 kg/m 0.2079 10 m 9.81 m/s 5000 tops W V g Nρ π − = = × × × × wasteor 2.21 kNW = PROBLEM 5.58 The top of a round wooden table has the shape shown. Determine how many liters of lacquer are required to finish 5000 tops knowing that each top is given three coats of lacquer and that 1 liter of lacquer covers 12 m2. SOLUTION Referring to the figure in solution of Problem 5.57 and using the first theorem of Pappus-Guldinus, we have ( ) ( ) ( ) ( ) surface top circle bottom circle edge 2 2 3 2 535 mm 529 mm 2 122 535 mm 12 mm 2 2 182 529 mm 18 mm 2 617.115 10 mm A A A A π π ππ π ππ π π = + +    = +         ×  + + ×           ×  + + ×         = × surface tops coats 3 2 2 Then # liters Coverage 1 liter617.115 10 m 5000 3 12 m A N N π − = × × × = × × × × or # liters 2424 L= PROBLEM 5.60 The reflector of a small flashlight has the parabolic shape shown. Determine the surface area of the inside of the reflector. SOLUTION First note that the required surface area A can be generated by rotating the parabolic cross section through 2π radians about the x axis. Applying the first theorem of Pappus-Guldinus, we have 2A yLπ= ( )22Now, since , at : 7.5x ky x a a k= = = or 56.25a k= (1) At ( ) ( ) 215 mm: 15 12.5x a a k= + + = or 15 156.25a k+ = (2) Then Eq. (2) 15 156.25: or 8.4375 mm Eq. (1) 56.25 a k a a k + = = 1Eq. (1) 0.15 mm k⇒ = 20.15 and 0.3dxx y y dy ∴ = = 2 2Now 1 1 0.09dxdL dy y dy dy   = + = +    So 2 andA yL yL ydLπ= = ∫ ( ) 12.5 2 7.5 12.5 3/22 7.5 2 1 0.09 2 1 2 1 0.09 3 0.18 A y y dy y π π ∴ = +   = +      ∫ 21013 mm= 2or 1013 mmA = PROBLEM 5.61 For the beam and loading shown, determine (a) the magnitude and location of the resultant of the distributed load, (b) the reactions at the beam supports. SOLUTION ( )( ) ( )( ) 1 2 1 2 Resultant ( ) Have 40 lb/ft 18 ft 720 lb 1 120 lb/ft 18 ft 1080 lb 2 R R R a R R = + = = = = or 1800 lbR = The resultant is located at the centroid C of the distributed load x Have ( ) ( )( )( ) ( )( )( )1: 1800 lb 40 lb/ft 18 ft 9 ft 120 lb/ft 18 ft 12 ft2AM xΣ = + or 10.80 ftx = 1800 lb 10.80 ft R x = = ( )b 0: 0x xF AΣ = = 0: 1800 lb 0, 1800 lby y yF A AΣ = − = = 1800 lb∴ =A ( )( )0: 1800 lb 10.8 ft 0A AM MΣ = − = 19.444 lb ftAM = ⋅ or 19.44 kip ftA = ⋅M PROBLEM 5.62 For the beam and loading shown, determine (a) the magnitude and location of the resultant of the distributed load, (b) the reactions at the beam supports. SOLUTION ( )( ) ( )( ) I II ( ) Have 300 N/m 6 m 1800 N 1 6 m 900 N/m 1800 N 3 a R R = = = = Then I II:yF R R RΣ − = − − or 1800 N 1800 N 3600 NR = + = ( ) ( )( ) ( )( ): 3600 N 3 m 1800 N 4.5 m 1800 NAM xΣ − = − − or 3.75 mx = 3600 NR = 3.75 mx = (b) Reactions 0: 0x xF AΣ = = ( ) ( )( )0: 6 m 3600 N 3.75 m 0A yM BΣ = − = or 2250 NyB = 2250 N=B 0: 2250 N 3600 Ny yF AΣ = + = or 1350 NyA = 1350 N=A PROBLEM 5.65 Determine the reactions at the beam supports for the given loading. SOLUTION ( )( )I 1Have 200 N/m 0.12 m 12 N 2 R = = ( )( )II 200 N/m 0.2 m 40 NR = = Then 0: 0x xF AΣ = = 0: 18 N 12 N 40 N 0y yF AΣ = + − − = or 34 NyA = 34.0 N=A ( )( ) ( )( ) ( )( )0: 0.8 m 12 N 0.22 m 40 N 0.38 m 18 NA AM MΣ = − − + or 2.92 N mAM = ⋅ 2.92 N mA = ⋅M PROBLEM 5.66 Determine the reactions at the beam supports for the given loading. SOLUTION First, replace the given loading with the loading shown below. The two loadings are equivalent because both are defined by a linear relation between load and distance, and the values at the end points are the same. ( )( )IHave 1.8 m 2000 3600 NN/mR = = ( )( )II 1 1.8 m 4500 N/m 4050 N 2 R = = Then 0: 0x xF AΣ = = ( ) ( )( ) ( )( )0: 3 m 2.1 m 3600 N 2.4 m 4050 NB yM AΣ = − − + or 270 NyA = 270 N=A 0: 270 N 3600 N 4050 N 0y yF BΣ = − + − = or 720 NyB = 720 N=B PROBLEM 5.67 Determine the reactions at the beam supports for the given loading. SOLUTION ( )( )I 1Have 4 m 2000 kN/m 2667 N 3 R = = ( )( )II 1 2 m 1000 kN/m 666.7 N 3 R = = Then 0: 0x xF AΣ = = 0: 2667 N 666.7 N 0y yF AΣ = − − = or 3334 NyA = 3.33 kN=A ( )( ) ( )( )0: 1 m 2667 N 5.5 m 666.7 NA AM MΣ = − − or 6334 N mAM = ⋅ 6.33 kN mA = ⋅M PROBLEM 5.70 Determine (a) the distance a so that the vertical reaction at support B is minimum, (b) the corresponding reactions at the supports. SOLUTION ( )( )I 1( ) Have ft 120 lb/ft 60 lb 2 a R a a= = ( ) ( ) ( )II 1 12 ft 40 lb/ft 240 20 lb 2 R a a = − = −  Then ( ) ( ) ( )0: ft 60 lb 240 20 lb 8 ft 12 ft 03 3A y a aM a a B     Σ = − − − + + =          or 210 20 160 9 3y B a a= − + (1) Then 20 20 0 9 3 ydB a da = − = or 3.00 fta = ( ) Eq. (1)b ( ) ( ) 210 203.00 3.00 160 9 3y B = − + 150 lb= 150.0 lb=B and 0: 0x xF AΣ = = ( ) ( )0: 60 3.00 lb 240 20 3.00 lb 150 lb 0y yF A    Σ = − − − + =    or 210 lbyA = 210 lb=A PROBLEM 5.71 Determine the reactions at the beam supports for the given loading when 0 1.5 kN/m.w = SOLUTION ( )( )I 1Have 9 m 2 kN/m 9 kN 2 R = = ( )( )II 9 m 1.5 kN/m 13.5 kNR = = Then 0: 0x xF CΣ = = ( )( ) ( )( ) ( )0: 50 kN m 1 m 9 kN 2.5 m 13.5 kN 6 m 0B yM CΣ = − ⋅ − − + = or 15.4583 kNyC = 15.46 kN=C 0: 9 kN 13.5 kN 15.4583 0y yF BΣ = − − + = or 7.0417 kNyB = 7.04 kN=B PROBLEM 5.72 Determine (a) the distributed load 0w at the end D of the beam ABCD for which the reaction at B is zero, (b) the corresponding reactions at C. SOLUTION ( ) ( ) ( )I 0 0 1Have 9 m 3.5 kN/m 4.5 3.5 kN 2 R w w = − = −  ( )( )II 0 09 m kN/m 9 kNR w w= = ( ) Thena ( ) ( ) ( )( )0 00: 50 kN m 5 m 4.5 3.5 kN 3.5 m 9 kN 0CM w w Σ = − ⋅ + − + =  or 09 28.75 0w + = so 0 3.1944 kN/mw = − 0 3.19 kN/mw = Note: the negative sign means that the distributed force 0w is upward. ( )b 0: 0x xF CΣ = = ( ) ( )0: 4.5 3.5 3.19 kN 9 3.19 kN 0y yF CΣ = − + + + = or 1.375 kNyC = 1.375 kN=C PROBLEM 5.75 The cross section of a concrete dam is as shown. For a dam section of unit width, determine (a) the reaction forces exerted by the ground on the base AB of the dam, (b) the point of application of the resultant of the reaction forces of part a, (c) the resultant of the pressure forces exerted by the water on the face BC of the dam. In the following problems, use γ = 62.4 lb/ft3 for the specific weight of fresh water and γc = 150 lb/ft 3 for the specific weight of concrete if U.S. customary units are used. With SI units, use ρ = 103 kg/m3 for the density of fresh water and ρc = 2.40 × 10 3 kg/m3 for the density of concrete. (See the footnote on page 222 for how to determine the specific weight of a material given its density.) SOLUTION The free body shown consists of a 1-m thick section of the dam and the triangular section BCD of the water behind the dam. Note: 1 6 mX = ( )2 9 3 m 12 mX = + = ( )3 15 2 m 17 mX = + = ( )4 15 4 m 19 mX = + = ( ) Now so thata W gVρ= ( )( ) ( )( )( )3 21 12400 kg/m 9.81 m/s 9 m 15 m 1 m 1589 kN2W  = =   ( )( ) ( )( )( )3 22 2400 kg/m 9.81 m/s 6 m 18 m 1 m 2543 kNW  = =  ( )( ) ( )( )( )3 23 12400 kg/m 9.81 m/s 6 m 18 m 1 m 1271 kN2W  = =   ( )( ) ( )( )( )3 24 12400 kg/m 9.81 m/s 6 m 18 m 1 m 529.7 kN2W  = =   ( )( ) ( )( )( )3 3 21 1Also 18 m 1 m 10 kg/m 9.81 m/s 18 m2 2P Ap   = =     1589 kN= Then 0: 1589 kN 0xF HΣ = − = or 1589 kNH = 1589 kN=H 0: 1589 kN 2543 kN 1271 kN 529.7 kNyF VΣ = − − − − or 5933 kNV = 5.93 MN=V PROBLEM 5.75 CONTINUED ( ) Haveb ( ) ( )( )0: 5933 kN 6 m 1589 kNAM XΣ = + ( )( ) ( )( ) ( )( ) ( )( ) 6 m 1589 kN 12 m 2543 kN 17 m 1271 kN 19 m 529.7 0 − − − − = or 10.48 mX = 10.48 mX = to the right of A (c) Consider water section BCD as the free body. Have 0Σ =F Then 1675 kN− =R 18.43° or 1675 kN=R 18.43° Alternative solution to part (c) Consider the face BC of the dam. 2 2Have 6 18 18.9737 mBC = + = 6tan 18.43 18 θ θ= = ° ( ) ( )( )( )3 2 2 and 1000 kg/m 9.81 m/s 18 m 176.6 kN/m p g hρ= = = ( )( ) ( )21 1Then 18.97 m 1 m 176.6 kN/m2 2 1675 kN R Ap  = =   = 1675 kN∴ =R 18.43° PROBLEM 5.76 The cross section of a concrete dam is as shown. For a dam section of unit width, determine (a) the reaction forces exerted by the ground on the base AB of the dam, (b) the point of application of the resultant of the reaction forces of part a, (c) the resultant of the pressure forces exerted by the water on the face BC of the dam. SOLUTION The free body shown consists of a 1-ft thick section of the dam and the parabolic section of water above (and behind) the dam. Note ( )1 5 16 ft 10 ft 8 x = = ( )2 116 6 ft 19 ft 2 x  = + =   ( )3 122 12 ft 25 ft 4 x  = + =   ( )4 522 12 ft 29.5 ft 8 x  = + =   PROBLEM 5.78 The 9 12-ft× side of an open tank is hinged at its bottom A and is held in place by a thin rod. The tank is filled with glycerine, whose specific weight is 380 lb/ft . Determine the force T in the rod and the reactions at the hinge after the tank is filled to a depth of 8 ft. SOLUTION Consider the free-body diagram of the side. ( )1 1Have 2 2 P Ap A dγ= = ( )( ) ( )( )31 8 ft 12 ft 80 lb/ft 8 ft 30,720 lb2  = =  Then 0: 0y yF AΣ = = ( ) ( ) 80: 9 ft ft 30,720 lb 0 3A M T  Σ = − =    or 9102.22 lbT = 9.10 kips=T 0: 30,720 lb 9102.22 lb 0x xF AΣ = + − = or 21,618 lbA = − 21.6 kips=A PROBLEM 5.79 The friction force between a 2 2-m× square sluice gate AB and its guides is equal to 10 percent of the resultant of the pressure forces exerted by the water on the face of the gate. Determine the initial force needed to lift the gate that its mass is 500 kg. SOLUTION Consider the free-body diagram of the gate. ( ) ( )( )( )2 3 3 2I I1 1Now 2 2 m 10 kg/m 9.81 m/s 3 m2 2P Ap   = = ×    58.86 kN= ( ) ( )( )( )2 3 3 2II II1 1 2 2 m 10 kg/m 9.81 m/s 5 m2 2P Ap   = = ×    98.10 kN= ( )I IIThen 0.1 0.1F P P P= = + ( )0.1 58.86 98.10 kN= + 15.696 kN= Finally ( )( )20: 15.696 kN 500 kg 9.81 m/s 0yF TΣ = − − = or 20.6 kN=T PROBLEM 5.80 The dam for a lake is designed to withstand the additional force caused by silt which has settled on the lake bottom. Assuming that silt is equivalent to a liquid of density 3 31.76 10 kg/msρ = × and considering a 1-m-wide section of dam, determine the percentage increase in the force acting on the dam face for a silt accumulation of depth 1.5 m. SOLUTION First, determine the force on the dam face without the silt. Have ( ) 1 1 2 2w w P Ap A ghρ= = ( )( ) ( )( )( )3 3 21 6 m 1 m 10 kg/m 9.81 m/s 6 m2   =     176.58 kN= Next, determine the force on the dam face with silt. ( )( ) ( )( )( )3 3 21Have 4.5 m 1m 10 kg/m 9.81 m/s 4.5 m2wP  ′  =     99.326 kN= ( ) ( )( ) ( )( )( )3 3 2I 1.5 m 1 m 10 kg/m 9.81 m/s 4.5 msP   =     66.218 kN= ( ) ( )( ) ( )( )( )3 3 2II 1 1.5 m 1 m 1.76 10 kg/m 9.81 m/s 1.5 m2sP   = ×    19.424 kN= Then ( ) ( )I II 184.97 kNw s sP P P P′ ′= + + = The percentage increase, % inc., is then given by ( )184.97 176.58% inc. 100% 100% 4.7503% 176.58 w w P P P −′ − = × = × = % inc. 4.75%= PROBLEM 5.82 The square gate AB is held in the position shown by hinges along its top edge A and by a shear pin at B. For a depth of water 3.5d = m, determine the force exerted on the gate by the shear pin. SOLUTION First consider the force of the water on the gate. Have ( ) 1 1 2 2 P Ap A ghρ= = Then ( ) ( )( )( )2 3 3 2I 1 18 m 10 kg/m 9.81 m/s 1.7 m2P = 26.99 kN= ( ) ( )( )( )2 3 3 2II 1 18 m 10 kg/m 9.81 m/s 1.7 1.8cos30 m2P = × ° 51.74 kN= Now ( ) ( )I II 1 20: 0 3 3A AB AB AB B M L P L P L FΣ = + − = ( ) ( )1 2or 26.99 kN 51.74 kN 0 3 3 B F+ − = or 43.49 kNBF = 4.35 kNB =F 30.0° PROBLEMS 5.83 AND 5.84 Problem 5.83: A temporary dam is constructed in a 5-ft-wide fresh water channel by nailing two boards to pilings located at the sides of the channel and propping a third board AB against the pilings and the floor of the channel. Neglecting friction, determine the reactions at A and B when rope BC is slack. Problem 5.84: A temporary dam is constructed in a 5-ft-wide fresh water channel by nailing two boards to pilings located at the sides of the channel and propping a third board AB against the pilings and the floor of the channel. Neglecting friction, determine the magnitude and direction of the minimum tension required in rope BC to move board AB. SOLUTION First, consider the force of the water on the gate. Have ( ) 1 1 2 2 P Ap A hγ= = ( )( ) ( )( )3I 1So that 1.5 ft 5 ft 62.4 lb/ft 1.8 ft2P   =     421.2 lb= ( )( ) ( )( )3II 1 1.5 ft 5 ft 62.4 lb/ft 3 ft2P   =     702 lb= 5.83 Find the reactions at A and B when rope is slack. ( ) ( )( ) ( )( )0: 0.9 ft 0.5 ft 421.2 lb 1.0 ft 702 lb 0AM BΣ = − + + = or 1014 lbB = 1014 lb=B ( ) ( )4 40: 2 421.2 lb 702 lb 0 5 5x x F AΣ = + + = or 449.28 lbxA = − Note that the factor 2 (2 )xA is included since xA is the horizontal force exerted by the board on each piling. ( ) ( ) 3 30: 1014 lb 421.2 lb 702 lb 0 5 5y y F AΣ = − − + = or 340.08 lbyA = − 563 lb∴ =A 37.1° PROBLEMS 5.83 AND 5.84 CONTINUED 5.84 Note that there are two ways to move the board: 1. Pull upward on the rope fastened at B so that the board rotates about A. For this case 0→B and BCT is perpendicular to AB for minimum tension. 2. Pull horizontally at B so that the edge B of the board moves to the left. For this case 0yA → and the board remains against the pilings because of the force of the water. Case (1) ( )( )0: 1.5 0.5 ft 421.2 lbA BCM TΣ = − + ( )( )1.0 ft 702 lb 0+ = or 608.4 lbBCT = Case (2) ( )( ) ( )( )0: 1.2 ft 2 0.5 ft 702 lbB xM AΣ = − − ( )( )1.0 ft 421.2 lb 0− = or 2 643.5 lbxA = − ( )40: 643.5 lb 421.2 lb 5x BC F TΣ = − − + ( ) 4 702 lb 0 5 + = or 255.06 lbBCT = ( )min 255 lbBC∴ =T