Vector Mechanics for Engineers: Statics
Vector Mechanics for Engineers: Statics
12th Edition
ISBN: 9781259977268
Author: Ferdinand P. Beer, E. Russell Johnston Jr., David Mazurek
Publisher: McGraw-Hill Education
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Textbook Question
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Chapter 3.4, Problem 3.140P

A flagpole is guyed by three cables. If the tensions in the cables have the same magnitude P, replace the forces exerted on the pole with an equivalent wrench and determine (a) the resultant force R, (b) the pitch of the wrench, (c) the point where the axis of the wrench intersects the xz-plane.

Chapter 3.4, Problem 3.140P, A flagpole is guyed by three cables. If the tensions in the cables have the same magnitude P,

(a)

Expert Solution
Check Mark
To determine

The resultant force.

Answer to Problem 3.140P

The resultant force is (3P/25)[2i^20j^k^]_.

Explanation of Solution

The diagram for the force-couple system is given below:

Vector Mechanics for Engineers: Statics, Chapter 3.4, Problem 3.140P

Refer fig 1.

Write the equation of resultant force.

R=PλBA+PλDC+PλDE=P[λBA+λDC+λDE] (I)

Here, the resultant force is R, the magnitude of the forces are P, the direction of the force at points is λBA,λDC,λDE

Conclusion:

Substitute, (45j^35k^) for λBA, (35i^45j^) for λDC, (35i^45j^) for λDE in equation (I)

R=P[(45j^35k^)+(35i^45j^)+(35i^45j^)]=3P25[2i^20j^k^]

The magnitude of the resultant force,

|R|=(3P/25)2[(2)2+(20)2+(1)2]=(275/25)P

Thus, the resultant force is (3P/25)[2i^20j^k^]_.

(b)

Expert Solution
Check Mark
To determine

The pitch of the wrench.

Answer to Problem 3.140P

The pitch of the wrench is 0.0988a_.

Explanation of Solution

Write the equation of pitch of the wrench.

p=M1R (II)

Here, the pitch of the wrench is p, the momentum along wrench is M1

Since, the R and MoR are not in same direction, so the form of the wrench is,

M1=λaxisMoR (III)

Here, the constant is λaxis.

Write the expression for the constant is,

λaxis=RR

Write the equation of momentum.

MoR=(r1×F1)+(r2×F2)+(r3×F3) (IV)

Here, the momentum is MBR,MA,MB, the distance is r1.

Rewrite the expression for the momentum of the wrench is,

M1=(RR)[(r1×F1)+(r2×F2)+(r3×F3)] (V)

Conclusion:

Substitute, (3P/25)[2i^20j^k^] for R, (275/25)P for R, (24a)j^ for r1, (4P53P5)k^ for F1, (20a)j^ for r2, (3P5i^4P5j^) for F2, (20a)j^ for r3, (9P25i^4P5j^+12P25k^) for F3 in equation (V).

M1=(3P/25)[2i^20j^k^](275/25)P[{(24a)j^×(4P53P5)k^}+{(20a)j^×(3P5i^4P5j^)}+{(20a)j^×(9P25i^4P5j^+12P25k^)}]=(8Pa675)(2i^20j^k^)=(8Pa155)

Substitute, (8Pa155) for M1, (275/25)P for R in equation (II)

p=(8Pa/155)(275/25)P=0.0988a

Thus, the pitch of the wrench is 0.0988a_.

(c)

Expert Solution
Check Mark
To determine

The point at which the axis of wrench intersects the xz-plane.

Answer to Problem 3.140P

The axis of wrench intersects the xz-plane at x=2a,z=1.99a_.

Explanation of Solution

Refer fig 1,

Write the equation for the force couple system for the wrench.

MBR=M1+M2 (VI)

Here, the momentum is M2.

Write the expression for the momentum at which the wrench intersects the xz-plane.

M2=rQ/O×R (VII)

Here, the position vector is rQ/O.

Write the expression for the position vector is,

rQ/O=xi^+zk^

Here, the coordinates are x,z.

Conclusion:

Substitute, (24a)j^ for r1, (4P53P5)k^ for F1, (20a)j^ for r2, (3P5i^4P5j^) for F2, (20a)j^ for r3, (9P25i^4P5j^+12P25k^) for F3 in equation (IV).

MoR=[{(24a)j^×(4P53P5)k^}+{(20a)j^×(3P5i^4P5j^)}+{(20a)j^×(9P25i^4P5j^+12P25k^)}]=[(24Pa5)(i^k^)]

Substitute, [(24Pa5)(i^k^)] for MoR, (8Pa675)(2i^20j^k^) for M1 in equation (VI).

[(24Pa5)(i^k^)]=[(8Pa675)(2i^20j^k^)]+M2M2={(8Pa675)[430i^20j^406k^]}

Substitute, {(8Pa675)[430i^20j^406k^]} for M2, {(3P25)[2i^+20j^k^]} for R, (xi^+zk^) for rQ/O in equation (VII).

{(8Pa675)[430i^20j^406k^]}=(xi^+zk^)×{(3P25)[2i^+20j^k^]}

Comparing the coefficients of the x and z components both sides,

x=2a,z=1.99a

Therefore, he axis of wrench intersects the xz-plane at x=2a,z=1.99a_.

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Chapter 3 Solutions

Vector Mechanics for Engineers: Statics

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