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Moments (Couples)

Statics · FE Reference Handbook section

Statics
3 formulas
10 exam-style examples
~51 min
All Statics lectures

Handbook notes for this section

Definitions and conditions exactly as the handbook states them.

  • A system of two forces that are equal in magnitude, opposite in direction, and parallel to each other is called a couple. A moment
  • M is defined as the cross product of the radius vector r and the force F from a point to the line of action of the force.

Core formulas for this FE topic

Definitions, applicability, units, assumptions and worked examples for each relation.

Worked exam-style examples

The four ways this section is written on the real exam — thoughts first, then equations, then substitution.

Example 1
Moment of a force — solve for moment — Moments (Couples)

A statics problem uses Moment of a force. Given force (F) = 659.0 lb; moment arm (d) = 11.8000 ft, determine the moment (M) in lb·ft.

Given

  • force(F)=659.0lbforce (F) = 659.0 lb
  • momentarm(d)=11.8000ftmoment arm (d) = 11.8000 ft

Find

moment (M), in lb·ft

Start with the thinking

  • The governing relation printed in this handbook section is Moment of a force.
  • Everything except M is given, so isolate M symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Statics items reward recognising the unknown before touching a calculator.

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=F×dM = F \times d
  2. Step 2 — Rearrange the relation so that M stands alone on the left-hand side.

  3. Step 3

    Listthegivens:force(F)=659.0lb,momentarm(d)=11.8000ftList the givens: force (F) = 659.0 lb, moment arm (d) = 11.8000 ft
  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    M=7776 lb⋅ftM = 7776\ \text{lb·ft}
  6. Step 6 — Check: returning M = 7,776 lb·ft to

    M=F×dM = F \times d

    reproduces the given quantities, and both sides carry the same units.

Answer:
M=7776 lb⋅ftM = 7776\ \text{lb·ft}

Why the other options are there

  • 15,552 — kept a factor of two that cancels in the correct rearrangement.
  • 3,888 — dropped that same factor in the other direction.
  • 8,554 — rounded an intermediate value before the final step.

Reference: FE Reference Handbook — Statics → Moments (Couples)

Example 2
Moment of a force about a point — solve for moment — Moments (Couples) (2)

a cantilevered sign arm Given force (F) = 3,153 N; perpendicular distance (d) = 3.9000 m, determine the moment (M) in N\cdot m.

Given

  • force(F)=3,153Nforce (F) = 3,153 N
  • perpendiculardistance(d)=3.9000mperpendicular distance (d) = 3.9000 m

Find

moment (M), in N\cdot m

Start with the thinking

  • The governing relation printed in this handbook section is Moment of a force about a point.
  • Everything except M is given, so isolate M symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • A force acting at a perpendicular distance d produces a moment.

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=FdM = F d
  2. Step 2 — Rearrange symbolically for M:

    M=M=FdM = M = F d
  3. Step 3

    Listthegivens:force(F)=3,153N,perpendiculardistance(d)=3.9000mList the givens: force (F) = 3,153 N, perpendicular distance (d) = 3.9000 m
  4. Step 4 — Substitute the given values:

    M=M=31533.9000M = M = 3153 3.9000
  5. Step 5 — Evaluate:

    M=12295 N\cdotmM = 12295\ \text{N\cdot m}
  6. Step 6 — Check: returning M = 12,295 N\cdot m to

    M=FdM = F d

    reproduces the given quantities, and both sides carry the same units.

Answer:
M=12295 N\cdotmM = 12295\ \text{N\cdot m}

Why the other options are there

  • 24,590 — kept a factor of two that cancels in the correct rearrangement.
  • 6,147 — dropped that same factor in the other direction.
  • 13,524 — rounded an intermediate value before the final step.

Reference: FE Handbook — Statics (Moments)

Example 3
Moment of a couple — solve for couple moment — Moments (Couples) (3)

Two tugboat lines apply a couple to rotate a barge. Given couple force (F) = 242.0 lbf; perpendicular distance (d) = 2.0000 ft, determine the couple moment (M) in lbf-ft.

Given

  • coupleforce(F)=242.0lbfcouple force (F) = 242.0 lbf
  • perpendiculardistance(d)=2.0000ftperpendicular distance (d) = 2.0000 ft

Find

couple moment (M), in lbf-ft

Start with the thinking

  • The governing relation printed in this handbook section is Moment of a couple.
  • Everything except M is given, so isolate M symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • A couple made of two equal, opposite, parallel forces produces a pure moment used to analyze moments (couples) on rigid bodies.
FFPlate

Figure 3 — schematic for Moment of a couple — solve for couple moment — Moments (Couples) (3)

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=FdM = F d
  2. Step 2 — Rearrange symbolically for M:

    M=FdM = F d
  3. Step 3

    Listthegivens:coupleforce(F)=242.0lbf,perpendiculardistance(d)=2.0000ftList the givens: couple force (F) = 242.0 lbf, perpendicular distance (d) = 2.0000 ft
  4. Step 4 — Substitute the given values:

    M=242.02.0000M = 242.0 2.0000
  5. Step 5 — Evaluate:

    M=484.0 lbf-ftM = 484.0\ \text{lbf-ft}
  6. Step 6 — Check: returning M = 484.0 lbf-ft to

    M=FdM = F d

    reproduces the given quantities, and both sides carry the same units.

Answer:
M=484.0 lbf-ftM = 484.0\ \text{lbf-ft}

Why the other options are there

  • 968.0 — kept a factor of two that cancels in the correct rearrangement.
  • 242.0 — dropped that same factor in the other direction.
  • 532.4 — rounded an intermediate value before the final step.

Reference: FE Handbook — Statics: Moments (Couples)

Example 4
Moment of a force — solve for force — Moments (Couples) (4)

A statics problem uses Moment of a force. Given moment arm (d) = 9.9000 ft; moment (M) = 2,481 lb·ft, determine the force (F) in lb.

Given

  • momentarm(d)=9.9000ftmoment arm (d) = 9.9000 ft
  • moment (M) = 2,481 lb·ft

Find

force (F), in lb

Start with the thinking

  • The governing relation printed in this handbook section is Moment of a force.
  • Everything except F is given, so isolate F symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Statics items reward recognising the unknown before touching a calculator.

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=F×dM = F \times d
  2. Step 2 — Rearrange the relation so that F stands alone on the left-hand side.

  3. Step 3 — List the givens: moment arm (d) = 9.9000 ft, moment (M) = 2,481 lb·ft.

  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    F=250.6 lbF = 250.6\ \text{lb}
  6. Step 6 — Check: returning F = 250.6 lb to

    M=F×dM = F \times d

    reproduces the given quantities, and both sides carry the same units.

Answer:
F=250.6 lbF = 250.6\ \text{lb}

Why the other options are there

  • 501.2 — kept a factor of two that cancels in the correct rearrangement.
  • 125.3 — dropped that same factor in the other direction.
  • 275.7 — rounded an intermediate value before the final step.

Reference: FE Reference Handbook — Statics → Moments (Couples)

Example 5
Moment of a force about a point — solve for force — Moments (Couples) (5)

a wrench applied to an anchor nut Given moment (M) = 3,106 N\cdot m; perpendicular distance (d) = 3.2000 m, determine the force (F) in N.

Given

  • moment(M)=3,106N⋅mmoment (M) = 3,106 N\cdot m
  • perpendiculardistance(d)=3.2000mperpendicular distance (d) = 3.2000 m

Find

force (F), in N

Start with the thinking

  • The governing relation printed in this handbook section is Moment of a force about a point.
  • Everything except F is given, so isolate F symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • A force acting at a perpendicular distance d produces a moment.

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=FdM = F d
  2. Step 2 — Rearrange symbolically for F:

    F=F=MdF = F = \dfrac{M}{d}
  3. Step 3

    Listthegivens:moment(M)=3,106N⋅m,perpendiculardistance(d)=3.2000mList the givens: moment (M) = 3,106 N\cdot m, perpendicular distance (d) = 3.2000 m
  4. Step 4 — Substitute the given values:

    F=F=31063.2000F = F = \dfrac{3106}{3.2000}
  5. Step 5 — Evaluate:

    F=970.7 NF = 970.7\ \text{N}
  6. Step 6 — Check: returning F = 970.7 N to

    M=FdM = F d

    reproduces the given quantities, and both sides carry the same units.

Answer:
F=970.7 NF = 970.7\ \text{N}

Why the other options are there

  • 1,941 — kept a factor of two that cancels in the correct rearrangement.
  • 485.4 — dropped that same factor in the other direction.
  • 1,068 — rounded an intermediate value before the final step.

Reference: FE Handbook — Statics (Moments)

Example 6
Moment of a couple — solve for couple force — Moments (Couples) (6)

A steering wheel resists a couple applied by the driver's hands. Given perpendicular distance (d) = 3.3000 ft; couple moment (M) = 928.5 lbf-ft, determine the couple force (F) in lbf.

Given

  • perpendiculardistance(d)=3.3000ftperpendicular distance (d) = 3.3000 ft
  • couplemoment(M)=928.5lbf−ftcouple moment (M) = 928.5 lbf-ft

Find

couple force (F), in lbf

Start with the thinking

  • The governing relation printed in this handbook section is Moment of a couple.
  • Everything except F is given, so isolate F symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • A couple made of two equal, opposite, parallel forces produces a pure moment used to analyze moments (couples) on rigid bodies.
FFPlate

Figure 6 — schematic for Moment of a couple — solve for couple force — Moments (Couples) (6)

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=FdM = F d
  2. Step 2 — Rearrange symbolically for F:

    F=MdF = \dfrac{M}{d}
  3. Step 3

    Listthegivens:perpendiculardistance(d)=3.3000ft,couplemoment(M)=928.5lbf−ftList the givens: perpendicular distance (d) = 3.3000 ft, couple moment (M) = 928.5 lbf-ft
  4. Step 4 — Substitute the given values:

    F=928.53.3000F = \dfrac{928.5}{3.3000}
  5. Step 5 — Evaluate:

    F=281.4 lbfF = 281.4\ \text{lbf}
  6. Step 6 — Check: returning F = 281.4 lbf to

    M=FdM = F d

    reproduces the given quantities, and both sides carry the same units.

Answer:
F=281.4 lbfF = 281.4\ \text{lbf}

Why the other options are there

  • 562.7 — kept a factor of two that cancels in the correct rearrangement.
  • 140.7 — dropped that same factor in the other direction.
  • 309.5 — rounded an intermediate value before the final step.

Reference: FE Handbook — Statics: Moments (Couples)

Example 7
Moment of a force — solve for moment arm — Moments (Couples) (7)

A statics problem uses Moment of a force. Given force (F) = 760.0 lb; moment (M) = 6,179 lb·ft, determine the moment arm (d) in ft.

Given

  • force(F)=760.0lbforce (F) = 760.0 lb
  • moment (M) = 6,179 lb·ft

Find

moment arm (d), in ft

Start with the thinking

  • The governing relation printed in this handbook section is Moment of a force.
  • Everything except d is given, so isolate d symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Statics items reward recognising the unknown before touching a calculator.

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=F×dM = F \times d
  2. Step 2 — Rearrange the relation so that d stands alone on the left-hand side.

  3. Step 3 — List the givens: force (F) = 760.0 lb, moment (M) = 6,179 lb·ft.

  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    d=8.1303 ftd = 8.1303\ \text{ft}
  6. Step 6 — Check: returning d = 8.1303 ft to

    M=F×dM = F \times d

    reproduces the given quantities, and both sides carry the same units.

Answer:
d=8.1303 ftd = 8.1303\ \text{ft}

Why the other options are there

  • 16.2605 — kept a factor of two that cancels in the correct rearrangement.
  • 4.0651 — dropped that same factor in the other direction.
  • 8.9433 — rounded an intermediate value before the final step.

Reference: FE Reference Handbook — Statics → Moments (Couples)

Example 8
Moment of a force about a point — solve for perpendicular distance — Moments (Couples) (8)

a bracket bolted to a column flange Given moment (M) = 795.1 N\cdot m; force (F) = 1,909 N, determine the perpendicular distance (d) in m.

Given

  • moment(M)=795.1N⋅mmoment (M) = 795.1 N\cdot m
  • force(F)=1,909Nforce (F) = 1,909 N

Find

perpendicular distance (d), in m

Start with the thinking

  • The governing relation printed in this handbook section is Moment of a force about a point.
  • Everything except d is given, so isolate d symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • A force acting at a perpendicular distance d produces a moment.

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=FdM = F d
  2. Step 2 — Rearrange symbolically for d:

    d=d=MFd = d = \dfrac{M}{F}
  3. Step 3

    Listthegivens:moment(M)=795.1N⋅m,force(F)=1,909NList the givens: moment (M) = 795.1 N\cdot m, force (F) = 1,909 N
  4. Step 4 — Substitute the given values:

    d=d=795.11909d = d = \dfrac{795.1}{1909}
  5. Step 5 — Evaluate:

    d=0.4166 md = 0.4166\ \text{m}
  6. Step 6 — Check: returning d = 0.4166 m to

    M=FdM = F d

    reproduces the given quantities, and both sides carry the same units.

Answer:
d=0.4166 md = 0.4166\ \text{m}

Why the other options are there

  • 0.8332 — kept a factor of two that cancels in the correct rearrangement.
  • 0.2083 — dropped that same factor in the other direction.
  • 0.4583 — rounded an intermediate value before the final step.

Reference: FE Handbook — Statics (Moments)

Example 9
Moment of a couple — solve for perpendicular distance — Moments (Couples) (9)

A wrench applies a couple to loosen a rusted bolt. Given couple force (F) = 246.0 lbf; couple moment (M) = 2,477 lbf-ft, determine the perpendicular distance (d) in ft.

Given

  • coupleforce(F)=246.0lbfcouple force (F) = 246.0 lbf
  • couplemoment(M)=2,477lbf−ftcouple moment (M) = 2,477 lbf-ft

Find

perpendicular distance (d), in ft

Start with the thinking

  • The governing relation printed in this handbook section is Moment of a couple.
  • Everything except d is given, so isolate d symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • A couple made of two equal, opposite, parallel forces produces a pure moment used to analyze moments (couples) on rigid bodies.
FFPlate

Figure 9 — schematic for Moment of a couple — solve for perpendicular distance — Moments (Couples) (9)

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=FdM = F d
  2. Step 2 — Rearrange symbolically for d:

    d=MFd = \dfrac{M}{F}
  3. Step 3

    Listthegivens:coupleforce(F)=246.0lbf,couplemoment(M)=2,477lbf−ftList the givens: couple force (F) = 246.0 lbf, couple moment (M) = 2,477 lbf-ft
  4. Step 4 — Substitute the given values:

    d=2477246.0d = \dfrac{2477}{246.0}
  5. Step 5 — Evaluate:

    d=10.0671 ftd = 10.0671\ \text{ft}
  6. Step 6 — Check: returning d = 10.0671 ft to

    M=FdM = F d

    reproduces the given quantities, and both sides carry the same units.

Answer:
d=10.0671 ftd = 10.0671\ \text{ft}

Why the other options are there

  • 20.1341 — kept a factor of two that cancels in the correct rearrangement.
  • 5.0335 — dropped that same factor in the other direction.
  • 11.0738 — rounded an intermediate value before the final step.

Reference: FE Handbook — Statics: Moments (Couples)

Example 10
Moment of a force — solve for moment (case 2) — Moments (Couples) (10)

A statics problem uses Moment of a force. Given force (F) = 1,480 lb; moment arm (d) = 14.5000 ft, determine the moment (M) in lb·ft.

Given

  • force(F)=1,480lbforce (F) = 1,480 lb
  • momentarm(d)=14.5000ftmoment arm (d) = 14.5000 ft

Find

moment (M), in lb·ft

Start with the thinking

  • The governing relation printed in this handbook section is Moment of a force.
  • Everything except M is given, so isolate M symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Statics items reward recognising the unknown before touching a calculator.

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=F×dM = F \times d
  2. Step 2 — Rearrange the relation so that M stands alone on the left-hand side.

  3. Step 3

    Listthegivens:force(F)=1,480lb,momentarm(d)=14.5000ftList the givens: force (F) = 1,480 lb, moment arm (d) = 14.5000 ft
  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    M=21460 lb⋅ftM = 21460\ \text{lb·ft}
  6. Step 6 — Check: returning M = 21,460 lb·ft to

    M=F×dM = F \times d

    reproduces the given quantities, and both sides carry the same units.

Answer:
M=21460 lb⋅ftM = 21460\ \text{lb·ft}

Why the other options are there

  • 42,920 — kept a factor of two that cancels in the correct rearrangement.
  • 10,730 — dropped that same factor in the other direction.
  • 23,606 — rounded an intermediate value before the final step.

Reference: FE Reference Handbook — Statics → Moments (Couples)

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