Moments (Couples)
Statics · FE Reference Handbook section
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.
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
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
Step 1 — State the governing relation:
Step 2 — Rearrange the relation so that M stands alone on the left-hand side.
Step 3
Step 4 — Substitute the given values into the rearranged relation.
Step 5 — Evaluate:
Step 6 — Check: returning M = 7,776 lb·ft to
reproduces the given quantities, and both sides carry the same units.
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)
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
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
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for M:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning M = 12,295 N\cdot m to
reproduces the given quantities, and both sides carry the same units.
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)
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
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.
Figure 3 — schematic for Moment of a couple — solve for couple moment — Moments (Couples) (3)
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for M:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning M = 484.0 lbf-ft to
reproduces the given quantities, and both sides carry the same units.
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)
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
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
Step 1 — State the governing relation:
Step 2 — Rearrange the relation so that F stands alone on the left-hand side.
Step 3 — List the givens: moment arm (d) = 9.9000 ft, moment (M) = 2,481 lb·ft.
Step 4 — Substitute the given values into the rearranged relation.
Step 5 — Evaluate:
Step 6 — Check: returning F = 250.6 lb to
reproduces the given quantities, and both sides carry the same units.
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)
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
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
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for F:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning F = 970.7 N to
reproduces the given quantities, and both sides carry the same units.
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)
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
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.
Figure 6 — schematic for Moment of a couple — solve for couple force — Moments (Couples) (6)
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for F:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning F = 281.4 lbf to
reproduces the given quantities, and both sides carry the same units.
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)
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
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
Step 1 — State the governing relation:
Step 2 — Rearrange the relation so that d stands alone on the left-hand side.
Step 3 — List the givens: force (F) = 760.0 lb, moment (M) = 6,179 lb·ft.
Step 4 — Substitute the given values into the rearranged relation.
Step 5 — Evaluate:
Step 6 — Check: returning d = 8.1303 ft to
reproduces the given quantities, and both sides carry the same units.
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)
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
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
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for d:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning d = 0.4166 m to
reproduces the given quantities, and both sides carry the same units.
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)
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
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.
Figure 9 — schematic for Moment of a couple — solve for perpendicular distance — Moments (Couples) (9)
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for d:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning d = 10.0671 ft to
reproduces the given quantities, and both sides carry the same units.
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)
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
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
Step 1 — State the governing relation:
Step 2 — Rearrange the relation so that M stands alone on the left-hand side.
Step 3
Step 4 — Substitute the given values into the rearranged relation.
Step 5 — Evaluate:
Step 6 — Check: returning M = 21,460 lb·ft to
reproduces the given quantities, and both sides carry the same units.
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)