Centroids of Masses, Areas, Lengths, and Volumes
Statics · FE Reference Handbook section
Handbook notes for this section
Definitions and conditions exactly as the handbook states them.
- The following formulas are for discrete masses, areas, lengths, and volumes:
- The moment of area (Ma) is defined as
- The centroid of area is defined as
- The first moment of area with respect to the y-axis and the x-axis, respectively, are:
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.
An angle bracket made of two rectangular plates needs its centroid located. Given area 1 (A_1) = 19.0000 in^2; centroid 1 x (x_1) = 3.5000 in; area 2 (A_2) = 36.5000 in^2; centroid 2 x (x_2) = 14.2000 in, determine the composite centroid x (x_bar) in in.
Given
Find
composite centroid x (x_bar), in in
Start with the thinking
- The governing relation printed in this handbook section is Composite centroid (areas).
- Everything except x_bar is given, so isolate x_bar 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.
- Locating the centroid of masses, areas, lengths, and volumes for a composite shape by weighting each part's area and centroid.
Figure 1 — schematic for Composite centroid (areas) — solve for composite centroid x — Centroids of Masses, Areas, Lengths, and Volumes
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for x_bar:
Step 3 — List the givens: area 1 (A_1) = 19.0000 in^2, centroid 1 x (x_1) = 3.5000 in, area 2 (A_2) = 36.5000 in^2, centroid 2 x (x_2) = 14.2000 in.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning x_bar = 10.5369 in to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 21.0739 — kept a factor of two that cancels in the correct rearrangement.
- 5.2685 — dropped that same factor in the other direction.
- 11.5906 — rounded an intermediate value before the final step.
Reference: FE Handbook — Statics: Centroids of Masses, Areas, Lengths, and Volumes
A composite bridge deck cross section is split into rectangles to find the overall centroid. Given area 1 (A_1) = 24.0000 in^2; area 2 (A_2) = 22.0000 in^2; centroid 2 x (x_2) = 5.3000 in; composite centroid x (x_bar) = 8.5800 in, determine the centroid 1 x (x_1) in in.
Given
Find
centroid 1 x (x_1), in in
Start with the thinking
- The governing relation printed in this handbook section is Composite centroid (areas).
- Everything except x_1 is given, so isolate x_1 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.
- Locating the centroid of masses, areas, lengths, and volumes for a composite shape by weighting each part's area and centroid.
Figure 2 — schematic for Composite centroid (areas) — solve for centroid 1 x — Centroids of Masses, Areas, Lengths, and Volumes (2)
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for x_1:
Step 3 — List the givens: area 1 (A_1) = 24.0000 in^2, area 2 (A_2) = 22.0000 in^2, centroid 2 x (x_2) = 5.3000 in, composite centroid x (x_bar) = 8.5800 in.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning x_1 = 11.5867 in to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 23.1733 — kept a factor of two that cancels in the correct rearrangement.
- 5.7933 — dropped that same factor in the other direction.
- 12.7453 — rounded an intermediate value before the final step.
Reference: FE Handbook — Statics: Centroids of Masses, Areas, Lengths, and Volumes
A machined bracket's area is decomposed into simple shapes to compute the centroid. Given area 1 (A_1) = 36.0000 in^2; centroid 1 x (x_1) = 6.7000 in; area 2 (A_2) = 7.5000 in^2; composite centroid x (x_bar) = 2.1900 in, determine the centroid 2 x (x_2) in in.
Given
Find
centroid 2 x (x_2), in in
Start with the thinking
- The governing relation printed in this handbook section is Composite centroid (areas).
- Everything except x_2 is given, so isolate x_2 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.
- Locating the centroid of masses, areas, lengths, and volumes for a composite shape by weighting each part's area and centroid.
Figure 3 — schematic for Composite centroid (areas) — solve for centroid 2 x — Centroids of Masses, Areas, Lengths, and Volumes (3)
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for x_2:
Step 3 — List the givens: area 1 (A_1) = 36.0000 in^2, centroid 1 x (x_1) = 6.7000 in, area 2 (A_2) = 7.5000 in^2, composite centroid x (x_bar) = 2.1900 in.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning x_2 = -19.4580 in to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- -38.9160 — kept a factor of two that cancels in the correct rearrangement.
- -9.7290 — dropped that same factor in the other direction.
- -21.4038 — rounded an intermediate value before the final step.
Reference: FE Handbook — Statics: Centroids of Masses, Areas, Lengths, and Volumes
An angle bracket made of two rectangular plates needs its centroid located. Given area 1 (A_1) = 11.0000 in^2; centroid 1 x (x_1) = 7.0000 in; area 2 (A_2) = 26.5000 in^2; centroid 2 x (x_2) = 3.3000 in, determine the composite centroid x (x_bar) in in.
Given
Find
composite centroid x (x_bar), in in
Start with the thinking
- The governing relation printed in this handbook section is Composite centroid (areas).
- Everything except x_bar is given, so isolate x_bar 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.
- Locating the centroid of masses, areas, lengths, and volumes for a composite shape by weighting each part's area and centroid.
Figure 4 — schematic for Composite centroid (areas) — solve for composite centroid x (case 2) — Centroids of Masses, Areas, Lengths, and Volumes (4)
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for x_bar:
Step 3 — List the givens: area 1 (A_1) = 11.0000 in^2, centroid 1 x (x_1) = 7.0000 in, area 2 (A_2) = 26.5000 in^2, centroid 2 x (x_2) = 3.3000 in.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning x_bar = 4.3853 in to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 8.7707 — kept a factor of two that cancels in the correct rearrangement.
- 2.1927 — dropped that same factor in the other direction.
- 4.8239 — rounded an intermediate value before the final step.
Reference: FE Handbook — Statics: Centroids of Masses, Areas, Lengths, and Volumes
A composite bridge deck cross section is split into rectangles to find the overall centroid. Given area 1 (A_1) = 12.0000 in^2; area 2 (A_2) = 16.5000 in^2; centroid 2 x (x_2) = 9.0000 in; composite centroid x (x_bar) = 6.1100 in, determine the centroid 1 x (x_1) in in.
Given
Find
centroid 1 x (x_1), in in
Start with the thinking
- The governing relation printed in this handbook section is Composite centroid (areas).
- Everything except x_1 is given, so isolate x_1 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.
- Locating the centroid of masses, areas, lengths, and volumes for a composite shape by weighting each part's area and centroid.
Figure 5 — schematic for Composite centroid (areas) — solve for centroid 1 x (case 2) — Centroids of Masses, Areas, Lengths, and Volumes (5)
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for x_1:
Step 3 — List the givens: area 1 (A_1) = 12.0000 in^2, area 2 (A_2) = 16.5000 in^2, centroid 2 x (x_2) = 9.0000 in, composite centroid x (x_bar) = 6.1100 in.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning x_1 = 2.1363 in to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 4.2725 — kept a factor of two that cancels in the correct rearrangement.
- 1.0681 — dropped that same factor in the other direction.
- 2.3499 — rounded an intermediate value before the final step.
Reference: FE Handbook — Statics: Centroids of Masses, Areas, Lengths, and Volumes
A machined bracket's area is decomposed into simple shapes to compute the centroid. Given area 1 (A_1) = 5.5000 in^2; centroid 1 x (x_1) = 4.1000 in; area 2 (A_2) = 14.0000 in^2; composite centroid x (x_bar) = 12.8100 in, determine the centroid 2 x (x_2) in in.
Given
Find
centroid 2 x (x_2), in in
Start with the thinking
- The governing relation printed in this handbook section is Composite centroid (areas).
- Everything except x_2 is given, so isolate x_2 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.
- Locating the centroid of masses, areas, lengths, and volumes for a composite shape by weighting each part's area and centroid.
Figure 6 — schematic for Composite centroid (areas) — solve for centroid 2 x (case 2) — Centroids of Masses, Areas, Lengths, and Volumes (6)
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for x_2:
Step 3 — List the givens: area 1 (A_1) = 5.5000 in^2, centroid 1 x (x_1) = 4.1000 in, area 2 (A_2) = 14.0000 in^2, composite centroid x (x_bar) = 12.8100 in.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning x_2 = 16.2318 in to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 32.4636 — kept a factor of two that cancels in the correct rearrangement.
- 8.1159 — dropped that same factor in the other direction.
- 17.8550 — rounded an intermediate value before the final step.
Reference: FE Handbook — Statics: Centroids of Masses, Areas, Lengths, and Volumes
An angle bracket made of two rectangular plates needs its centroid located. Given area 1 (A_1) = 31.5000 in^2; centroid 1 x (x_1) = 1.3000 in; area 2 (A_2) = 24.5000 in^2; centroid 2 x (x_2) = 9.8000 in, determine the composite centroid x (x_bar) in in.
Given
Find
composite centroid x (x_bar), in in
Start with the thinking
- The governing relation printed in this handbook section is Composite centroid (areas).
- Everything except x_bar is given, so isolate x_bar 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.
- Locating the centroid of masses, areas, lengths, and volumes for a composite shape by weighting each part's area and centroid.
Figure 7 — schematic for Composite centroid (areas) — solve for composite centroid x (case 3) — Centroids of Masses, Areas, Lengths, and Volumes (7)
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for x_bar:
Step 3 — List the givens: area 1 (A_1) = 31.5000 in^2, centroid 1 x (x_1) = 1.3000 in, area 2 (A_2) = 24.5000 in^2, centroid 2 x (x_2) = 9.8000 in.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning x_bar = 5.0188 in to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 10.0375 — kept a factor of two that cancels in the correct rearrangement.
- 2.5094 — dropped that same factor in the other direction.
- 5.5206 — rounded an intermediate value before the final step.
Reference: FE Handbook — Statics: Centroids of Masses, Areas, Lengths, and Volumes
A composite bridge deck cross section is split into rectangles to find the overall centroid. Given area 1 (A_1) = 8.0000 in^2; area 2 (A_2) = 13.5000 in^2; centroid 2 x (x_2) = 5.0000 in; composite centroid x (x_bar) = 4.5200 in, determine the centroid 1 x (x_1) in in.
Given
Find
centroid 1 x (x_1), in in
Start with the thinking
- The governing relation printed in this handbook section is Composite centroid (areas).
- Everything except x_1 is given, so isolate x_1 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.
- Locating the centroid of masses, areas, lengths, and volumes for a composite shape by weighting each part's area and centroid.
Figure 8 — schematic for Composite centroid (areas) — solve for centroid 1 x (case 3) — Centroids of Masses, Areas, Lengths, and Volumes (8)
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for x_1:
Step 3 — List the givens: area 1 (A_1) = 8.0000 in^2, area 2 (A_2) = 13.5000 in^2, centroid 2 x (x_2) = 5.0000 in, composite centroid x (x_bar) = 4.5200 in.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning x_1 = 3.7100 in to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 7.4200 — kept a factor of two that cancels in the correct rearrangement.
- 1.8550 — dropped that same factor in the other direction.
- 4.0810 — rounded an intermediate value before the final step.
Reference: FE Handbook — Statics: Centroids of Masses, Areas, Lengths, and Volumes
A machined bracket's area is decomposed into simple shapes to compute the centroid. Given area 1 (A_1) = 4.5000 in^2; centroid 1 x (x_1) = 2.4000 in; area 2 (A_2) = 19.0000 in^2; composite centroid x (x_bar) = 9.9200 in, determine the centroid 2 x (x_2) in in.
Given
Find
centroid 2 x (x_2), in in
Start with the thinking
- The governing relation printed in this handbook section is Composite centroid (areas).
- Everything except x_2 is given, so isolate x_2 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.
- Locating the centroid of masses, areas, lengths, and volumes for a composite shape by weighting each part's area and centroid.
Figure 9 — schematic for Composite centroid (areas) — solve for centroid 2 x (case 3) — Centroids of Masses, Areas, Lengths, and Volumes (9)
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for x_2:
Step 3 — List the givens: area 1 (A_1) = 4.5000 in^2, centroid 1 x (x_1) = 2.4000 in, area 2 (A_2) = 19.0000 in^2, composite centroid x (x_bar) = 9.9200 in.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning x_2 = 11.7011 in to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 23.4021 — kept a factor of two that cancels in the correct rearrangement.
- 5.8505 — dropped that same factor in the other direction.
- 12.8712 — rounded an intermediate value before the final step.
Reference: FE Handbook — Statics: Centroids of Masses, Areas, Lengths, and Volumes
An angle bracket made of two rectangular plates needs its centroid located. Given area 1 (A_1) = 4.5000 in^2; centroid 1 x (x_1) = 9.3000 in; area 2 (A_2) = 9.0000 in^2; centroid 2 x (x_2) = 6.7000 in, determine the composite centroid x (x_bar) in in.
Given
Find
composite centroid x (x_bar), in in
Start with the thinking
- The governing relation printed in this handbook section is Composite centroid (areas).
- Everything except x_bar is given, so isolate x_bar 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.
- Locating the centroid of masses, areas, lengths, and volumes for a composite shape by weighting each part's area and centroid.
Figure 10 — schematic for Composite centroid (areas) — solve for composite centroid x (case 4) — Centroids of Masses, Areas, Lengths, and Volumes (10)
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for x_bar:
Step 3 — List the givens: area 1 (A_1) = 4.5000 in^2, centroid 1 x (x_1) = 9.3000 in, area 2 (A_2) = 9.0000 in^2, centroid 2 x (x_2) = 6.7000 in.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning x_bar = 7.5667 in to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 15.1333 — kept a factor of two that cancels in the correct rearrangement.
- 3.7833 — dropped that same factor in the other direction.
- 8.3233 — rounded an intermediate value before the final step.
Reference: FE Handbook — Statics: Centroids of Masses, Areas, Lengths, and Volumes