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Centroids of Masses, Areas, Lengths, and Volumes

Statics · FE Reference Handbook section

Statics
16 formulas
10 exam-style examples
~60 min
All Statics lectures

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.

Example 1
Composite centroid (areas) — solve for composite centroid x — Centroids of Masses, Areas, Lengths, and Volumes

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

  • area1(A1)=19.0000in2area 1 (A_1) = 19.0000 in^2
  • centroid1x(x1)=3.5000incentroid 1 x (x_1) = 3.5000 in
  • area2(A2)=36.5000in2area 2 (A_2) = 36.5000 in^2
  • centroid2x(x2)=14.2000incentroid 2 x (x_2) = 14.2000 in

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.
Composite sectioncentroidal axisb = 6h = 10

Figure 1 — schematic for Composite centroid (areas) — solve for composite centroid x — Centroids of Masses, Areas, Lengths, and Volumes

Step-by-step solution

  1. Step 1 — State the governing relation:

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}
  2. Step 2 — Rearrange symbolically for x_bar:

    xbar=A1x1+A2x2A1+A2x_{bar} = \dfrac{A_1 x_1 + A_2 x_2}{A_1 + A_2}
  3. 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.

  4. Step 4 — Substitute the given values:

    xbar=A1x1+A2x2A1+A2x_{bar} = \dfrac{A_1 x_1 + A_2 x_2}{A_1 + A_2}
  5. Step 5 — Evaluate:

    xbar=10.5369 inx_{bar} = 10.5369\ \text{in}
  6. Step 6 — Check: returning x_bar = 10.5369 in to

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}

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

Answer:
xbar=10.5369 inx_{bar} = 10.5369\ \text{in}

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

Example 2
Composite centroid (areas) — solve for centroid 1 x — Centroids of Masses, Areas, Lengths, and Volumes (2)

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

  • area1(A1)=24.0000in2area 1 (A_1) = 24.0000 in^2
  • area2(A2)=22.0000in2area 2 (A_2) = 22.0000 in^2
  • centroid2x(x2)=5.3000incentroid 2 x (x_2) = 5.3000 in
  • compositecentroidx(xbar)=8.5800incomposite centroid x (x_bar) = 8.5800 in

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.
Composite sectioncentroidal axisb = 6h = 10

Figure 2 — schematic for Composite centroid (areas) — solve for centroid 1 x — Centroids of Masses, Areas, Lengths, and Volumes (2)

Step-by-step solution

  1. Step 1 — State the governing relation:

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}
  2. Step 2 — Rearrange symbolically for x_1:

    x1=xˉ(A1+A2)−A2x2A1x_{1} = \dfrac{\bar{x}(A_1+A_2) - A_2 x_2}{A_1}
  3. 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.

  4. Step 4 — Substitute the given values:

    x1=xˉ(A1+A2)−A2x2A1x_{1} = \dfrac{\bar{x}(A_1+A_2) - A_2 x_2}{A_1}
  5. Step 5 — Evaluate:

    x1=11.5867 inx_{1} = 11.5867\ \text{in}
  6. Step 6 — Check: returning x_1 = 11.5867 in to

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}

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

Answer:
x1=11.5867 inx_{1} = 11.5867\ \text{in}

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

Example 3
Composite centroid (areas) — solve for centroid 2 x — Centroids of Masses, Areas, Lengths, and Volumes (3)

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

  • area1(A1)=36.0000in2area 1 (A_1) = 36.0000 in^2
  • centroid1x(x1)=6.7000incentroid 1 x (x_1) = 6.7000 in
  • area2(A2)=7.5000in2area 2 (A_2) = 7.5000 in^2
  • compositecentroidx(xbar)=2.1900incomposite centroid x (x_bar) = 2.1900 in

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.
Composite sectioncentroidal axisb = 6h = 10

Figure 3 — schematic for Composite centroid (areas) — solve for centroid 2 x — Centroids of Masses, Areas, Lengths, and Volumes (3)

Step-by-step solution

  1. Step 1 — State the governing relation:

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}
  2. Step 2 — Rearrange symbolically for x_2:

    x2=xˉ(A1+A2)−A1x1A2x_{2} = \dfrac{\bar{x}(A_1+A_2) - A_1 x_1}{A_2}
  3. 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.

  4. Step 4 — Substitute the given values:

    x2=xˉ(A1+A2)−A1x1A2x_{2} = \dfrac{\bar{x}(A_1+A_2) - A_1 x_1}{A_2}
  5. Step 5 — Evaluate:

    x2=−19.4580 inx_{2} = -19.4580\ \text{in}
  6. Step 6 — Check: returning x_2 = -19.4580 in to

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}

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

Answer:
x2=−19.4580 inx_{2} = -19.4580\ \text{in}

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

Example 4
Composite centroid (areas) — solve for composite centroid x (case 2) — Centroids of Masses, Areas, Lengths, and Volumes (4)

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

  • area1(A1)=11.0000in2area 1 (A_1) = 11.0000 in^2
  • centroid1x(x1)=7.0000incentroid 1 x (x_1) = 7.0000 in
  • area2(A2)=26.5000in2area 2 (A_2) = 26.5000 in^2
  • centroid2x(x2)=3.3000incentroid 2 x (x_2) = 3.3000 in

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.
Composite sectioncentroidal axisb = 6h = 10

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

  1. Step 1 — State the governing relation:

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}
  2. Step 2 — Rearrange symbolically for x_bar:

    xbar=A1x1+A2x2A1+A2x_{bar} = \dfrac{A_1 x_1 + A_2 x_2}{A_1 + A_2}
  3. 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.

  4. Step 4 — Substitute the given values:

    xbar=A1x1+A2x2A1+A2x_{bar} = \dfrac{A_1 x_1 + A_2 x_2}{A_1 + A_2}
  5. Step 5 — Evaluate:

    xbar=4.3853 inx_{bar} = 4.3853\ \text{in}
  6. Step 6 — Check: returning x_bar = 4.3853 in to

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}

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

Answer:
xbar=4.3853 inx_{bar} = 4.3853\ \text{in}

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

Example 5
Composite centroid (areas) — solve for centroid 1 x (case 2) — Centroids of Masses, Areas, Lengths, and Volumes (5)

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

  • area1(A1)=12.0000in2area 1 (A_1) = 12.0000 in^2
  • area2(A2)=16.5000in2area 2 (A_2) = 16.5000 in^2
  • centroid2x(x2)=9.0000incentroid 2 x (x_2) = 9.0000 in
  • compositecentroidx(xbar)=6.1100incomposite centroid x (x_bar) = 6.1100 in

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.
Composite sectioncentroidal axisb = 6h = 10

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

  1. Step 1 — State the governing relation:

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}
  2. Step 2 — Rearrange symbolically for x_1:

    x1=xˉ(A1+A2)−A2x2A1x_{1} = \dfrac{\bar{x}(A_1+A_2) - A_2 x_2}{A_1}
  3. 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.

  4. Step 4 — Substitute the given values:

    x1=xˉ(A1+A2)−A2x2A1x_{1} = \dfrac{\bar{x}(A_1+A_2) - A_2 x_2}{A_1}
  5. Step 5 — Evaluate:

    x1=2.1363 inx_{1} = 2.1363\ \text{in}
  6. Step 6 — Check: returning x_1 = 2.1363 in to

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}

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

Answer:
x1=2.1363 inx_{1} = 2.1363\ \text{in}

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

Example 6
Composite centroid (areas) — solve for centroid 2 x (case 2) — Centroids of Masses, Areas, Lengths, and Volumes (6)

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

  • area1(A1)=5.5000in2area 1 (A_1) = 5.5000 in^2
  • centroid1x(x1)=4.1000incentroid 1 x (x_1) = 4.1000 in
  • area2(A2)=14.0000in2area 2 (A_2) = 14.0000 in^2
  • compositecentroidx(xbar)=12.8100incomposite centroid x (x_bar) = 12.8100 in

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.
Composite sectioncentroidal axisb = 6h = 10

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

  1. Step 1 — State the governing relation:

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}
  2. Step 2 — Rearrange symbolically for x_2:

    x2=xˉ(A1+A2)−A1x1A2x_{2} = \dfrac{\bar{x}(A_1+A_2) - A_1 x_1}{A_2}
  3. 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.

  4. Step 4 — Substitute the given values:

    x2=xˉ(A1+A2)−A1x1A2x_{2} = \dfrac{\bar{x}(A_1+A_2) - A_1 x_1}{A_2}
  5. Step 5 — Evaluate:

    x2=16.2318 inx_{2} = 16.2318\ \text{in}
  6. Step 6 — Check: returning x_2 = 16.2318 in to

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}

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

Answer:
x2=16.2318 inx_{2} = 16.2318\ \text{in}

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

Example 7
Composite centroid (areas) — solve for composite centroid x (case 3) — Centroids of Masses, Areas, Lengths, and Volumes (7)

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

  • area1(A1)=31.5000in2area 1 (A_1) = 31.5000 in^2
  • centroid1x(x1)=1.3000incentroid 1 x (x_1) = 1.3000 in
  • area2(A2)=24.5000in2area 2 (A_2) = 24.5000 in^2
  • centroid2x(x2)=9.8000incentroid 2 x (x_2) = 9.8000 in

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.
Composite sectioncentroidal axisb = 6h = 10

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

  1. Step 1 — State the governing relation:

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}
  2. Step 2 — Rearrange symbolically for x_bar:

    xbar=A1x1+A2x2A1+A2x_{bar} = \dfrac{A_1 x_1 + A_2 x_2}{A_1 + A_2}
  3. 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.

  4. Step 4 — Substitute the given values:

    xbar=A1x1+A2x2A1+A2x_{bar} = \dfrac{A_1 x_1 + A_2 x_2}{A_1 + A_2}
  5. Step 5 — Evaluate:

    xbar=5.0188 inx_{bar} = 5.0188\ \text{in}
  6. Step 6 — Check: returning x_bar = 5.0188 in to

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}

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

Answer:
xbar=5.0188 inx_{bar} = 5.0188\ \text{in}

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

Example 8
Composite centroid (areas) — solve for centroid 1 x (case 3) — Centroids of Masses, Areas, Lengths, and Volumes (8)

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

  • area1(A1)=8.0000in2area 1 (A_1) = 8.0000 in^2
  • area2(A2)=13.5000in2area 2 (A_2) = 13.5000 in^2
  • centroid2x(x2)=5.0000incentroid 2 x (x_2) = 5.0000 in
  • compositecentroidx(xbar)=4.5200incomposite centroid x (x_bar) = 4.5200 in

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.
Composite sectioncentroidal axisb = 6h = 10

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

  1. Step 1 — State the governing relation:

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}
  2. Step 2 — Rearrange symbolically for x_1:

    x1=xˉ(A1+A2)−A2x2A1x_{1} = \dfrac{\bar{x}(A_1+A_2) - A_2 x_2}{A_1}
  3. 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.

  4. Step 4 — Substitute the given values:

    x1=xˉ(A1+A2)−A2x2A1x_{1} = \dfrac{\bar{x}(A_1+A_2) - A_2 x_2}{A_1}
  5. Step 5 — Evaluate:

    x1=3.7100 inx_{1} = 3.7100\ \text{in}
  6. Step 6 — Check: returning x_1 = 3.7100 in to

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}

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

Answer:
x1=3.7100 inx_{1} = 3.7100\ \text{in}

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

Example 9
Composite centroid (areas) — solve for centroid 2 x (case 3) — Centroids of Masses, Areas, Lengths, and Volumes (9)

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

  • area1(A1)=4.5000in2area 1 (A_1) = 4.5000 in^2
  • centroid1x(x1)=2.4000incentroid 1 x (x_1) = 2.4000 in
  • area2(A2)=19.0000in2area 2 (A_2) = 19.0000 in^2
  • compositecentroidx(xbar)=9.9200incomposite centroid x (x_bar) = 9.9200 in

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.
Composite sectioncentroidal axisb = 6h = 10

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

  1. Step 1 — State the governing relation:

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}
  2. Step 2 — Rearrange symbolically for x_2:

    x2=xˉ(A1+A2)−A1x1A2x_{2} = \dfrac{\bar{x}(A_1+A_2) - A_1 x_1}{A_2}
  3. 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.

  4. Step 4 — Substitute the given values:

    x2=xˉ(A1+A2)−A1x1A2x_{2} = \dfrac{\bar{x}(A_1+A_2) - A_1 x_1}{A_2}
  5. Step 5 — Evaluate:

    x2=11.7011 inx_{2} = 11.7011\ \text{in}
  6. Step 6 — Check: returning x_2 = 11.7011 in to

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}

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

Answer:
x2=11.7011 inx_{2} = 11.7011\ \text{in}

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

Example 10
Composite centroid (areas) — solve for composite centroid x (case 4) — Centroids of Masses, Areas, Lengths, and Volumes (10)

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

  • area1(A1)=4.5000in2area 1 (A_1) = 4.5000 in^2
  • centroid1x(x1)=9.3000incentroid 1 x (x_1) = 9.3000 in
  • area2(A2)=9.0000in2area 2 (A_2) = 9.0000 in^2
  • centroid2x(x2)=6.7000incentroid 2 x (x_2) = 6.7000 in

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.
Composite sectioncentroidal axisb = 6h = 10

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

  1. Step 1 — State the governing relation:

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}
  2. Step 2 — Rearrange symbolically for x_bar:

    xbar=A1x1+A2x2A1+A2x_{bar} = \dfrac{A_1 x_1 + A_2 x_2}{A_1 + A_2}
  3. 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.

  4. Step 4 — Substitute the given values:

    xbar=A1x1+A2x2A1+A2x_{bar} = \dfrac{A_1 x_1 + A_2 x_2}{A_1 + A_2}
  5. Step 5 — Evaluate:

    xbar=7.5667 inx_{bar} = 7.5667\ \text{in}
  6. Step 6 — Check: returning x_bar = 7.5667 in to

    xˉ=∑Aixi∑Ai\bar{x} = \dfrac{\sum A_i x_i}{\sum A_i}

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

Answer:
xbar=7.5667 inx_{bar} = 7.5667\ \text{in}

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

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