Solving the Weight Puzzle: Explaining Why and How

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SUMMARY

The discussion centers on the calculation of the weight of a bar using integrals, specifically addressing the relationship between the integral of the length of the bar and its weight per unit length of 6 lb/ft. The integral is confirmed to represent the total weight of the bar in pounds. Additionally, the term ##\bar{x}## is identified as the x-coordinate of the centroid, which is crucial for understanding the weight distribution along the bar. The participants seek clarity on how these concepts interrelate and the necessity of multiplying by 6.

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  • Understanding of integral calculus, particularly in relation to physical applications.
  • Familiarity with the concept of centroids in mechanics.
  • Knowledge of weight distribution and linear density in physics.
  • Basic understanding of units of measurement, specifically pounds per foot.
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  • Study the application of integrals in calculating physical properties, focusing on weight and density.
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Homework Statement
The uniform bar is bent in the form of a parabola and has a weight per unit length of 6 lb>ft. parabola and has a weight per unit length of 6 lb>ft. Determine the reactions at the fixed support A.
Relevant Equations
## dL=\sqrt{\left(\frac{dx}{dy}\right)^2+1}dy ##
## \overline{x}=\frac{\int _{ }^{ }\tilde{x}dL}{\int _{ }^{ }dL} ##
1635021706807.png

My solution is
<math xmlns=http://www.w3.org/1998/Math/MathML display=block data-is-equatio=1 data-latex=\begin{array}{l}Data\\
\begin{array}{l}\left(dL\right)^2=\left(dx\right)^2+\left(dy\right)^2\\
dL=\sqrt{\left(dx\right)^2+\left(dy\right)^2}\\
dL=\sqrt{\left(\left(\frac{dx}{dy}\right)^2dy^2+\left(1\right)\left(dy\right)^2\right)}\\
dL=\sqrt{\left(\frac{dx}{dy}\right)^2+1}dy\\
y^2=3x\\
\frac{y^2}{3}=x\ →\frac{2y}{3}dy=dx\\
\frac{y^2}{3}=x\ →\frac{2y}{3}=\frac{dx}{dy}\\
\overline{x}=\frac{\int_0^3\tilde{x}dL}{\int_0^3dL}\ →\left[\textcolor{#E94D40}{\ dL=\sqrt{\left(\frac{dx}{dy}\right)^2+1}dy}\right]\\
\overline{x}=\frac{\int_0^3\tilde{x}\sqrt{\left(\frac{dx}{dy}\right)^2+1}dy}{\int_0^3\sqrt{\left(\frac{dx}{dy}\right)^2+1}dy}\ →\left[\textcolor{#E94D40}{\ \frac{dx}{dy}=\frac{2y}{3}}\right]\end{array}\\
\overline{x}=\frac{\int_0^3x\sqrt{\left(\frac{2y}{3}\right)^2+1}dy\ →\ \left[\textcolor{#E94D40}{\frac{y^2}{3}=x}\ \right]\ }{\int_0^3\sqrt{\left(\frac{2y}{3}\right)^2+1}dy}\\
\overline{x}=\frac{\int_0^3\frac{y^2}{3}\sqrt{\left(\frac{2y}{3}\right)^2+1}dy\ \ }{\int_0^3\sqrt{\left(\frac{2y}{3}\right)^2+1}dy}\\
\overline{x}=\frac{5.46}{4.44}p\\
\overline{x}\approx1.23p\\
w=\int_0^36\sqrt{\left(\frac{2y}{3}\right)^2+1}dy\\
w=\textcolor{#28AE61}{\frac{6}{3}}\int_{ }^{ }\sqrt{4y^2+9}dy\ \left(y=\frac{3}{2}\tan\theta,\ dy=\frac{3}{2}\sec^2\theta\ d\theta\right)\\
w=2\int_{ }^{ }\sqrt{9\tan^2\theta+9}\frac{3}{2}\sec^2\theta\ d\theta\\
w=\frac{\cancel{\textcolor{#E94D40}{2}}\cdot3\cdot3}{\cancel{\textcolor{#E94D40}{2}}}\int_0^3\sqrt{\tan^2\theta+1}\sec^2\theta\ d\theta\\
w=9\int_{ }^{ }\sec^3\theta\ d\theta\ \left(u=\sec\theta,\ du=\sec\theta\tan\theta,\ dv=\sec^2\theta,\ v=\tan\theta\right)\\
w=9\left\{\sec\theta\tan\theta-\int_{ }^{ }\sec\theta\tan\theta\tan\theta d\theta\right\}\\
w=9\left\{\sec\theta\tan\theta-\int_{ }^{ }\sec\theta\tan^2\theta d\theta\right\}\\
w=9\left\{\sec\theta\tan\theta-\int_{ }^{ }\left(\sec^3\theta-\sec\theta\right)d\theta\right\}\\
w=9\left\{\sec\theta\tan\theta-\int_{ }^{ }\sec^3\theta d\theta+\int_{ }^{ }\sec\theta d\theta\right\}\\
w=\frac{9}{2}\sec\theta\tan\theta+\frac{9}{2}\ln\left|\sec\theta+\tan\theta\right|\ \left[y=\frac{3}{2}\tan\theta,\ \theta=\arctan\left(\frac{2y}{3}\right)\right]\\
w=\frac{9}{2}\frac{\sqrt{4y^2+9}}{3}\frac{2y}{3}+\frac{9}{2}\ln\left|\frac{\sqrt{4y^2+9}}{3}+\frac{2y}{3}\right|\\
w=y\sqrt{4y^2+9}+\frac{9}{2}\ln\left|\frac{2y+\sqrt{4y^2+9}}{3}\right|\ \begin{bmatrix}3\\
0\end{bmatrix}\\
w=3\sqrt{4\left(3\right)^2+9}+\frac{9}{2}\ln\left|\frac{2\left(3\right)+\sqrt{4\left(3\right)^2+9}}{3}\right|-\left(\cancel{\textcolor{#E94D40}{0\sqrt{4\left(0\right)^2+9}}}+\frac{9}{2}\ln\left|\frac{\cancel{\textcolor{#E94D40}{2\left(0\right)}}+\sqrt{\cancel{\textcolor{#E94D40}{4\left(0\right)^2}}+9}}{3}\right|\right)\\
w=9\sqrt{5}+\frac{9\ln\left(2+\sqrt{5}\right)}{2}-\left(\textcolor{#E94D40}{\cancel{\frac{9}{2}\ln\left|1\right|}}\right)\\
w=26.62lb\end{array}><mtable columnalign=left columnspacing=1em rowspacing=4pt><mtr><mtd><mi>D</mi><mi>a</mi><mi>t</mi><mi>a</mi></mtd></mtr><mtr><mtd><mtable columnspacing=1em rowspacing=4pt><mtr><mtd><msup><mrow data-mjx-texclass=INNER><mo data-mjx-texclass=OPEN>(</mo><mi>d</mi><mi>L</mi><mo data-mjx-texclass=CLOSE>)</mo></mrow><mn>2</mn></msup><mo>=</mo><msup><mrow data-mjx-texclass=INNER><mo data-mjx-texclass=OPEN>(</mo><mi>d</mi><mi>x</mi><mo data-mjx-texclass=CLOSE>)</mo></mrow><mn>2</mn></msup><mo>+</mo><msup><mrow data-mjx-texclass=INNER><mo data-mjx-texclass=OPEN>(</mo><mi>d</mi><mi>y</mi><mo data-mjx-texclass=CLOSE>)</mo></mrow><mn>2</mn></msup></mtd></mtr><mtr><mtd><mi>d</mi><mi>L</mi><mo>=</mo><msqrt><msup><mrow data-mjx-texclass=INNER><mo data-mjx-texclass=OPEN>(</mo><mi>d</mi><mi>x</mi><mo data-mjx-texclass=CLOSE>)</mo></mrow><mn>2</mn></msup><mo>+</mo><msup><mrow data-mjx-texclass=INNER><mo data-mjx-texclass=OPEN>(</mo><mi>d</mi><mi>y</mi><mo data-mjx-texclass=CLOSE>)</mo></mrow><mn>2</mn></msup></msqrt></mtd></mtr><mtr><mtd><mi>d</mi><mi>L</mi><mo>=</mo><msqrt><mrow data-mjx-texclass=INNER><mo data-mjx-texclass=OPEN>(</mo><msup><mrow data-mjx-texclass=INNER><mo data-mjx-texclass=OPEN>(</mo><mfrac><mrow><mi>d</mi><mi>x</mi></mrow><mrow><mi>d</mi><mi>y</mi></mrow></mfrac><mo data-mjx-texclass=CLOSE>)</mo></mrow><mn>2</mn></msup><mi>d</mi><msup><mi>y</mi><mn>2</mn></msup><mo>+</mo><mrow data-mjx-texclass=INNER><mo data-mjx-texclass=OPEN>(</mo><mn>1</mn><mo data-mjx-texclass=CLOSE>)</mo></mrow><msup><mrow data-mjx-texclass=INNER><mo data-mjx-texclass=OPEN>(</mo><mi>d</mi><mi>y</mi><mo data-mjx-texclass=CLOSE>)</mo></mrow><mn>2</mn></msup><mo data-mjx-texclass=CLOSE>)</mo></mrow></msqrt></mtd></mtr><mtr><mtd><mi>d<
-hhWsBfNmxOB99DZJZQyRA2Qwj_HTwpdlPJ74SIZozf1=s1600.png

<math xmlns=http://www.w3.org/1998/Math/MathML display=block data-is-equatio=1 data-latex=\begin{array}{l}↶_+ΣM_A=0\\
M_A-\overline{x}\cdot26.6=0\\
M_A-1.23\cdot26.6=0\\
\left[\textcolor{#E94D40}{M_A=32.72↶_+}\right]\\
\overrightarrow{+}Σfx=0\\
\left[\textcolor{#E94D40}{A_x=\overrightarrow{0}}\right]\\
_+↑Σfy=0\\
A_y-26.6=0\\
\left[\textcolor{#E94D40}{A_y=26.6_+↑}\right]\\
Solutions\\
\left[\textcolor{#E94D40}{A_x=\overrightarrow{0}}\right]\left[\textcolor{#E94D40}{A_y=26.6_+↑}\right]\\
\left[\textcolor{#E94D40}{M_A=32.72↶_+}\right]\end{array}><mtable columnalign=left columnspacing=1em rowspacing=4pt><mtr><mtd><msub><mo>↶</mo><mo>+</mo></msub><mi>Σ</mi><msub><mi>M</mi><mi>A</mi></msub><mo>=</mo><mn>0</mn></mtd></mtr><mtr><mtd><msub><mi>M</mi><mi>A</mi></msub><mo>−</mo><mover><mi>x</mi><mo accent=true>―</mo></mover><mo>⋅</mo><mn>26.6</mn><mo>=</mo><mn>0</mn></mtd></mtr><mtr><mtd><msub><mi>M</mi><mi>A</mi></msub><mo>−</mo><mn>1.23</mn><mo>⋅</mo><mn>26.6</mn><mo>=</mo><mn>0</mn></mtd></mtr><mtr><mtd><mrow data-mjx-texclass=INNER><mo data-mjx-texclass=OPEN>[</mo><mstyle mathcolor=#E94D40><msub><mi>M</mi><mi>A</mi></msub><mo>=</mo><mn>32.72</mn><msub><mo>↶</mo><mo>+</mo></msub></mstyle><mo data-mjx-texclass=CLOSE>]</mo></mrow></mtd></mtr><mtr><mtd><mover><mo>+</mo><mo>→</mo></mover><mi>Σ</mi><mi>f</mi><mi>x</mi><mo>=</mo><mn>0</mn></mtd></mtr><mtr><mtd><mrow data-mjx-texclass=INNER><mo data-mjx-texclass=OPEN>[</mo><mstyle mathcolor=#E94D40><msub><mi>A</mi><mi>x</mi></msub><mo>=</mo><mover><mn>0</mn><mo>→</mo></mover></mstyle><mo data-mjx-texclass=CLOSE>]</mo></mrow></mtd></mtr><mtr><mtd><msub><mi/><mo>+</mo></msub><mo stretchy=false>↑</mo><mi>Σ</mi><mi>f</mi><mi>y</mi><mo>=</mo><mn>0</mn></mtd></mtr><mtr><mtd><msub><mi>A</mi><mi>y</mi></msub><mo>−</mo><mn>26.6</mn><mo>=</mo><mn>0</mn></mtd></mtr><mtr><mtd><mrow data-mjx-texclass=INNER><mo data-mjx-texclass=OPEN>[</mo><mstyle mathcolor=#E94D40><msub><mi>A</mi><mi>y</mi></msub><mo>=</mo><msub><mn>26.6</mn><mo>+</mo></msub><mo stretchy=false>↑</mo></mstyle><mo data-mjx-texclass=CLOSE>]</mo></mrow></mtd></mtr><mtr><mtd><mi>S</mi><mi>o</mi><mi>l</mi><mi>u</mi><mi>t</mi><mi>i</mi><mi>o</mi><mi>n</mi><mi>s</mi></mtd></mtr><mtr><mtd><mrow data-mjx-texclass=INNER><mo data-mjx-texclass=OPEN>[</mo><mstyle mathcolor=#E94D40><msub><mi>A</mi><mi>x</mi></msub><mo>=</mo><mover><mn>0</mn><mo>→</mo></mover></mstyle><mo data-mjx-texclass=CLOSE>]</mo></mrow><mrow data-mjx-texclass=INNER><mo data-mjx-texclass=OPEN>[</mo><mstyle mathcolor=#E94D40><msub><mi>A</mi><mi>y</mi></msub><mo>=</mo><msub><mn>26.6</mn><mo>+</mo></msub><mo stretchy=false>↑</mo></mstyle><mo data-mjx-texclass=CLOSE>]</mo></mrow></mtd></mtr><mtr><mtd><mrow data-mjx-texclass=INNER><mo data-mjx-texclass=OPEN>[</mo><mstyle mathcolor=#E94D40><msub><mi>M</mi><mi>A</mi></msub><mo>=</mo><mn>32.72</mn><msub><mo>↶</mo><mo>+</mo></msub></mstyle><mo data-mjx-texclass=CLOSE>]</mo></mrow></mtd></mtr></mtable></math>
Pu86lhm6bdm8enE8Nd-Ruc3yuopJ5st3T-gWcQEX1fIv=s1600.png

However can someone explain in detail why
7rEJAKNbTVFKW4R9L7Wcvr_qv5KBnDlvK5ll2q-9m7_h=s1600.png

will be equal to the weight? and why does it have to be multiplied by 6?
 

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The integral is length of the bar multiplied by weight per unit length of 6 lb>ft so should be equal to weight of bar in lb. I do not see if it equals to ##\bar{x}## which I do not catch its definition.
 
anuttarasammyak said:
The integral is length of the bar multiplied by weight per unit length of 6 lb>ft so should be equal to weight of bar in lb. I do not see if it equals to ##\bar{x}## which I do not catch its definition.
##\bar{x}## should be the x coordinate of the centroid. I can't see anywhere in the image that equates that to w or to the integral expression for w.
 

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