Solving the Weight Puzzle: Explaining Why and How

In summary, the solution to the problem involves finding the weight of a bar using the integral expression for weight. The weight is equal to the length of the bar multiplied by the weight per unit length of 6 lb/ft. The value of ##\bar{x}## is the x coordinate of the centroid, but it is unclear how it relates to the weight or the integral expression for weight.
  • #1
Tapias5000
46
10
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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  • #2
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.
 
  • #3
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.
 

1. What is the weight puzzle?

The weight puzzle is a phenomenon where individuals struggle to lose weight or maintain a healthy weight despite their efforts to do so. It is a complex issue that involves a combination of biological, psychological, and environmental factors.

2. Why is it important to solve the weight puzzle?

Solving the weight puzzle is important because obesity and weight-related health problems, such as diabetes and heart disease, are on the rise globally. Understanding why and how individuals struggle with weight can help develop more effective strategies for weight management and improve overall health.

3. What are the main factors that contribute to the weight puzzle?

The weight puzzle is a multifaceted issue, but some of the main factors that contribute to it include genetics, lifestyle habits, psychological factors, and environmental influences. These factors can interact and influence each other, making it challenging to pinpoint one specific cause.

4. Can the weight puzzle be solved?

While there is no one-size-fits-all solution to the weight puzzle, it can be managed and improved with the right approach. This may include a combination of healthy eating habits, regular physical activity, stress management, and seeking support from healthcare professionals.

5. What are some potential solutions for the weight puzzle?

Some potential solutions for the weight puzzle include developing personalized weight management plans, promoting healthy lifestyle habits in schools and workplaces, providing access to affordable and nutritious food options, and addressing societal and cultural norms around weight and body image.

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