Chapter 3: Problem 23
Suppose a spherical floating buoy has radius \(1 \mathrm{m}\) and density \(\frac{1}{4}\) that of sea water. Given that the formula for the volume of a spherical cap is \(V_{\mathrm{cap}}=\frac{\pi x}{6}\left(3 a^{2}+x^{2}\right),\) to what depth does the buoy sink in sea water?
Short Answer
Expert verified
The buoy sinks to a depth of approximately 0.8 meters.
Step by step solution
01
- Determine Volume and Density
First, note that the radius of the sphere is \( r = 1 \text{ meter} \). The formula for the volume of a sphere is \( V = \frac{4}{3} \pi r^3 \). Plug in \( r = 1 \) to get \( V = \frac{4}{3} \pi \times 1^3 = \frac{4}{3} \pi \text{ cubic meters} \). The density of the buoy relative to sea water is \( \frac{1}{4} \), leading to the buoy having \( \rho_b = \frac{1}{4} \rho_{sw} \), where \( \rho_{sw} \) is the density of sea water.
02
- Calculate Buoyant Force
The buoyant force is equal to the weight of the displaced water. The weight of the displaced water is calculated by the formula \( F_b = \text{Volume} \times \text{Density} \times g \), where g is the acceleration due to gravity. Let the volume of displaced water be \( V_{displaced}\) and its density be \( \rho_{sw} \), then the buoyant force \( F_b = V_{displaced} \rho_{sw} g \).
03
- Find the Weight of the Buoy
The weight \( W_b \) of the buoy can be computed using density and volume: \( W_b = V \rho_b g \). Substituting the values we get, \( W_b = \frac{4}{3} \pi \times \frac{1}{4} \rho_{sw} \times g = \frac{1}{3} \pi \rho_{sw} g \).
04
- Equate Buoyant Force to Weight of Buoy
For the buoy to float, the buoyant force must equal the weight of the buoy: \( F_b = W_b \). So, \( V_{displaced} \rho_{sw} g = \frac{1}{3} \pi \rho_{sw} g \). This simplifies to \( V_{displaced} = \frac{1}{3} \pi \).
05
- Use the Volume of Spherical Cap Formula
We use the given formula for the volume of the spherical cap: \( V_{\text{cap}} = \frac{\pi x}{6}(3a^{2}+x^{2}) \), where \(a = 1\) (radius), to equate \( V_{\text{cap}} = \frac{1}{3} \pi\). Substituting \(a = 1\), we get: \( \frac{\pi x}{6}(3(1)^2 + x^2) = \frac{1}{3} \pi \). Simplifying, we obtain \( \frac{\pi x}{6}(3 + x^2) = \frac{1}{3} \pi \).
06
- Solve for x (Depth)
Solve the equation: \( \frac{\pi x}{6}(3 + x^2) = \frac{1}{3} \pi \). Cancel \( \pi \) and multiply both sides by 6 to get \( x(3 + x^2) = 2 \). This simplifies to: \( x^3 + 3x - 2 = 0 \). Use numerical methods or algebraic factorization to solve the cubic equation for \(x\).
07
- Approximate Solution
By solving the cubic equation \( x^3 + 3x - 2 = 0 \), we find that \( x \approx 0.8 \) meters after using a numerical solver or graphing method.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Volume of a Sphere
The volume of a sphere is a crucial concept in many physics and engineering problems. It's calculated using the formula: \( V = \frac{4}{3} \pi r^3 \), where \( r \) is the radius of the sphere.
To find the volume, just plug in the value of the radius and follow the formula.
For instance, with a radius of 1 meter, the volume calculation would look like this:
To find the volume, just plug in the value of the radius and follow the formula.
For instance, with a radius of 1 meter, the volume calculation would look like this:
- Plug in \( r = 1 \)
- Calculate \( (1)^3 = 1 \)
- Multiply by \( \pi \)
- Then multiply by \frac{4}{3}\text{ yielding } V = \frac{4}{3}\pi \text{ cubic meters}
Density
Density is another core concept, defined as mass per unit volume: \( \rho = \frac{m}{V} \). It's an essential property of materials that tells us how much mass is packed into a given volume.
In our problem, the buoy has a density that is \( \frac{1}{4} \) of sea water's density. This means if sea water has a density \( \rho_{sw} \), then the buoy's density will be \( \rho_b = \frac{1}{4} \rho_{sw} \).
Understanding density helps in determining why some objects float while others sink. And it forms the basis of calculating buoyant forces.
In our problem, the buoy has a density that is \( \frac{1}{4} \) of sea water's density. This means if sea water has a density \( \rho_{sw} \), then the buoy's density will be \( \rho_b = \frac{1}{4} \rho_{sw} \).
Understanding density helps in determining why some objects float while others sink. And it forms the basis of calculating buoyant forces.
Spherical Cap Volume Calculation
The volume of a spherical cap is a bit more involved but equally important. A spherical cap is essentially a 'cap' or 'hat' on top of a sphere, sliced off by a plane.
The formula for the volume of a spherical cap is: \( V_{\text{cap}} = \frac{ \pi x}{6}(3a^2 + x^2) \), where \( x \) is the height of the cap and \( a \) is the radius of the sphere.
The formula for the volume of a spherical cap is: \( V_{\text{cap}} = \frac{ \pi x}{6}(3a^2 + x^2) \), where \( x \) is the height of the cap and \( a \) is the radius of the sphere.
- Plug in the height \( x \) and radius \( a = 1 \)
- Compute \( 3(1)^2 + x^2 = 3 + x^2 \)
- Multiply and simplify to find the volume of the displaced water