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(III) Determine the \(\mathrm{CM}\) of a machine part that is a uniform cone of height \(h\) and radius \(R\), Fig. 9-46. [Hint: Divide the cone into an infinite number of disks of thickness \(d z,\) one of which is shown.

Short Answer

Expert verified
The center of mass is \( \frac{3h}{4} \) from the base.

Step by step solution

01

Understand the Problem

We need to find the center of mass of a uniform cone with height \(h\) and base radius \(R\). Using calculus, we will divide the cone into an infinite number of thin disks of thickness \(dz\) and determine how these disks contribute to the center of mass.
02

Set Up the Geometry

The cone has its base on the horizontal plane and its apex at the origin. We will position the cone along the \(z\)-axis. For a point \(z\) along the height of the cone, the radius of the disk is proportional to \(z\), thus, \(r = \frac{R}{h} z\).
03

Consider an Elemental Disk

At height \(z\), each disk has thickness \(dz\), radius \(r = \frac{R}{h}z\), and therefore a circular area of \(\pi r^2 = \pi \left(\frac{R}{h} z\right)^2\). The volume of this disk element is \(dV = \pi \left(\frac{R}{h} z\right)^2 dz\).
04

Determine the Mass of the Disk

Assume the density of the cone is uniform, \(\rho\). The mass of a disk is \(dm = \rho dV = \rho \pi \left(\frac{R}{h} z\right)^2 dz\).
05

Express the Center of Mass in Integral Form

The center of mass \( \overline{z} \) along the \(z\)-axis is given by \( \overline{z} = \frac{1}{M} \int_0^h z \cdot dm\), where \(M\) is the total mass of the cone. Substituting \(dm = \rho \pi \left(\frac{R}{h} z\right)^2 dz\), we have \(M = \int_0^h dm\).
06

Evaluate the Integrals

The total mass is \(M = \rho \pi \int_0^h \left(\frac{R}{h} z\right)^2 dz = \rho \pi \left(\frac{R}{h}\right)^2 \int_0^h z^2 dz = \rho \pi \left(\frac{R}{h}\right)^2 \left[ \frac{z^3}{3} \right]_0^h = \rho \pi \frac{R^2 h}{3}\).
07

Compute the Center of Mass

The center of mass is \( \overline{z} = \frac{1}{M} \int_0^h z \cdot dm = \frac{1}{M} \rho \pi \left(\frac{R}{h}\right)^2 \int_0^h z^3 dz.\ Now, \int_0^h z^3 dz = \left[ \frac{z^4}{4} \right]_0^h = \frac{h^4}{4} \). Thus, \( \overline{z} = \frac{3}{h^4} \int_0^h z^3 dz = \frac{3}{4}h \frac{h^4}{4} = \frac{3h}{4}.\)
08

Conclusion

The center of mass (CM) of the cone is located along the height of the cone at \( \frac{3h}{4} \) from the cone's base.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Calculus Integration
Calculus integration is a crucial mathematical tool used to find values like areas, volumes, and other quantities that are accumulated over an interval. In the context of finding the center of mass of a cone, integration allows us to sum the contributions of infinitesimally small elements of mass spread throughout the cone.

We can think of integration like a continuous summation. By integrating, we calculate the total mass and determine how this total distributes along a certain axis. For our exercise, we used \[M = \rho \pi \int_0^h \left(\frac{R}{h} z\right)^2 dz\]to find the total mass of the cone. This formula sums up the contributions of tiny disks along the height \(z\) of the cone.

The integration process also helped in calculating the center of mass, \(\overline{z}\). The expression \[\overline{z} = \frac{1}{M} \int_0^h z \cdot dm\]combines the influence of the location of each disk and its mass to find a weighted average position. Understanding integration is key to solving such problems in physics and engineering efficiently.
Uniform Cone
A uniform cone is a three-dimensional geometric shape that has a circular base and tapers smoothly to a point, called the apex. The term 'uniform' here indicates that the density of the cone is consistent throughout its entire volume, meaning no part is denser or lighter than another.

Why is understanding a uniform cone important for our exercise? Firstly, the uniform density simplifies our calculations. We do not need to worry about varying mass distributions. In our cone, the geometry is pivotal as it dictates the size and dimensions of every elemental disk we considered during integration. Each of these disks contributes to the total mass and also to the location of the center of mass.

For the cone in our problem, its height \(h\) and base radius \(R\) are known parameters. By leveraging these dimensions, we calculate the radius of any disk at height \(z\) as \(r = \frac{R}{h}z\). This relationship helps us detail the cone's geometry step-by-step to proceed with calculations of center of mass using integration. If the cone were non-uniform, this process would become significantly more complex.
Density and Mass Distribution
Density refers to how much mass exists within a given volume. In our uniform cone, the density \(\rho\) is constant throughout. By knowing the density, we can ascertain the mass of different sections of the cone and factor these into the calculations for the center of mass.

Mass distribution is crucial when evaluating how mass is spread across the object. For the cone, this involves determining how each thin disk contributes to the overall mass and its position along the height. The uniform density ensures that each disk's mass depends solely on its volume. Thus, the mass of a disk is calculated as:\[dm = \rho \pi \left(\frac{R}{h} z\right)^2 dz\]Given the linear distribution of radius with height, this formula describes how the mass increases quadratically as we move from the apex to the base.

By analyzing the density and mass distribution, we ensure that the center of mass aligns with our calculated value \(\overline{z} = \frac{3h}{4} \), giving insight into the physical balance of the cone.

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