How do I find the mass of a cone with changing density?

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    Integrating Mass
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Discussion Overview

The discussion revolves around calculating the mass of a cone with a variable density defined by the function ρ=3h^2+4h, where h represents the height. Participants explore different methods for integrating the density over the volume of the cone, considering its geometry and the implications of changing density.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant suggests using symmetry and slicing the cone into horizontal disks to find the mass, proposing the volume of each slice as dV=πr^2(h)dh.
  • Another participant attempts to compute the integral for mass, expressing uncertainty about their reasoning and the orientation of the cone.
  • A different participant provides a detailed approach, defining the radius as a function of height and deriving the area of circular disks to set up the integral for mass.
  • One participant questions the orientation of the cone and suggests an alternative definition of the cone's geometry, indicating a potential misunderstanding of the problem setup.

Areas of Agreement / Disagreement

Participants present multiple methods for calculating the mass, and there is no consensus on the correct approach or the orientation of the cone. The discussion remains unresolved with competing views on how to define the geometry.

Contextual Notes

Some participants express uncertainty about the integration steps and the assumptions regarding the cone's orientation, which may affect the calculations. The discussion reflects different interpretations of the cone's geometry and density function.

xykouxai
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Hi i was wondering if i could get help with a question.

A cone has a radius of 0.75m and a height of 4m. The density changes throughout the cone and can be modeled by the function ρ=3h^2+4h (h for height). Find the mass of the cone.

Please help
 
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Welcome to PF;

There are several ways of doing this - the easiest method is to exploit the symmetry.

work out how the horizontal radius of the cone varies with height (h) so you have r(h).
Cut the cone into slices of thickness dh - so the volume of each slice will be ##dV=\pi r^2(h)dh## and so you can find the mass of each slice as a function of h, and then do the integration $$m = \int dm = \int_0^4 f(h)dh$$
 
so i'll get
[o]\int[/4] 3h^4 x π x 9/256 + 4h^3 x π x 9/256 dh
=96.13kg right?

the x's are multiplication, still learning how to use the symbols
 
Man that's hard to read ... here, let me help:
$$ \int_{0}^{4} \left ( 3h^{4}\pi\frac{9}{256} + 4h^{3}\pi\frac{9}{256} \right ) dh$$ ... is that what you meant?

(If you click the "quote" button at the bottom of this reply, you'll get to see how I did that ;) )

Frankly I cannot tell if that is right or not - I haven't seen your reasoning, and I don't know which way up the cone goes. However, I was expecting a 4th order polynomial in h so it seems OK.
 
xykouxai said:
Hi i was wondering if i could get help with a question.

A cone has a radius of 0.75m and a height of 4m. The density changes throughout the cone and can be modeled by the function ρ=3h^2+4h (h for height). Find the mass of the cone.

Please help
Looking at the cone from positive z axis of an xyz coordinate system, we have two lines, one from (.75, 0) to (0, 4). That has equation y= (-16/3)(x- .75). Solving that for x, the radius of a circle when the line is rotated around the y-axis, x= .75- (3/16)y. The area of a circle of radius x is [itex]\pi x^2= \pi(.75- (3/16)y)^2[/itex] and the volume of a circular disk of that radius and height "dy" is [itex]\pi(.75- (3/16)y)^2dy[/itex]. The mass of such an object with density [itex]3y^2+ 4y[/itex] is [itex]\pi(.75- (3/16)y)^2(3y^2+ 4y)dy[/itex]. To find the total mass, integrate from y= 0 to y= 4.
 
Why can't the lines be from (0,0) to (0.75,4) etc. r(h)=3h/16 in that case.
It is quite common to define cones like that, though I'd normally think of the point as "higher" than the wide part. Hopefully OP knows which way up the cone goes ;) In fact - that seems to be the way around OP did it.
 

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