How can I locate the coordinates of the centroid of a cone in Z?

In summary, the conversation discusses finding the centroid of a cone and the formula for its volume. The solution involves using the formula for the centroid and understanding the concept of finding the mass centre of an object by representing it as the sum or difference of simpler objects. The range of integration should not extend beyond where the expression for the integrand represents a contribution to the desired total. The conversation also touches on knowing which standard results are allowed to be quoted and the importance of understanding the method to get from the simplest case to a more general one.
  • #1
Tapias5000
46
10
Homework Statement
Locate the centroid Z of the homogeneous solid that is formed by rotating the area formed by rotating the area shaded in blue with respect to the z-axis. the z-axis.
Relevant Equations
## \overline{z}=\frac{\int _{ }^{ }\tilde{z}dA}{\int _{ }^{ }dA} ##

## A_b=πr^2 ##
This is the picture of the problem.
Imagen2.png

My solution is:
Imagen3.png

I'm not sure if the limit is 0 to 2 or 0 to 4...
 
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  • #2
Tapias5000 said:
I'm not sure if the limit is 0 to 2 or 0 to 4...
Would your expressions for the integrands be valid above z=2?

There is a way to solve it without calculus if you know the formula for the centroid of a cone.
 
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  • #3
haruspex said:
Would your expressions for the integrands be valid above z=2?
ummm I don't understand what you mean by this, could you give me another example?

haruspex said:
There is a way to solve it without calculus if you know the formula for the centroid of a cone.
I saw that there were several formulas depending on the case, but I doubt that my professor would like me to use them.
 
  • #4
Tapias5000 said:
could you give me another example?
The denominator in your fifth equation gives the volume as ##\int_0^2\pi y^2dz##. That is based on an element thickness dz at height z having volume ##\pi y^2dz##.
In the figure, does the solid have a volume element of that size between ##z## and ##z+dz## for all z in ##(0,4)##?
Tapias5000 said:
there were several formulas depending on the case, but I doubt that my professor would like me to use them.
Yes, it can be hard knowing which standard results you are allowed to quote. But students would be expected to know the formula for the simplest case, and that is all you need here. The method to get from the simplest case to this more general one is certainly worth knowing. Do you see how to do it?
 
  • #5
haruspex said:
The denominator in your fifth equation gives the volume as ##\int_0^2\pi y^2dz##. That is based on an element thickness dz at height z having volume ##\pi y^2dz##.
In the figure, does the solid have a volume element of that size between ##z## and ##z+dz## for all z in ##(0,4)##?
Hmm, the problem tells us to locate the centroid of the area shaded in blue in Z, and according to the figure that is a known data from 0 to 2, right?
haruspex said:
Yes, it can be hard knowing which standard results you are allowed to quote. But students would be expected to know the formula for the simplest case, and that is all you need here. The method to get from the simplest case to this more general one is certainly worth knowing. Do you see how to do it?
There are several formulas, so I would not know which one to apply here.
 
  • #6
Tapias5000 said:
the problem tells us to locate the centroid of the area shaded in blue in Z,
Yes, but you asked what the bounds should be on the integrations, (0,2) or (0,4).
The range of integration should not extend beyond where the expression for the integrand represents a contribution to the desired total. In the region z>2, are there volume elements contributing to the total volume of the shaded region? If not, the range should stop at 2.
Tapias5000 said:
There are several formulas
Of which the simplest is that for a conical form extending from a plane base to a point the distance of the mass centre from the base is 1/4 the distance of the point from the base.
Can you see how to use that here to get the answer quickly?
 
  • #7
haruspex said:
Yes, but you asked what the bounds should be on the integrations, (0,2) or (0,4).
The range of integration should not extend beyond where the expression for the integrand represents a contribution to the desired total. In the region z>2, are there volume elements contributing to the total volume of the shaded region? If not, the range should stop at 2.
aaa already understand.
haruspex said:
Of which the simplest is that for a conical form extending from a plane base to a point the distance of the mass centre from the base is 1/4 the distance of the point from the base.
Can you see how to use that here to get the answer quickly?
you mean the formula ## h / 4 ##?
If I replace values I do not get the same result so I assume that I am missing some concept
If you solve the integral in the form of its symbols, I will obtain the "formula" for this case, right?
 
  • #8
Tapias5000 said:
you mean the formula h/4?
If I replace values I do not get the same result so I assume that I am missing some concept
Yes. The concept is finding the mass centre of an object by representing it as the sum or difference of simpler objects.
In this case, the object is the difference between two complete cones.
You can write down where the mass centre of each complete cone is, and you know their relative masses.
Can you take it from there?
 
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  • #9
haruspex said:
Yes. The concept is finding the mass centre of an object by representing it as the sum or difference of simpler objects.
In this case, the object is the difference between two complete cones.
You can write down where the mass centre of each complete cone is, and you know their relative masses.
Can you take it from there?
no, I really couldn't understand it, can you show me a different example?
 
  • #10
Tapias5000 said:
no, I really couldn't understand it, can you show me a different example?
Should I look up the definition of the center of mass for a cone?
 
  • #11
Tapias5000 said:
Should I look up the definition of the center of mass for a cone?
The definition of the centre of mass does not depend on the shape. Do you mean the formula for the center of mass for a cone? It's one quarter of the way from the base to the tip.
Suppose you had two cubes of the same material but different sizes, 1x1x1 sitting centrally on a 2x2x2 say. Could you work out where the mass centre of the combination is?
 
  • #12
haruspex said:
The definition of the centre of mass does not depend on the shape. Do you mean the formula for the center of mass for a cone? It's one quarter of the way from the base to the tip.
Suppose you had two cubes of the same material but different sizes, 1x1x1 sitting centrally on a 2x2x2 say. Could you work out where the mass centre of the combination is?
damn I suck at this lmao
I think it's better if I just give up and that's it.

Do you have any video on yt that you can share with me to illustrate what you mean?
 
  • #13
Tapias5000 said:
damn I suck at this lmao
I think it's better if I just give up and that's it.

Do you have any video on yt that you can share with me to illustrate what you mean?
For the two cubes problem, take moments about a point in the middle of the base of the lower cube. The result should be the same treating it as one object or two.
The masses of the cubes are 1 and 8 for a total of 9. The mass centre heights are 1 and 2.5. The combined moment is 1x2.5+8x1=11.5.
If the overall mass centre is height Y then 11.5=9Y, so Y=11.5/9.
The frustrated cone in the problem in post 1 can be represented as the difference of two cones.
Can you take it from there?
 
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1. How do I find the centroid of a cone in Z?

To find the centroid of a cone in Z, you will need to use the formula:
x = (1/4)h
y = (1/4)h
z = (1/3)h
Where h is the height of the cone.

2. What is the significance of finding the centroid of a cone in Z?

The centroid of a cone in Z is an important point as it represents the center of mass of the cone. This point can be used to calculate the moment of inertia and other important physical properties of the cone.

3. Can I use any point to find the centroid of a cone in Z?

No, you cannot use any point to find the centroid of a cone in Z. The formula mentioned above only works for cones that have their base on the xy-plane and their axis along the z-axis. For other types of cones, you will need to use different formulas.

4. How can I verify if I have correctly located the centroid of a cone in Z?

You can verify if you have correctly located the centroid of a cone in Z by checking if the coordinates you have calculated satisfy the equation of the centroid mentioned in the first question. You can also use the coordinates to calculate the moment of inertia and compare it to the known value for a cone to check for accuracy.

5. Are there any alternative methods for finding the centroid of a cone in Z?

Yes, there are alternative methods for finding the centroid of a cone in Z. One method is by using integration to find the volume and then calculating the moment of the cone with respect to each axis. Another method is by dividing the cone into smaller, simpler shapes and finding the centroid of each shape, then using the weighted average method to find the overall centroid of the cone.

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