# Calculate the center of mass of a non-uniform metal rod

• VitaminK
In summary: The two parts have a different surface area to mass ratio. You are trying to solve for the center of mass of two masses, which is not the same as the center of mass of a part with the same volume and surface area.
VitaminK
Homework Statement
A metal rod consists of two parts. one part is a 1.2m metal with the density of 5kg/m3. The other part is a 1,8m metal with the density of 7kg/m3. Calculate the center of mass for this rod.
Relevant Equations
Density=m/v

I know that if they had the same density they would have the center of mass at 1,5 m. But now that they don't the center of mass will be shifted towards the part of the rod with higher density. they will have their center of mass where they
have equal mass
p1*v=p2*v

now i don't know how to move forward

calculate the COM for each side then treat it as a 2-body system.

FactChecker
Do you mean
1.2/2=0.6m
1.8/2= 0.9m
(0.6m+0.9m)

im not familiar with 2-body system (high school physics)

If you have a 100lb man standing at -10 on the x axix and a 200lb man standing at +10 on the x axis, where is the center of gravity of the two men considered as one system?

300lb/20 = 15 ?

VitaminK said:
Do you mean
1.2/2=0.6m
1.8/2= 0.9m
(0.6m+0.9m)

im not familiar with 2-body system (high school physics)
Alternatively, try to find the point ##x## along the rod where there is equal mass on both sides.

PS Ignore this post!

Last edited:
PeroK said:
Alternatively, try to find the point ##x## along the rod where there is equal mass on both sides.
you mean p1*v=p2*v --> 5*pi*r^2*l=7*pi*r^2*l now I am confused about the length l

There are rod two parts, each with a uniform density, where the center of mass of each part is simple to determine. The original problem can be converted to a problem with the entire mass of the two parts located at their individual centers of mass. That problem is then simple to solve.

PeroK said:
Alternatively, try to find the point ##x## along the rod where there is equal mass on both sides.
That won't work. You are not after the point where the mass on both sides is the same. You need the leverage of the two masses to be the same.

Worded differently, you are not after the median [mass-weighted] position. You are after the mean [mass-weighted] position.

PeroK

Is this correct?

VitaminK said:
you mean p1*v=p2*v --> 5*pi*r^2*l=7*pi*r^2*l now I am confused about the length l
Ignore what I said! I wasn't thinking.

VitaminK said:
View attachment 260260
Is this correct?
That doesn't look right.

Can you break down your calculation so we can see where you've gone wrong?

Last edited:
That is not the answer I get. To make your calculations more clear, I think you should specifically calculate the two masses, ##M_1, M_2## at two locations, ##x_1,x_2##. Then put them into a final equation to find the center of mass of two masses.

EDIT: Sorry, I see that you can not calculate specific numbers for the masses. You must leave the density in. But I think you can make your calculations more clear. I still think that there is a mistake in your calculations.

jbriggs444 and PeroK
I see the problem. You need to be more careful about the volume. Assuming that the diameter of both parts is identical, the two parts do not have the same volume.

## What is the center of mass of a non-uniform metal rod?

The center of mass of a non-uniform metal rod is the point at which the entire mass of the rod is evenly distributed. It is the balance point of the rod, where it would remain suspended if it were placed on a fulcrum.

## How is the center of mass of a non-uniform metal rod calculated?

The center of mass of a non-uniform metal rod can be calculated by dividing the rod into small segments and finding the center of mass of each segment. Then, the weighted average of these individual centers of mass is taken to determine the overall center of mass of the rod.

## Why is it important to calculate the center of mass of a non-uniform metal rod?

Calculating the center of mass of a non-uniform metal rod is important because it helps us understand the distribution of mass in the rod. This information is useful in various applications, such as designing structures or predicting the behavior of the rod under different forces.

## What factors can affect the center of mass of a non-uniform metal rod?

The shape, size, and mass distribution of the rod can affect its center of mass. Additionally, external forces acting on the rod, such as gravity or applied forces, can also alter its center of mass.

## Can the center of mass of a non-uniform metal rod be outside of the physical boundaries of the rod?

Yes, it is possible for the center of mass of a non-uniform metal rod to be outside of the physical boundaries of the rod. This can happen if the mass is unevenly distributed, causing the center of mass to be closer to one end of the rod than the other.

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