Finding z component of center of mass of a complex shape

In summary, the rigidly connected unit consists of a 2.5-kg circular disk, a 2.8-kg round shaft, and a 4.2-kg square plate. The z-coordinate of the mass center of the unit is 26.5 mm.
  • #1
Ella Tankersley
6
0

Homework Statement



453786-5-63IP1.png
The rigidly connected unit consists of a 2.5-kg circular disk, a 2.8-kg round shaft, and a 4.2-kg square plate. Determine the z-coordinate of the mass center of the unit.

Homework Equations


∑zm/∑m

The Attempt at a Solution


Circular disk:
mass = 2.5 kg
z = 0
zm = 0
Round Shaft:
mass = 2.8 kg
z = 180/2 = 90
zm = 90(2.8) = 252
Square plate:
mass = 4.2 kg
z = 0
zm = 0

Total mass = 2.5+2.8+4.2 = 9.5 kg
Total mz = 252

∑zm/∑m = 252/9.5 = 26.5 mm
 

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  • #2
Ella Tankersley said:
The rigidly connected unit consists of a 2.5-kg circular disk, a 2.8-kg round shaft, and a 4.2-kg square plate. Determine the z-coordinate of the mass center of the unit.
I think we'll want to see a diagram or better description of the orientations, placement, and dimensions of the objects. We shouldn't have to fish about in your solution equations to find the statement of the problem.
 
  • #3
gneill said:
I think we'll want to see a diagram or better description of the orientations, placement, and dimensions of the objects. We shouldn't have to fish about in your solution equations to find the statement of the problem.
I added a diagram, can you see it?
 
  • #4
Ella Tankersley said:
I added a diagram, can you see it?
Ah! Much better! Thanks.
 
  • #5
Ella Tankersley said:
Square plate:
mass = 4.2 kg
z = 0
zm = 0
I don't understand this calculation. Why is the plate at z = 0?
 
  • #6
gneill said:
I don't understand this calculation. Why is the plate at z = 0?
I was assuming that the plate is very thin so there is no thickness in the z direction
 
  • #7
Ella Tankersley said:
I was assuming that the plate is very thin so there is no thickness in the z direction
What has the plate's thickness have to do with its center of mass position with respect to the xy plane?

Edit: removed spurious thought that I thought I'd removed before posting. Sorry about that.
 
Last edited:
  • #8
Ella Tankersley said:
I was assuming that the plate is very thin so there is no thickness in the z direction

OK, so assume the mass of the plate is all at the same z value. And that's the z coordinate of the center of mass. What is that z value?

Let me put it a different way. You figure the center of mass of the circular disk is at z = 0. Fine. Now suppose I pick the disk up, raise it 20 meters. Is its center of mass still at z = 0?
 

1. How do you calculate the z component of the center of mass of a complex shape?

To calculate the z component of the center of mass of a complex shape, you need to divide the moment of inertia about the z-axis by the total mass of the object. This will give you the distance from the z-axis to the center of mass.

2. What is the importance of finding the z component of the center of mass of a complex shape?

Finding the z component of the center of mass is important because it helps determine the stability and rotational motion of an object. It also allows for accurate calculations of forces and torques acting on the object.

3. Can the z component of the center of mass be negative?

Yes, the z component of the center of mass can be negative if the majority of the object's mass is located below the z-axis. This indicates that the center of mass is below the z-axis.

4. How does the shape of an object affect the z component of its center of mass?

The shape of an object can greatly affect the z component of its center of mass. Objects with more mass located above the z-axis will have a higher z component, while objects with more mass located below the z-axis will have a lower z component.

5. What are some methods for finding the z component of the center of mass of a complex shape?

There are several methods for finding the z component of the center of mass of a complex shape, including the use of integrals, the parallel axis theorem, and the center of mass formula. It is important to choose the appropriate method based on the complexity and symmetry of the object.

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