Closed System and Center of Mass

AI Thread Summary
A closed system cannot change the position of its center of mass without external forces acting on it, as it will move with constant velocity. Internal forces can affect the distribution of mass within the system, but they do not alter the overall center of mass unless the system is redefined to exclude certain components, like exhausted fuel. The discussion highlights that while the total system's center of mass remains constant, internal explosions can create localized changes in mass distribution. The concept of a closed system is debated, particularly regarding what constitutes its boundaries. Ultimately, the center of mass can change in specific scenarios, such as violent internal explosions, but not in a traditional closed system context.
MHD93
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Hello,

Can a closed system change the position of its center of mass if no external force is exerted on it?
 
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Position with respect to what? Is it moving?
 
hmmmmm
I'd better have said "...change the velocity of its center of mass..."
 
Mohammad_93 said:
hmmmmm
I'd better have said "...change the velocity of its center of mass..."
That's better. If there's no external force on a system, its center of mass will move with constant velocity.
 
Mohammad_93 said:
Hello,

Can a closed system change the position of its center of mass if no external force is exerted on it?

Yes. BUT, an internal force must be destructive in effect in order to work.
This violates nothing, as energy is required and expended.
Think of a rocket with side thrusters.
There is no external force. The center of mass changes.
 
pallidin said:
Yes. BUT, an internal force must be destructive in effect in order to work.
This violates nothing, as energy is required and expended.
Think of a rocket with side thrusters.
There is no external force. The center of mass changes.
No. The center of mass of the system--including the exhausted fuel--remains moving with constant velocity.
 
Doc Al said:
No. The center of mass of the system--including the exhausted fuel--remains moving with constant velocity.

True, of course.
However, exhausted fuel is often conventionally regarded as not being a current part of the system at some point.
"Collectively" definitely yes, but with separation, other, effective aspects arise, including COM changes in the primary mass system.
 
pallidin said:
However, exhausted fuel is often conventionally regarded as not being a current part of the system at some point.
"Collectively" definitely yes, but with separation, other, effective aspects arise, including COM changes in the primary mass system.
If you regard the fuel as not being part of the system, then the force it exerts is an external force.
 
Doc Al said:
If you regard the fuel as not being part of the system, then the force it exerts is an external force.

Doc, you're missing the point.
Internal explosion CAN create off-centered mass.
That the "total system" is centered is NOT the issue here.
What is the issue is that the primary mass system changes.
That's the point.
 
  • #10
pallidin said:
Doc, you're missing the point.
Internal explosion CAN create off-centered mass.
That the "total system" is centered is NOT the issue here.
What is the issue is that the primary mass system changes.
That's the point.
Reread the question that started this thread:
Mohammad_93 said:
Can a closed system change the position of its center of mass if no external force is exerted on it?
Seems pretty clear to me that the 'total system' is exactly the issue.

Obviously, if a 'system' breaks apart so that the parts push against each other, those parts can change their motion. That's hardly a closed system.
 
  • #11
OK, so what is physically accepted as a "closed system"?
I suppose we should start with that.
COM changes can and do occur in violent internal explosions.
 

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