# Closed System and Center of Mass

#### MHD93

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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#### Doc Al

Mentor
Position with respect to what? Is it moving?

#### MHD93

hmmmmm
I'd better have said "...change the velocity of its center of mass..."

#### Doc Al

Mentor
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.

#### pallidin

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.

#### Doc Al

Mentor
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.

#### pallidin

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.

#### Doc Al

Mentor
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.

#### pallidin

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.

#### Doc Al

Mentor
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.
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.

#### pallidin

OK, so what is physically accepted as a "closed system"?
COM changes can and do occur in violent internal explosions.

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