What Defines a Closed System Free from External Forces?

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Homework Help Overview

The discussion revolves around the concept of a closed system in physics, particularly in relation to momentum conservation and external forces. The original poster questions the definition of a closed system, especially in the context of a toy car colliding with a wall, and whether anything can truly be considered isolated from external influences.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the definition of a closed system and discuss the implications of external forces on momentum conservation. Questions arise about whether walls and other structures can be considered part of the system and how they interact with external forces.

Discussion Status

The discussion is ongoing, with participants offering insights into the nature of systems and external forces. There is recognition that the concept of a closed system can be somewhat artificial, and various interpretations are being explored without a clear consensus.

Contextual Notes

Participants note the complexity of defining a closed system, particularly in real-world scenarios where objects are interconnected. The original poster expresses difficulty in grasping these concepts, indicating a need for further clarification.

Drizzy
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Homework Statement



apparently momentum is only sure to be conserved in a closed system - no external forces. So that means if a toy car collides with a wall then the total momentum efter won't be the same as before. That is because the wall would
be joined to something else and ultimately to the Earth.

What exactly is a closed system with no external forces? And isn't everything on the Earth connected to earth?

2. Homework Equations
3. The Attempt at a Solution
 
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A closed system is exactly what you said. No external forces.

The idea of "system" is fairly artificial. You draw a "bag" around the things you are interested in. That's your system. For purposes of conservation of momentum, you have to take notice if any forces go in or out through the "bag."

So if your system is just the toy car, then it is not closed. Forces involving the wall penetrate in through the "bag."

But suppose instead of a wall it was a big block of concrete. And both the car and the stone were floating in space with nothing contacting them. And you include both masses as part of your "system." Then for your system there are no forces penetrating in or out. And momentum is conserved. It may be very hard to accurately measure the change in velocity of the big block of concrete when a small toy car hits it. If the toy car masses 100 grams, and the concrete masses 1E8 grams, for example, the change in velocity of the concrete will be correspondingly small.
 
So you mean that there are things that can affect the wall other than the car?
 
Drizzy said:
So you mean that there are things that can affect the wall other than the car?

^_o
 
Sorry I am going to sound stupid but this is hard
 
The wall is attached to other walls that make up a building, the building is attached to the earth. Yes, there are a lot of things that 'affect the wall other than the car'.
 

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