# About Isolated System

1. Nov 18, 2007

### jack1234

For the following question, the answer given is b. Non-Isolated

http://tinyurl.com/2s3waq

But may I know why it is not a. Isolated?

2. Nov 18, 2007

### Staff: Mentor

If is non-isolated, it certainly can't be isolated.

What forces are acting on the block?

3. Nov 18, 2007

### jack1234

One of them is friction force, the other one not sure...

By the way, from the other sources, I found the answer is a, I am really confused now, what is the correct answer??

4. Nov 19, 2007

### jack1234

Hmm, can anybody shed me some light?

5. Nov 19, 2007

### Staff: Mentor

The correct answer is non-isolated, because the block is affected by gravity which acts on the mass of the block giving it weight. In turn, friction is an external force related to the weight.

On a horizontal surface, the only way the block could be moving is to have had an external force applied to it. If the driving force has been removed, it must be decelerating due to friction.

The forces are not internal to the block.

Last edited: Nov 19, 2007
6. Nov 19, 2007

### HallsofIvy

Staff Emeritus
If the "system" had been the block and[\b] the surface, then the system would be isolated (since the surface is horizontal, gravity only acts to affect friction so doesn't have to be considered as a separate force).

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