Why is the system not considered isolated in this scenario?

[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?

 

[Astronuc]

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

What forces are acting on the block?

 

[jack1234]

What forces are acting on the block?

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??

 

[jack1234]

Hmm, can anybody shed me some light?

 

[Astronuc]

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.

 

[HallsofIvy]

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).