Why would Newton's 2nd law be meaningless here?

[autodidude]

If an isolated body starts to accelerate, why would saying the system isn't properly isolated render Newton's second law meaningless?

pg. 60 of K&K:

http://books.google.com.au/books?id... carelessness in isolating the system&f=false

 

[Drakkith]

As the example says, if the object starts to accelerate you have no way of telling whether or not it is actually isolated unless you can find something else to measure. Is the system isolated and the laws of physics broken? Or is it that you haven't isolated the system well? You don't know! You need an independent way of telling.

 

[autodidude]

Thanks Drakkith...I think I might've misinterpreted what it was saying a bit