bearycute said:er... if the string is in tact (not broken) and in tension (not slack), can u tell me how 1 mass can move without affecting the other 2?
They move wrt each other, not without affecting each other...
The whole problem is like looking at the 2kg mass sitting on the table top without the other two 1kg mass. Thus it's like asking what will happen when there's a 8N force applied to the left of the 2kg mass.
The 1 kg mass P and Q are there to confuse the students. But then again, if the mass at both end are different, then we have to consider these mass when working out the problems.