Question about analyzing forces

1. Mar 1, 2012

jaja1990

In a try to test my knowledge of Newton's laws, I have posed the following question to myself.

My attempt:

(The arrows indicate direction.)

I have been informed that the solution is invalid, and that the question itself is wrong. Can you tell me why the problem is wrong/invalid?
Someone tried to explain to me, but I didn't really understand.

2. Mar 1, 2012

Staff: Mentor

For one thing:
This contradicts Newton's 3rd law.

A and B will exert equal and opposite forces on each other.

3. Mar 1, 2012

jaja1990

Thanks for the simple answer.