I came across this example question in my mechanics book:
It's a light scale pan with negligible mass, with two blocks A and B, with masses 0.4kg and 0.6kg respectively. The pan is attached to a string.
A tension T (10.3N) is then applied to the string, causing the scale pan to accelerate at 0.5m/s/s.
The question is to work out the force exerted on B by A.
The book explains that the best way to go about this is to work out the force exerted on A by B, and then use Newton's 3rd Law to then say that A will exert an equal and opposite force on B.
So it's just a simple matter of working out R, giving 4.12N, meaning 4.12N is exerted on A by B, thus meaning A exerts the same force on B.
What I don't understand this, is why do we ignore the gravitational acceleration of A? If A is sitting on B, does that not mean that A will also be exerting a force of 0.4g on B? Since this isn't the case, why isn't this so? And what would it entail if I did include A's gravitational acceleration?