Tough question about reaction force Newton's third law -- Please help

[bomberkid]

Homework Statement

. question is as attached. answer is 2f/5..how do u get this?

Homework Equations

F=ma
F= (5m)a
a= F/(5m)

The Attempt at a Solution

gotten a=F/5m , but why the answer is 2f/5?

 

[Joffan]

What force is necessary to produce your calculated acceleration in the second block?

 

[bomberkid]

F?

 

[Joffan]

No... you calculated the specific $a$, now put that back into $F=ma$ along with the second block's mass

Unfortunately the confusion between the abstract "F" and the specific "F" in this problem is perhaps making understanding a little harder than it should be.

 

[bomberkid]

I see. Why not use the mass 3m?

 

[haruspex]

I see. Why not use the mass 3m?

Either will do provided you bring all relevant forces acting on the mass into the equation.

 

[bomberkid]

but if i were to use f=3m, and a is F/5m, i wouldnt get 2f/5 as the answer

 

[haruspex]

but if i were to use f=3m, and a is F/5m, i wouldnt get 2f/5 as the answer

You get the right answer if you bring in all forces acting on the mass and do the algebra correctly. Please post your working.