What Is the Contact Force Between Two Boxes with Different Frictions?

[fahlim003]

Homework Statement

Box A
Mass=3.2kg
Force of Friction=2 Newtons

Box B
Mass=2.1kg
Force of Friction=1 Newton

Force applied 10.5 Newtons [horizontally].

http://img162.imageshack.us/img162/7697/f1qd1.png

Q. Find the magnitude of the force of contact between the two boxes.

Homework Equations

F=ma

The Attempt at a Solution

I need to find acceleration first since the Force of Friction of both boxes is not greater than the Force Applied.

Fnet=(mA+mB)a

(mA+mB)a=Fa-(FfA+FfB)
(3.2kg+2.1kg)a=10.5N-(2N+1N)
(5.3kg)a=10.5N-(3N)
a=10.5N-(3N)/5.3kg
a=7.5N/5.3kg
a=1.415m/s/s

Now, I tired many, many methods of solving for this from here on, including the example from this page:https://www.physicsforums.com/showthread.php?t=202002

However, the result seems still to be wrong.
I got 7.5N as the internal force both boxes act upon each other.

Any help is appreciated although not dire.

 

[alphysicist]

Hi fahlim003,

The Attempt at a Solution

I need to find acceleration first since the Force of Friction of both boxes is not greater than the Force Applied.

Fnet=(mA+mB)a

(mA+mB)a=Fa-(FfA+FfB)
(3.2kg+2.1kg)a=10.5N-(2N+1N)
(5.3kg)a=10.5N-(3N)
a=10.5N-(3N)/5.3kg
a=7.5N/5.3kg
a=1.415m/s/s

Now, I tired many, many methods of solving for this from here on, including the example from this page:https://www.physicsforums.com/showthread.php?t=202002

However, the result seems still to be wrong.
I got 7.5N as the internal force both boxes act upon each other.

Any help is appreciated although not dire.

I'm not getting the answer of 7.5N. Can you show how you got it?