Find the force that the larger box exerts on the smaller box

[rrosa522]

Homework Statement

Two boxes of books are stacked and placed on the floor of an elevator.
the masses of the bottom and top boxes are 9.5kg and 2.5kg respectively. The normal force between the floor and the bottom box is 70N.
Determine the force that the larger box exerts on the smaller box.

Homework Equations

f=ma

The Attempt at a Solution

The answer is 15N {up} . I got 93 N. (9.81m/s^2 *9.5kg)

 

[billy_joule]

First, draw a free body diagram.
What is the acceleration of the elevator? And more importantly, what is it's direction?
You've been given the normal force and the mass...

 

[rrosa522]

First, draw a free body diagram.
What is the acceleration of the elevator? And more importantly, what is it's direction?
You've been given the normal force and the mass...

its not given

 

[billy_joule]

What's not given? The acceleration of the elevator? Clearly that's not given.

You need to find it yourself, it's the second step to solving the problem, after drawing a diagram.
Like I said, you've been given a force and a mass, you've already shown you know how to use Newtons second law and that you know "g". Once you have your diagram solve for a_elevator.