Box of Mass Friction Problem

[Kakateo]

Homework Statement

A box with a mass of 2.2 kg sits on top of another box with a mass of 3.8 kg. The coefficient of friction between the two boxes is 0.25, and the coefficient or kinetic friction between the larger box and the horizontal surface is 0.32. Determine the largest horizontal force that can be applied to the larger box so that the smaller box does not slip off.

Homework Equations

Let fs be friction static
Let fs be friction kinetic

Fnet = Fa - Ffk
Fnet = Fa - Ffs

The Attempt at a Solution

I made Fnet for equation #2 0. I isolated each for force applied for each of the equations and set them equal to each other.

My equation before plugging numbers in for variables was this:

Ffk = Fnet + Ffs

I tried expanding the equation then solving for acceleration in which I got a negative value so I know I did something wrong.

I'd like to know the logic behind solving this equation and the process itselfThanks so much

 

[NascentOxygen]

Hi Kakateo http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

Determine the largest force that can be transferred (via friction) to the top box.