Maximum Acceleration for Flatbed Truck with Jet-Powered Crate

[Antendo.]

Homework Statement

A jet-powered flatbed truck has a crate of mass m, which is strapped down to the truck with a tension three times the weight of the box. Find the maximum acceleration without having the box slide. Us= .9, Uk=.7

Homework Equations

Fun= SUM
Fun= ma
Ff= -(Us) Fn

The Attempt at a Solution

I've figured out the Y components, and resulted in the Ff= 35.3m, my only problem now is where to go from here. I can't for the life of me figure out the other horizontal force other than Ff.

when I attempt the solution, I get

Fun=SUM
= Ff
=35.3m

Fun=ma
35.3m=ma
35.3=a

I know the answer is -7.7 but I can't seem to think of the opposing horizontal force.

 

[Dr.D]

You need to start off with a Free Body Diagram for the crate.

By the way, what is SUM? Fun? Ff? It would really help to put some words with these symbols.

 

[Antendo.]

I apologize, SUM is the sum of all the forces, and Fun is the unbalanced force, in this case it would be force of friction (Ff) and something else. Also Fn is the normal force, and Fg is the force of gravity, and Fa would be an applied force.

My free body diagram includes the box with the force of gravity, and tension acting downward, and force normal pushing it up. Horizontally it has the force of friction resisting the box's movement, but I don't know what it's opposing :(

 

[Antendo.]

bump.