What Coefficient of Friction Keeps a Crate from Sliding on a Decelerating Truck?

[LittleWing]

Homework Statement

A truck is traveling down a 15º incline. The truck then begind to break (decelerate) at a rate of 4 m/s. A crate sits on top of the truck bed..Determine the coefficient of friction to keep the crate from sliding on the truck bed. If the truck were to accelerate up a 15º incline at 4m/s^2 would the required coefficient of friction be the same?

Homework Equations

The Attempt at a Solution

I know I have to calculate the forces acting on the crate but I can't figure outwhere to start without a mass..Did my prof leave something out?

 

[da_nang]

What does Newton's second law of motion tell you about the forces acting on the box in the direction of the incline? And in what different ways can you define the forces acting on the box defined in terms of the data you have?

Spoiler

Specifically, what's the definition of the frictional force? The answer should then come to you.

Spoiler

Again, using Newton's second law of motion, you'll see the equation for the coefficient of friction is independent of mass.