How Do You Calculate Mass and Frictional Force in Physics Problems?

[pixies6]

Homework Statement

A force of 400 Newtons is applied horizontally to a box. The kinetic coefficient of friction is .64 and the acceleration of the box is 4 m/s^2. Find the mass and the force of friction.

Homework Equations

assume that acceleration due to gravity or g=10 m/s^2

The Attempt at a Solution

please explain!

 

[Simon Bridge]

Welcome to PF.
Did you draw the free-body diagram?

There should be four forces on it.
Are the forces balanced?
Are any of them related to each other?

 

[pixies6]

yeah i drew the free body diagram and have that there are four forces but I am having trouble finding the normal force without knowing the mass since i need it to find the force of fricition

 

[Simon Bridge]

Write everything out as symbols - use algebra to get an equation that says "m=" a bunch of symbols. Then put the numbers in.