Physics
posted by Eileen .
a force of 400 N Pushes on a 30 kg box. Starting from rest, the box attains a speed of 4 m/s in 5 seconds. determine the mu between the box and floor.

net force= ma
400forcefriction=ma
400mu*mg=ma
where a= 4/5 m/s^2
solve for mu 
I've tried over and over and get mu = .36 which I know is wrong. (Supposed to be .51)
What am I doing wrong?
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