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.

Answer 1

net force= ma
400-forcefriction=ma
400-mu*mg=ma
where a= 4/5 m/s^2
solve for mu

Answer 2

I've tried over and over and get mu = .36 which I know is wrong. (Supposed to be .51)

What am I doing wrong?