Physics help in friction?

[Jonathan]

The coefficient of static and kinetic between a 50.0 kg box and a horizontal surface are .5 and .4 respectively
a. what is the acceleration of the object if a 250 N horizontal force is applied to the box ?
b. what is the acceleration if the applied force is 235 N

[physics_philosopher]

F_app + .9(u_k+u_s)mgcos180 = ma

just plug and play, same with part b

[jim m]

a.static friction force opposes horizontal force
Ff=.5*N=-250, just balances FApp out so constant velocity. However in motion, kinetic friction takes over
Fk=.4*500=-200 and net force of 50 moving the box.
acceleration is 1m/s^2

b) FApp not enough to overcome Ff. acc=0