physics question

Question

Albert pushes a 12 kg box across a level floor. He pushes with a force of 45 N and observes that the box accelerates in the direction of his push at a rate of 3 m / s 2 . What is the net force acting on the box?

Assuming that Albert's push and the friction with the floor are the only horizontal forces acting, what is the magnitude of the frictional force pushing backward on the box?

Answer 1

Net Force = mass x acceleration

or F=12kg x 3ms^-2

F= 12x3 kgms^-2

F=36 kgms^-2 or 36N

Since applied force is 45N, then net force F=applied force -resistive force

or 36N=45N-F_f

i.e. F_f=(45-36)N

F_f=9N