Question

Two forces are the only forces acting on a 7.8 kg object which moves with an accelera- tion of 4.5 m/s2 in the positive y direction. One of the forces acts in the positive x direc- tion and has a magnitude of 14 N.
What is the magnitude of the other force f2?
Answer in units of N

Answer 1

the net force is in the directon of acceleration.
net force=ma
net force=7.8*4.5 in y direction

So F2 mus have a x component that cancels the 14N force, as there is no x acceleration
F2 in the y direction is 7.8*4.5 N

Magnitude=sqrt(14^2+(7.8*4.5)^2 )

Answer 2

F1+F2 = M*a.
14+F2 = 7.8*4.5i = 35.1i.
F2 = -14 + 35.1i = 37.8 N[68.3o] N. of W.