When unbalanced force is acting on the system of mass then as per Newton's II law we can say
here we have
now since the force and mass both given to us so we can find acceleration of object from this
now when 50 N force is applied on a box then this 50 N force is unbalanced force on it and it will produce acceleration to the box
So here we can say that box will accelerate with constant acceleration and hence its velocity will linearly increase with time.
Now the displacement of the object will be given as
so here the displacement will be a quadratic function of time and increases with time as well
so here overall the motion will be of constant acceleration, uniformly increasing velocity and displacement will be a quadratic function of time