Let t=0 represent the moment when the person reduces its force to 10N.
In that moment, the forces acting over the box are shown in the diagram:
The free body diagram for this object is:
Assuming that the initial velocity is 5.0 m/s towards the right, the net force acting on the box is 5N towards the left. Then, the box wil decelerate until it comes to a stop.
The acceleration of the box is constant and negative, until the box stops and the acceleration is 0.
The velocity of the box is 5.0m/s initially, and it decreases linearly until it reaches 0.0 m/s.
The position of the box grows as a quadratic function of time until it reaches a maximum displacement and then it stops.
These qualitative graphs, force diagram and free body diagram correspond to the described situation.
