They will only see the door move when both people exert the same force so that the forces are unbalanced. neither person exerts a force on the door so that the net force is zero. one person exerts more force than the other so that the forces are unbalanced. both people exert the same force so that the forces are balanced.

The correct answer is when one person exerts more force than the other so that the forces are unbalanced.

Explanation:

In fact, the door is initially at rest. In order to move the door, a net force different from zero should be applied, according to Newton's second law:

                                        ∑F = ma

where the term on the left is the resultant of the forces acting on the door, m is the door mass and a its acceleration. In order to move the door, the acceleration must be different from zero. But this means that the resultant of the forces acting on it must be different from zero: this is possible only if  the forces applied by the two persons are unbalanced, i.e. one person exerts more force than the other.

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