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Understanding the electric force is fundamental in the realm of electrostatics, the study of stationary electric charges. Electric force is a vector quantity, which means it has both magnitude and direction, and it arises due to the presence of electric charges. It can be attractive or repulsive depending on whether the charges in question are opposite or like, respectively.

When considering electric forces between point charges, Coulomb's Law provides the equation necessary to calculate this force: \[ F = K \cdot \frac{{Q^2}}{{r^2}} \], where \(F\) is the magnitude of the force, \(K\) is Coulomb's constant (approximately \(8.99 \times 10^9\) Nm^2/C^2), \(Q\) is the magnitude of each charge, and \(r\) is the distance between the charges. This equation highlights that the force is directly proportional to the product of the magnitudes of charges and inversely proportional to the square of the distance between them, an example of the inverse square law.

In the given exercise, this law is used to calculate the forces acting on one charge due to the other three placed at the corners of a square. Each pairwise interaction is governed by Coulomb's Law, leading to forces that are central—directly along the line connecting the two charges.

A free body diagram is a powerful tool used in physics to visualize the forces acting on an object and how they influence its motion. In essence, it's a simple drawing that represents the object of interest as a dot or a small box and uses arrows to depict the various forces exerted on it. Each arrow's length symbolizes the magnitude of force, while the direction indicates the direction in which the force is applied.

For electric forces, the free body diagram enables us to see how forces due to electric charges interact. To create an accurate free body diagram of a charge in a system of multiple charges, like the square arrangement described in the exercise, one must draw all the electric forces acting on the selected charge due to the presence of other charges. It's essential to represent both the magnitude and the direction correctly to understand how these forces balance or combine to affect the charge's motion.

Completing such a diagram is not merely an exercise in illustration but the basis for further calculation and understanding. By breaking down complex force interactions into manageable components, the diagram makes it easier to apply principles like Coulomb's Law for each individual force involved.

To analyze the combined effect of multiple forces acting on a single point, we must consider their vector sum or resultant force. This concept is especially relevant when dealing with electric forces because these forces can act in different directions and their combined effect determines the motion of the charge.

The vector sum is a method of adding multiple vectors, taking into account both their magnitudes and directions. When forces are not aligned—which is often the case with multiple charges, like in the exercise provided—the resultant force requires us to use vector addition principles, involving both the vertical and horizontal components of each force.

For the example of charges at the corners of a square, the forces from adjacent charges can be combined by finding their components along perpendicular axes and summing those to find the total force in each direction. The diagonal force is already along one axis, so it simplifies the calculation. Upon summing, the charge's movement will follow the direction of the resultant force, illustrating the collective influence of all individual forces at play.