what is the sum of all forces acting on an object?
The sum of all the forces acting on an object is the net force or resultant force. If all the forces cancel out, the resultant force is zero, else, the unbalanced force will cause acceleration of the object. Force is a vector and hence has both a direction and magnitude. For example, gravitational force (on earth) acts in downward direction and has a magnitude equal to mass of the object and acceleration due to gravity. Once we know all the forces that are acting on a body, we resolve all of them into components (typically horizontal and vertical) and take summation of forces in each direction. If such forces cancel out (in each direction), the object maintains its state of rest or constant motion. If however, forces do not cancel out, there is a net resultant force on the object that will cause acceleration of the object (as given by Newton's second law of motion).
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