The sum of the three forces acting on the center point of the rope is assumed to be zero because The sum of the three forces acting on the center point of the rope is assumed to be zero because This point has a very small mass. Tension forces in a rope always cancel. This point is not accelerating. The angle of deflection is very small.

An object can be acted on by several forces at the same time, these forces are vector quantities (with magnitude and direction associated). The vector sum of all of the forces acting on a body is a single force called the net force when an object is in equilibrium this net force is equal to zero.

According to Newton's second law of motion, the acceleration of an object as produced by a net force is directly proportional to the net's force, direction, and force and inversely proportional to the mass of the object. This means that if the net force is zero an object will have zero acceleration.

Therefore we can conclude that the sum of the three forces acting on the center point of the rope is assumed to be zero this point is not accelerating.

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