Final answer:
In Newton's second law of motion, when an object is in equilibrium, the net force is zero. Therefore, in a situation where a wall exerts a normal force and there is a known downward force of 495 N, the missing variable in the equation must be the weight force of the object to balance the forces in the vertical y-direction.
Step-by-step explanation:
According to Newton's second law of motion, the net force acting on an object is equal to the mass of the object multiplied by its acceleration (Fnet = ma). When the forces are balanced (such as an object at constant velocity), the net force is zero. In the case you are studying, since the wall is frictionless but exerts a horizontal normal force, the sum of the forces in the y-direction must also be zero for an object in equilibrium. Thus, the normal force (N) is likely being counteracted by the force of gravity and any other vertical forces present. If the wall exerts a normal force, and there is a downward force due to gravity or weight (495 N in this case), these forces must balance for net force to be zero. Expressing this with the equation you provided:
ΣFy = N - weight force (mg or any other vertical forces) - 495 N = 0
This gives us the equation N = weight force + 495 N to solve for the normal force. Therefore, in the blank, the weight force of the object must be included to ensure the sum of the forces equals zero.