A falling object that weighs 12 N encounters 3 N of upwards air resistance. What is the size of the net force on the object?

Assume the downward direction is positive and upward direction is negative

W = weight of the object = 12 N

f = air resistance on the object = - 3 N

F = net force on the object

net force is the sum of forces acting on the object, hence the net forces is given as

net force = sum of forces

F = W + f

F = 12 + ( - 3)

F = 12 - 3

F = 9 N

hence the size of net force on the object comes out to be 9 N