Final answer:
The net force acting on the object is 21 Newtons in the southward direction, which is determined by subtracting the total northward force from the southward force.
Step-by-step explanation:
To calculate the net force when multiple forces are acting upon an object, we take into account the direction and magnitude of each force. In this instance, we have a northward force (7n + 12n = 19n) and a southward force (40n) acting in opposite directions. Since these forces are along the same line of action, we can subtract the smaller force from the larger force to find the net force. The net force would therefore be the southward force minus the northward force:
Net force = 40n - 19n = 21n
The result shows that the net force is 21 Newtons to the south, indicating that's the direction of the acceleration according to Newton's second law of motion.