Final answer:
Option b) 40 N could not be a final magnitude of the net force of two different forces with magnitudes of 40 N and 30 N, as there would always be some contribution from both forces unless they're perpendicular, resulting in a net force greater than 40 N.
Step-by-step explanation:
The student has asked what could not be a final magnitude of their net force when two different forces of magnitude 40 N and 30 N act on the same object with unspecified direction. The possible magnitudes of the net force depend on the angle between the two forces. If the forces are in the same direction, the net force is their sum, which would be 70 N. If the forces are opposite, the net force is the difference, which would be 10 N. For any other angle between the two forces, the net force will be somewhere between 10 N and 70 N, inclusive. Therefore, option d) 70 N could indeed be the magnitude of the net force if the forces are in the same direction, making it a possible result.
The magnitude that could not be the net force is option b) 40 N, because it is not possible for the combination of a 40 N force and a 30 N force to solely result in a 40 N net force – there will always be some contribution from the 30 N force, whether it be additive or subtractive, unless the 30 N force is acting perpendicularly to the 40 N force, in which case the resultant would be more than 40 N due to the Pythagorean theorem.