Final answer:
The correct statement according to Newton's Laws is (c) There must be a net force to change a system's motion. Newton's first law supports this by asserting that an object remains in constant motion unless acted upon by a net external force, while his second law explains that a net force brings about a change in motion.
Step-by-step explanation:
According to Newton's Laws, which of the following is a correct statement? The correct statement is (c) There must be a net force to change a system's motion. This is supported by Newton's first law, often called the law of inertia, which states that an object will maintain its state of motion (either at rest or moving at a constant velocity) unless acted upon by a net external force. Thus, motion can indeed exist without a net force, as long as that motion is constant and unchanging.
Newton's second law further clarifies that a net force causes a change in motion, not motion itself. To illustrate this, consider a book resting on a table. The forces holding the book together (internal forces) are neglected when applying Newton's second law because they do not contribute to the motion of the book as a system. If a force, such as a push, were applied to the book, resulting in a net force, the book would accelerate in the direction of that force, demonstrating a change in motion.
Finally, when considering the 'system of interest', Newton's second law requires an evaluation of all the external forces acting on the system to understand how motion might change. For example, when viewing a car as a system, the engine's force pushing against the ground and the resulting frictional force are external forces that would need to be considered.