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Two children push a wagon carrying a third child, and the system of interest is the wagon plus its rider. How is the grip of the rider on the wagon classified?
AAn external force, so it changes the motion
BAn internal force, so it does not change the motion
CAn external force, but too small to matter here
DA fictitious force, since the wagon accelerates
Answer & Solution
Correct answer: B. An internal force, so it does not change the motion
1. An external force acts from outside the system of interest.
2. An internal force acts between elements within the system.
3. The rider and the wagon are both inside the system, so the grip between them is internal.
4. Only external forces affect the motion of a system, so this force does not change it.
5. The pushes from the other two children come from outside, so those are the external forces.
_Source: OpenStax College Physics (CC BY 4.0), Ch 4 "Dynamics: Force and Newton's Laws of Motion", section 4.3 Newton's Second Law of Motion: Concept of a System_
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