AP Physics 1 Question 201: Answer and Explanation
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1. A person standing on a horizontal floor feels two forces: the downward pull of gravity and the upward supporting force from the floor. These two forces
- A. have equal magnitudes and form an action/reaction pair
- B. have equal magnitudes but do not form an action/reaction pair
- C. have unequal magnitudes and form an action/reaction pair
- D. have unequal magnitudes and do not form an action/reaction pair
Correct Answer: B
Because the person is not accelerating, the net force he feels must be zero. Therefore, the magnitude of the upward normal force from the floor must balance that of the downward gravitational force. Although these two forces have equal magnitudes, they do not form an action/reaction pair because they both act on the same object (namely, the person). The forces in an action/reaction pair always act on different objects. The correct action/reaction pair in this situation is the Earth's pull on the person (the weight) and the person's pull on the Earth.