14) Q: When a person on Earth stands on a floor, the upward force the floor exerts on her feet is the force which is directly responsible her perception or sensation of weight not the downward force of gravity. (Assume the person is supported only by the floor.) True or false?
A: (True.) When a person stands on the floor she has both a weight force acting on her in a downward direction and a normal force which is the force the floor exerts on her in the upward direction. If we remove the normal force by springing a trap door located over a bottomless pit the person will fall. During the fall she will feel weightless even though the weight force has not changed. Since the weight force is present both when the person had a sensation of weight and when she didn’t, we have to conclude that the attractive force of gravity is not directly responsible for its perception. The perception of weight exists only when a normal force is present.
We might say that the sensation of weight requires both a normal force and a weight force but it also turns out that the sensation, of weight can occur even when there is virtually no weight, or gravitational attraction force. This is the basis of so-called artificial gravity on space stations in science fiction movies. The spinning motion of the space station creates a normal force on the space station dwellers’ feet. This normal force acts as the centripetal force, which keeps the station inhabitants rotating in a circle along with the space station. If the station rotates at the right speed, the normal force on the inhabitants will be the same as on Earth, and they will feel like they have a gravitational force acting on them even though they don’t. Hence, the perception of weight depends on the normal force.
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