Apparent weight

The apparent weight of an object is the value a spring weighing scale would measure. It is the reaction force by the floor that prevents the object from falling through it (see Normal force), keeping the acceleration of the object and the floor the same. Unlike your weight, your apparent weight always corresponds to how heavy you feel.

Related Topics:
Spring - Weighing scale - Normal force - Weight

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Most of the time, the apparent weight is the same as the weight. For example, if Alice has a mass of 65 kilograms and is standing at rest on the floor, gravity will pull her downward with a force of:

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where m is the mass and g is the acceleration due to gravity. The floor exerts an equal but opposite force on Alice, preventing her from falling through the floor. By definition,

Related Topics:
Mass - Acceleration due to gravity

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F_{mbox{gravity}} is Alice's weight while her apparent weight is the reaction force exerted by the floor. In this case both quantities are thus equal to each other.

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It is the reaction force of the floor that makes Alice feel heavy and not gravity pulling on her. If the floor on which she is standing on were suddenly to collapse, gravity would continue to pull on her, but the reaction force of the ground would suddenly have disappeared. She would then be in a freefall toward the ground. During this freefall she would experience weightlessness. Alice's apparent weight during the freefall is zero, while her weight remains unchanged.

Related Topics:
Freefall - Weightlessness

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The crucial difference between gravity and reaction forces is that gravity is a long range force that acts uniformly throughout a body, while reaction forces are short range forces that only act on the boundary of an object. Reaction forces are transfered throughout the body by causing the body to deform slightly. This causes stresses to build up within the body. Gravity, because it acts uniformly, does not cause stresses to build up. If one could let the reaction force act in a uniform way like gravity one could experience weightlessness on Earth. For example, strong magnetic fields have been used to let frogs float, see here!

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