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Weight |
When studying the motion of earthly objects (vector kinematics), and the forces
that make them change speed/direction (dynamics), there is always one force at work: the
gravitational pull of earth (Fg). |
Fg = mg |
where g = acceleration
caused by gravity's pull. It equals approximately 9.81 m/s2 for objects
near the earth. We will later on derive the value of g in the gravitation section.
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| Any object has a mass that is universal in
any part of the universe, although its weight depends on the local gravity. For
example, an apple has mass of 0.1 kg anywhere in the universe, but its weight on the
surface of the Earth is approximately 1 N (0.1 kg X 9.8 m/s2 = 0.98 N). Weight acts down, it behaves as an external force acting on an object.
In effect, the Earth pulls down on the object and the object pulls up on the Earth
as an reaction force or normal force, FN.
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| The free-body diagram shows the two forces
acting on the standing person. The normal force is distributed over both feet. Normal force, FN is actually the force exerted
perpendicularly on a supporting surface (such as floor, pillar, or roadway) supporting the
weight. The weight of a brick resting on a floor numerically equals the downward
load on the floor, and the equal and opposite force on the brick exerted by the floor is
the upward normal force. |
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