FPS System: Why 1 Slug = 15.59 kg?

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Tahira Firdous
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Why 1 slug is equal to 15.59 kg ? In FPS system we use slug as a unit of mass and pounds as a unit of force but sometimes we also use pounds as mass .I know that pound-force and pound-mass are almost equal at Earth's surface . But both these quantities have differelnt dimensions then how can we use it in place of each other and why don't we use slug for mass?
 

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1 slug is the mass that sees an acceleration of 1 ft/s2 when subject to a force of 1 pound-force. It is the imperial equivalent of "1 kg is accelerated by 1 m/s2 when subject to a force of 1 N".

Tahira Firdous said:
I know that pound-force and pound-mass are almost equal at Earth's surface
They are not equal. 1 pound-mass feels about 1 pound-force as gravitational force on the surface of Earth.
Tahira Firdous said:
and why don't we use slug for mass?
The unit system is overly complicated even without the slug, introducing one more unit doesn't help.
Going metric would be the right thing. The trend is there, it is just very slow.