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Q & A: Difference between weight and mass

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Most recent answer: 06/20/2008
Q:
If meathode of measuring mass of Air directly available (for Ex by chart and density),then why in caculating jet engine's thrust engineer uses weight of air and then after the convert it in into mass of air by deviding weight by gravity= 9.8 meter/sec 2............?
- Amit Patil (age 22)
mumbai,maharashtra,india
A:
Mass is a quantity that relates the reaction acceleration due to a force, F.  One of Newton's Laws is F = M*a   or M = F/a where a is the acceleration.   Weight has dimensions of a force.  On the surface of the earth the amount of force pushing down on a scale is W = M*g where g = 9.8 m/sec is the acceleration of an object dropped at the surface of the earth.
Now the standard value of g is evaluated at the surface of the earth. If you go above the surface of the earth, g falls off as R^2/r^2 where r is the distance to the center of the earth and R is the radius of the earth. Likewise if you would measure the weight at the bottom of a very deep hole you would find that g also falls off as r/R.

Lee H 

(published on 06/20/2008)

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