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Mass is Inherent but Weight is Practical

 

Since mass is an absolute property of matter, you would think mass is correct to use for all purposes, including day-to-day applications such as shipping.  But mass itself is not so easy to measure and requires lab conditions to determine.   Weight, however, is quite easy to measure and everyone knows what a weight scale is.  Weight scales come in an assortment of types including the common spring scale, ones with counterweights, or a modern day digital scale.  Weight is practical to measure but mass is not.  Only the measure of weight is practical to work with on a day-to-day basis.1

Some contend that since the acceleration of gravity, or g, is able to vary then weight will not always be true.  The acceleration g varies about 1/2 percent from locations at the Earth's equator (where Earth slightly bulges) to a location at the poles (where it slightly flattens) or numerically, 9.75 m/s2  to  9.85 m/s2.   When rounded to two significant figures, this is 9.8 m/s2, which is good enough for day-to-day practical usage.  Moreover, even though g varies per location, a calibrated weigh scale will always show the correct weight.  The measured weight of an item may vary according to its location on Earth, but then this is the actual measured weight.  (Weight can change depending on its location).  

Some further contend that since the accleration g is less on a mountain top or when flying on an airplane that it will change the weight (make it less).  Although this is scientifically true, it does not justify from a practical standpoint.  Although an item may weigh less at a mountain top, it did not start there.  A shipped item, for example, will most likely begin at ground-sea level before it makes its journey up a mountain.  Likewise, any aircraft must first take off from the ground before it reaches a higher altitude.  The load-bearing weight of any structure, whether it is a freight truck or a cargo aircraft, must always bear the weight at ground level (where its voyage begins) before it journeys upwards to an altitude and its weight becomes less.  In other words, it must safely bear the maximum weight (be structurally capable) at its beginning point which begins at ground level.  The weight at ground level (where cargo is weighed) is then what only really matters,  where g is 9.8 m/s2.   Therefore, from a completely practical standpoint, the acceleration g is always a constant 9.8 m/s2 and the measurement of Weight is a practical true measurement anywhere on Earth.

 

     

Freight or cargo transportation travelling to higher altitudes will begin and return to ground level where gravity's acceleration g is the greatest and weight is a maximum

 

Since weight is the practical entity, only weight should be discussed on a routine day-to-day basis including shipping customs.  Mass should never really enter the picture, it is only an entity for the science laboratory.  This points out an issue in human history where somebody was opposed to the word "slugs" used as a unit of mass.  But not mass nor the unit "slugs" should ever be used in common trade discussions, only weight.  Therefore the opposition of the word "slugs" was erroneous in history.  The unit "slugs" was only meant for a scientist.  If only history could be corrected !

Mass .. is only an entity for the science laboratory.

Another possible blunder is that someone didn't want to use "Newtons" as a weight unit because it was named after a person.  But there are many other units named after people:  volts (Alessandro Volta), amperes (Andre-Marie Ampere), watts (James Watt), or tesla (Nikola Tesla) - just to name a few.  It could be perhaps that weight was thought to be used more commonly than other units, and the name "Newton" would become too known.  That's possible, but the simple solution is to use "pounds" for a weight unit more often than "Newtons".  If this was explained back in time, it might have saved a lot of trouble !  Now days, we have to tolerate another unit of weight (or force) that is called kilograms-force, or kgf.  This begins mixing weight with mass, what was previously warned as a possible source of malpractice, with potential of serious error.  (Think of the aircraft weight-placard and any incidents related to overweighted aircraft).

For the record, the relation W = mg is from Isaac Newton's second law of motion, F = ma.  'W' is substituted for 'F' since weight is a type of force (sometimes called a static force) and 'g' is substituted for acceleration 'a' since 'g' is a specific type of acceleration caused by the Earth's pull of gravity.  Newton was rightfully credited for the force unit named "Newton" but perhaps this was not fully realized.

F = ma
W = mg

Weight is practical to measure but mass is not.

  The relation W = mg is derived as a special case of Newton's Second Law  F = ma

 

 

 

1.  A weigh-scale most people are familiar with is the common bath scale.  Although the unit of pounds is correct on a bath scale, if there are units of kilograms also showing on the scale, it is considered not really correct.  (It should be Newtons).  It is attempting to convert your weight to kilograms mass.  If you weighed yourself on a mountain top or on an aircraft, you will find that you weigh less, but your mass has not changed since it is intrinsic.  In this case, the scale that is attempting to read your weight in kilograms is incorrect.  This is not the same as shipping customs since you are trying to obtain a true reading in a different gravitational field.  The baseball that you throw on a mountain top weighs less, but has the same impact of force when caught by the catcher.  The weight is less but the mass is the same.  The scale that is showing less kilograms on a mountain top than at ground level is incorrect (since mass does not change).

2.  See also Mass is Not Measured But Calculated (8/15/23)