Why $\rm kg$ in daily life but not $\rm kg~m/s^2$?

Question

I had a question regarding weight so here I begin W=mg (where w=weight,m=mass,g=gravitational acceleration) w= kg*m/s^2(kg is the si unit for mass and weight , m/s^2 is the si unit of gravitation) w= kg m/s^2 doesn't it look like force formula like 1 newton is equal to 1 kg m/s^2 so why in daily life is weight measured in kg rather than in kg m/s^2? and also why not in newton if the value is equal? my father asked me to find this out as homework while solving the derivation as

Answer 1

Because the everyday colloquial use is wrong. And you are perfectly right.

When we in daily life say:

my weight is 75 kg,

then we should actually to be technically accurate have said:

My weight is 750 N (approximately),

or

my mass is 75 kg.

The reason is, as you have also noticed, that weight is a force. It is the gravitational force that Earth exerts in us. It is calculated as $w=mg$ giving units of $\mathrm{N=kg\cdot m/s^2}$, a unit bundle called Newton, $\mathrm N$.

So, don't blame science for this ambiguity. Blame our layman use of units.

Answer 2

You are correct weight should be measured in units of $kg$ $m/s^2$, and mass should be measured in units of $kg$.

This type of problem occurs occasionally, where two words that mean the same thing in common language, but have different definitions/meanings in science.

Scales technically tell you the mass, by measuring the weight and since the gravitational field, $g$, is approximately constant around the world, scales are calibrated for it, so that they give the mass.

Its is not flawless, because of the variations in the gravitational force sometimes scales have to be calibrated for the location.