What’s the difference between mass & weight?
The first time I had heard of this question was in a Doordarshan serial Brahmand – universe. Universe being my super favorite topic. I still remember it like yesterday.
Aakash Bahut uncha hai aur door bahut hain taare, tairte hue sainkdo sooraj, brahmand ke ye banjaare.
The sky is far too high, & far are the numerous stars. Myriads of swimming suns, these are Wanderers of the universe.
Wow, still sends a chill down my spine.
The story follows 2 students. One of them puts a coin in the machine promising weight on the moon.
He feels cheated because the slip from the machine reads 5kg! He’s the heaviest in his class ;).
How did he lose 25kg? Does the journey drain him? Or is the weighing machine faulty? I like to start my lecture on units & dimensions with such very questions. Even after so many years.
Then I proceed to explain the curious & confused lot.
Weight is the force with which earth pulls an object towards its center. This force is called as the Gravitational Force.
i.e: Weight = Gravitational Force
So the first mistake is the unit of weight. Weight is a force & so its unit must be Newton & not kg. If at all we have to use kg for weight, we must use it as Kg-Force & not just kg.
Coming back to our story, the gravitational force follows the inverse square law. In simple words, it increases with the mass of the object. Also, it decreases with the increase in distance.
So, because the moon has a lot lesser mass in comparison with the lesser distance. The net result is, that the force of gravity on the moon is much less compared to that on earth.
So does that mean we can happily lose ⅚ of our weight if we move to the moon. Well, Psychologically speaking – yes; But Physically speaking – No.
In fact, if you pack up & shift to the moon, there are chances you will end up gaining kilograms. Because you’ll need lesser energy in everything you do. Which translates to more energy getting stored as fat.
That brings our discussion to mass.
Mass is the amount of matter.
By amount I mean the number of subatomic particles. Basically, that means the number of protons & neutrons. Electrons have a negligible mass after all.
Because the number of the massed particles would almost be same when one moves to the moon. Their mass wouldn’t change.
What changes is just the force with which that same mass is pulled by gravity. Even on earth this force or weight changes from the poles to equator.
Now to some misconceptions about mass vs weight.
Technically speaking, Is it right to ask – “what’s your weight?”. Or “give me a kilo of tomatoes? How do we measure mass? How do we measure weight?
What’s your weight?
It may seem rude, but in this discussion, our concern must be technical aspects & not ethical values.
Generally, it would hold true. We all stay in the region where gravity is same. So mass & weight would be one & the same.
Let’s say you are talking to someone who lives near North or south pole. Now the gravity at their place is more than at yours. So a weight of 60 kg at your place would actually weigh greater near the pole.
In such a case is better to shift the discussion to mass rather than weight.
And if humans in the future move to colonize other planets, the weight would just become a relative quantity. Then it would be better off to talk about mass & not about weight.
Give me a kilo of tomatoes?
Wrong again. Kilo is merely a ratio. Kilo = 1000. Kilograms are a 1000 grams. Kilolitres are 1000 litres. 2k17 is 2000 & 17.
You can even have 10 Kilorupees. Don’t get me wrong, I am not weighing the notes. 10 Kilorupees = 10 thousand rupees ;).
So if your sabziwaala has done his BTech, be sure to ask him 1 Kilogram of tomatoes instead of just a Kilo. Chances are that he will be technically strong & pour 1000 tomatoes over you. But if you like the La Tomatina festival held in the Valencian town of spain, go ahead. 😉
Hmm… That brings us to the next question.
How do we measure weight?
If you want to measure weight, you must be measuring the pulling force of gravity. So, your instrument must be able to sense the force. Any instrument which uses springs is actually measuring your weight in Kg-Force.
If you take this instrument to the poles or to another planet, that instrument will give a different Kilogram reading than actual. Unless of course, the gravity on that planet matches that of earth ;).
How do we measure Mass?
To measure mass, you’ll need a 2 side balance with standard weighing pieces. When you add 1 Kilogram on left side, it will balance only when the same 1 Kilogram is on the right side as well.
What is gravity changes? It will change for both sides. Proportionately. Whatever mass is on the left side, the same mass you’ll get on the right side.
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