Somebody said they are different because of gravity.
In: 33
**Mass**
An object’s mass is what you get when you add up the “heaviness” of all the atoms in the object. We could call that the “atomic mass” and measure it in “atomic mass units” or amu’s.
A water (H2O) molecule, for example, contains 2 Hydrogen atoms and 1 Oxygen atom, and the “heaviness” of all the bits and bobs in that water molecule will be the same all over the universe. Of course it’d be the same! The number of amu’s hasn’t changed, has it?! Nope, because there’s still 2 hydrogens and 1 oxygen whether you’re on Jupiter, in your garage, or in the dead of space.
Obviously amu’s are pretty tiny, so for human-relevant things, we prefer to measure the mass in kilograms instead. Kg.
**Speed**
As you know, if you drop a water balloon either on Earth or on the moon it won’t just fall at a constant speed, instead it will accelerate towards the ground, getting faster and faster as it goes.
Just like the “miles per hour” you know and love, speed can be formatted as “meters per second” or “m/s”. (And this can also be written as “ms^(-1)”.)
**Acceleration**
Now, since the object is getting faster and faster and faster as it falls, it’s gaining some extra speed “per second” that it’s falling.
You might even ask “How many meters-per-second is it gaining *per second*?” And that would be you asking for it’s acceleration.
On Earth, as things fall, they get 10m/s faster *per second* that they’re falling. (“10 meters per second *per second*”). Whereas on the moon, the acceleration of Gravity is only 1.6 meters per second *per second.*
“Meters per second *per second*” can be written as “ms^(-2)”.
**Weight**
So, since the same object will accelerate downwards more slowly on the moon than on the Earth (ie. the “gravity” is different), the combinations of Mass and Acceleration are different.
In other words, the same mass (kg) will be affected by the different accelerations/gravities (ms^(-2)) depending on where it is. Ie. the “kg” x “ms^(-2)” will be different.
kg ms^(-2)
And these “kg ms^(-2)” are your units of “weight”. You can call them “Newtons” or “N” for short.
So weight is actually a “force”. When you hold that 1kg object in your hand, you’re not so much feeling its mass (in kg), you’re feeling it *force*fully pressing down into your hand because of gravity sucking it into your flesh (in Newtons).
If there was no gravity, it wouldn’t “force” itself down onto your hand… and you might even say it was “weightless”… despite it still having a “mass”.
So, “Weight” is the vertical force exerted by a “mass” as a result of gravity.
**Summary**
So, the long story short is that “Mass” is the “heaviness” of all the atoms, no matter what those atoms are doing, where they are or where they’re going.
And it’s measured in kg. Or kilograms.
Whereas the “Weight” is the force of those atoms pressing into your kitchen scales when they’re being sucked down onto those scales by your chosen gravity.
And it’s measured in kg ms^(-2). Or kilogram meters per second *per second*. Or Newtons.
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