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What is MASS and how it's not quite the same as WEIGHT
When people in the physics business talk about "mass" and how many kilogrammes something is, it sounds as if they are talking about how much it WEIGHS. But there's a distinction between mass and weight which is worth considering.
If you put something on a scale on surface of the earth and find out what it weighs, that's likely to also be the mass of the object. But it would be only a localised science if it only worked on the earth! So, the idea of MASS is used, so the idea works everywhere, instead of just locally. Here's a set of thoughts which help to explain it:
* WEIGHT: Weight is how an object is affected by gravity. Put it on a weighing scale, and the force of gravity attracting the object is registered on the scale.
* MASS: Swing a bag round your head, or push a car in neutral along the road, and you can feel there is a way the object has something heavy about it which has nothing to do with gravity! It would feel the same on the moon, or in space. The way objects have an inertia and respond to being pushed into acceleration; what you're feeling there is the mass of the objects.
You can see this has to make sense by the fact that in space, things are described as "weightless", for example in orbit. But, they aren't "massless", and still have mass regardless of where they are. Rockets pushing a spacecraft forwards have more to push if the spacecraft has more stuff onboard. That's because of mass and inertia. For more about mass and inertia see Ship Shifting
Density is mass divided by volume. How much mass of stuff there is inside a space. The more stuff you can cram into a space, the higher the density.
It's commonly believed that objects with a particular mass always have the same appropriate weight on earth, and that all objects are affected by gravity in the same way they are affected by inertia. That is the conventional belief in science. I am a heretic and I don't think that's necessarily the case! It might be that some stuff has a different inertial mass in relation to its gravitational attractiveness. This would be very interesting, as it might be possible to base a space travel system on it! You can see more about this crazy idea at the page of alternative ideas about Dark Energy