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Inertial mass :- "resistingness"
You're weightless, floating forward in space - in a spacesuit - and you see a large thing, similarly weightless, floating toward you.
It might be a good idea to find out what sort of impact a collision would produce!
(Your own "gravitational mass" or "weight" on Earth would be about 70 kilos.)
If you both met - each traveling at say 5 mph (opposite directions) - then, as you've both got 70 kilos of inertial mass, you'd both stop dead!
'Cos it'd be like running full tilt into a brick wall at 10 mph. - Ouch!
So the important thing is `inertial mass' - Not weight?
Well, in space you haven't got any weight. But you've still got the same body - so you've still got the same `inertial mass'.
"Weight" is how much your `inertial mass' gets pushed towards another body. It's a vector quantity.
The various "masses" defined (and discussed)
Maybe also see Reader's Mail 1 & Reader's Mail 2
Reaction of mass held in rotation, or released:
"Centripetal or Centrifugal Force"
Reaction of mass to varying radius of rotation:
"Conservation of Angular Momentum"
Reaction of mass to varying angles of rotation:
Reactions of mass to other movements: