It is strong enough to penetrate through flesh but not bone so we can see if there are fractures or breaks in our skeleton
You look up the element in the periodic table. Subtract the atomic number (small number) from the mass number (big number). The answer is the number of neutrons.
Well I can't see the following physical properties you talked about in the question.
Mass per unit volume ratio is called density.




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I'm not really sure but I do know that it's not 0 because the object is still moving, even if it's only moving at 1 m/s.