Basic fluid mechanics

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What is matter?
Matter has following properties
1. occupies space 
2. All things are made of because . 
3. Matters have mass.

What is mass?

Mass is a measure of the quantity of matter in an object. Mass determines inertia of matter. To be more specific the mass depends purely on the intensity of atoms present in the object. Atoms have electrons,protons and Neutrons.

What is weight?
Whatever may be the intensity of mass (whether it is heavy or light), that mass will be attracted by bigger mass, Earth.Biggest mass in the universe is Sun. If you refer universal law of gravitation, you will find the physics behind it that is Bigger mass always attracts lesser mass. The attraction force depends upon the distance between objects in interest. When mass is attracted by some vector like gravitational force, it also becomes a vector and that vector is called as Weight.

                     Weight = mass x acceleration due to gravity

Why astronauts are freely falling on space though they have mass?
Mass won't change with respect to its position on universe. But weight can differ since because of acceleration due to gravity of earth decreases when the altitude that is height increases. This is the reason for astronauts free fall in space.

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