Mass and Weight

Weight are mass are not the same.
The mass of an object is the 'amount of matter it contains'. The mass of an object stays constant if you move it. Mass measures the resistance of a body to a force, and is equal to the ratio of the applied force to the acceleration the mass experiences as a result of the applied force.
\[Mass =\frac{Force}{Acceleration}\]

This is commonly written as  
  and is known as Newton's Second Law.
The weight of a body is a very different concept from mass. A body only has a weight because it is in the gravitational field of another body. Thing on the Earth experience the Earth' gravity. Everything on the Earth experiences a force towards the centre of the Earth of 9.8 Newtons for every kilogram of mass.
For this reason we can write the weight of a body on the Earth as  
\[g=9,8 m/s^2\]
In the absence of a gravitational field, a body is weightless, even though the mass is the same as if it were in a field eg on Earth.

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