Can you define mass

Answer 1

This is a much more complicated question than it appears. In everyday use, and indeed for most science upto A-level, the intuitive notion of 'amount of stuff' works pretty well - things with more mass have got more 'stuff' in them.

At a higher level there are several different but equivalent definitions. In mechanics you define mass as a measure of the resistance of an object to acceleration; when dealing with high speeds or high precisions you can use E=mc^2 to describe mass in terms of the energy of an object; you can describe mass as 'gravitational charge', measuring how strongly an object is influenced by gravity. There are also several quantum definitions.

Note: mass should not be confused with weight! Mass measures the amount of material present but weight measures how strongly an object is affected by gravity: your weight pulls you down to the surface of the planet when you jump and would be less on the moon (which has a weaker gravity field) whereas your mass would be the same. Mass is measured in kg and weight (as a force) in Newtons. On earth the gravitational field strength, g, is about 10 so 1kg=10N on earth. On the moon 1kg would have a weight closer to 2N.