Before that time different solids, liquids and gases were all thought to be only loosely connected in conceptual terms. As with energy, the idea that mass is common to all objects is relatively new and again dates back to around the nineteenth century. The reason for this is that while the mass of the astronauts hasn’t changed, the pull of the Moon’s gravity is only one sixth of what the Earth’s gravitational pull is. For example, astronauts walking on the surface of the Moon have the same mass as on Earth but only weigh one sixth of what they would do back home. Weight is actually a measure of the gravitational force (pull) felt by a body and is measured in newtons (N) (note that scientific units that are named after people are almost always in lower case when spelled out fully, hence newtons and not Newtons, watts and not Watts etc.). Note that mass isn’t the same as weight, although it's often thought to be. This latter definition isn’t strictly true, but is good enough for our purposes here. Another and simpler way of defining mass is to say that it's the total amount of matter in an object. Mass is strictly defined as a measure of a body’s inertia, i.e. letter) in the equation in turn before addressing the question of what the equation means, but if you want to see worked examples of the equations you can do so here. He could have chosen any number of symbols but chose two parallel lines because, as he himself put it, "noe 2 thynges can be moare equalle". This allows us to be write the equation in another, slightly unusual, but equally correct way: E = m x c x c As a matter of interest, and to complete the terms used in the equation, the equals sign was only invented during the 16th century, by the Welsh mathematician Robert Recorde, apparently unhappy having to write out "is equal to" in his work. we multiply the speed of light by itself hence c 2 is the same as c times c. In order for the equation to be correct we need to "square" the term c (the speed of light), i.e. This is because physicists use the case of letters as well as the letters themselves to denote particular physical entities, quantities and constants in equations. Note that the case of each letter is important and it would be incorrect to show the equation as, for example, e = MC². In other words: E = energy (measured in joules, J) m = mass (measured in kilograms, kg) c = the speed of light (measured in metres per second, ms -1 ), but this needs to be "squared".
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