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Well, in most situations weight and mass are used to mean the same thing - the amount of matter in an object. So, you can say using this meaning that something weighs x kilograms or has a mass of x kilograms.
A kilogram, rather wonderfully, is defined as the measurement of mass equivalent to the mass of the international prototype of the kilogram, which is a cylinder of a platinum alloy kept in Paris. Before that was made, it was defined as the mass of a litre of pure water at standard atmospheric pressure and at a temperature of 3.98 degrees celsius. This was, as you might expect, a bugger to measure.
So, that's weight-as-synonym-for-mass. You can describe a diamond with a molecular mass of, say, one thousandth of the mass of that platinum-alloy cylinder as having a mass of one gram, or a weight of one gram, or as being a five-carat diamond.
However, weight also has a specific meaning, which is force, as Tsuga says. This is measured in Newtons, a newton being the SI measurement of force, and defined as the force required, as Tsuga says, to move an object with a mass of one kilogram forwards at an accelerating rate of one meter per second per second. So, if you have an object with a mass of one kilogram on the floor, the action of gravity on the mass would accelerate it at a rate of (not quite) one meter per second per second, were it not for things like the floor getting in the way.
A way to think about this is freefall. Using the rough rule-of-thumb 1 Newton = the force of earth's gravity on 100g of mass at sea level, imagine an object with a mass of 80 kilograms - say, a human - suddenly appearing some distance above the ground. The force of gravity would pull that body downwards at all-other-factors-excluded constant acceleration (but then affected by drag, increasing air pressure, differential gravity pull and so on), so the body increases in velocity until it reaches terminal velocity - that is, the point at which drag factors such as wind resistance have increased to equal the downward pull of gravity, and downward velocity stops increasing - the downward force is still about the same, but the drag force has increased to match it. Tsuga's carabiners show the ability of the carabiner's tensile strength to resist the downward pull of gravity on the mass of one's body - in effect, they are doing the same job (resisting downwards acceleration caused by gravity's action on the weight of our bodies) that the floor does when we are not hanging from anything.
So, weight has two meanings - one being a measure of mass and one being a measure of force.
In fact, there's a relatable example from archery, with which I believe you may be familiar, Olulabelle. A bow might weigh twenty pounds, in the sense that you would need to be able to lift twenty pounds to lift it off the ground. So, if you asked "what is the weight of that bow", the answer "twenty pounds" would be quite correct. However, the question might be asking about how much force needs to be applied to the bowstring to pull it back fully. If the bowstring needed fifty pounds of force to pull it back and hold it against the elasticity (that is, the force of the bow trying to pull it back to its normal shape), it would be said to have a fifty pound draw weight (or pull), as I understand it. So, two different meanings of "weight", one traditional and one scientific. |
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