So why don't we call it "mass"? Well, according to, "weight" is a very old word, The only time we care about weight is when we're about to snap the cables in the elevator (too much sweetened coffee?) or have some other engineering task where we care about the actual force of gravity (as opposed to the quantity of material). Newton Meter Per Kilogram Per Kelvin (N-m/kg-K) has a dimension of L2T-2Q-1 where L is length, T is time, and Q is temperatur. We use the kilogram because it is a more useful metric in "daily life". Well - with Blue Mountain I might not need sugar but that's another story. You can now express relative weights as a ratio to the reference.Īll I need to do when I move to Jamaica (would that I could…) is recalibrate my scales - and my coffee will taste just as sweet as before. So if I have a 1 kg calibration weight, it might read 9.81 N in one place, and 9.78 N in another place but if I put the reference weight on the scales and then say "if you feel this force, call it 1 kg" - that is what I get. If I calibrate scales using a reference weight, they will indicate (at that location) the amount of mass present in a sample relative to the calibration (reference). When you are interested in "how much" of something there is - say, a bag of sugar - you really don't care about the local force of gravity on the bag: you want to know how many cups of coffee you can sweeten with it. This is in part caused by the rotation of the earth, and in part by the fact that the earth's surface is not (quite) a sphere. It is described as the amount of force required to accelerate a kilogram of mass by one meter per second. There are exceptions however, such as water's density increasing between 0☌ and 4☌.īelow is a table of units in which density is commonly expressed, as well as the densities of some common materials.The problem is that while mass is the same everywhere on earth, weight is not - it can vary as much as 0.7% from the North Pole (heavy) to the mountains of Peru (light).
Increases in temperature tend to decrease density since the volume will generally increase. An increase in pressure decreases volume, and always increases density. If you have a torque value in Newton-meters (Nm) and your torque wrench is in pound-feet (lb-ft, ft-lb) youll need to convert units. However, when regarding gases, density is largely affected by temperature and pressure. To convert from N into units in the left column divide by the value in the right column or, multiply by the reciprocal, 1/x. In the case of solids and liquids, the change in density is typically low. To simply convert from any unit into newtons, for example, from 20 kilogram-force, just multiply by the conversion value in the right column in the table below. Note that density is also affected by pressure and temperature. It is useful to carefully write out whatever values are being worked with, including units, and perform dimensional analysis to ensure that the final result has units of mass There are many different ways to express density, and not using or converting into the proper units will result in an incorrect value. Although almost the entire world uses the newton for the unit of force, in the United States, the most familiar unit of force is the pound (lb), where 1 N 0.225 lb. However, it is important to pay special attention to the units used for density calculations. The calculation of density is quite straightforward.
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