Gravity on Earth varies by about 0.1% between poles and equator. If someone was buying/selling something mass critical e.g. gold, where is the standard place on Earth where a 1 kg mass produces a 1 kg reading as measured by a device like the following:
Based on some responses, I should give a very specific example. We take the International Prototype Kilogram mass from its repository in Paris. We then take it to one of the poles and measure it using this type of device (also shown above). We then take it to the equator and do the measurement again. The numbers are different.
Is there a place on Earth specified where the scales will read exactly 1kg?
Best Answer
EDIT: Though most of the comments have now been deleted, I state the following for completeness:
The kg is a unit of mass. Weight is a force, and like other forces it is measured (in the SI system) in Newtons. The weight of a body is given by the equation f=ma, where a is acceleration (in this case the local acceleration due to gravity.) Therefore we can rewrite this as f=mg
In the SI system, to say that something weighs 1kg is nonsense, because weight is a force, and should be quoted in units of force.
Nevertheless, there is a standard value for standard gravity
g.According to http://en.wikipedia.org/wiki/Standard_gravity:
According to the same reference, it is chosen as the gravitational acceleration at a latitude of 45 degrees at sea level.
This is of course absurdly precise given the global variation of
gbut there has to be an accepted standard value. It's also arguably not the most useful definition, since the majority of the earth's surface is below that latitude and of so it's likely to be slightly higher than what is needed in practice. If I had been given the opportunity to pick the value, I would have picked 9.8m/s² as a nice round number. http://en.wikipedia.org/wiki/Gravity_of_Earth shows that not just latitude but also continental landmasses influence the local gravitational acceleration.So a 1kg mass weighs 9.80665N at standard conditions.
There is a unit called kgf. This unit is deprecated, because it complicates all kinds of calculations. Nevertheless it is an easy way for most ordinary people to visualise force, and like it or not, this unit is used in the real world. 1kgf = 9.80665N. If you buy a rope or fishing line from a local store, its breaking tension will likely be stated in kgf.
Let's look at some widely used units of pressure: Pa, Bar, atm, kgf/cm², mmHg, mmH₂O. The first two are based on the SI system. Numbers 2-4 (Bar, atm, kgf/cm²) are within a few percent of each other but all are widely used. The last three on the list all depend on the value of standard gravity in their definition. One reason for the perpetuation of units like mmH₂O is that they are easy to measure directly, for example in a water manometer. Another reason is that they simplify certain specific calculations, such as the design of water distribution systems.
When it comes to quoting quantities of gas by volume, the situation is even worse. People do measure gas in this way and it is important to know what temperature and pressure they are considering. If everyone could measure all quantities of substance in kg (mass!) and avoid talking about weight and volume, everything would be a lot less ambiguous.