# B How come weight difference doesn't cause problems in trade?

1. Aug 9, 2016

### Andreas C

This is not strictly a physics question, I just figured this is the best place to ask it.

I think everyone is familiar with the subtle difference in the effects of gravity from the poles to the equator. At or near the equator, objects weigh about 0.5% less than they do at or near the poles. My question is, how come this doesn't cause any problems in trade?

Let me explain: say you run a company based on the Netherlands or Sweden or some place similar and another company based in some country near the equator orders 10kg of something precious, say gold. You weigh it precisely, but when it is shipped to whomever ordered it, guess what he will tell you: "Look, you a-holes, I ordered precisely 10kg, but received 9.95kg instead, are you trying to scam me?".

I guess that if I owned a big business I would be bummed to know that I consistently lose 0.5% of my earnings in that manner! How come there exist no such problems?

2. Aug 9, 2016

### Staff: Mentor

Trades happen based on mass (kg), not on weight (N). Scales have to be adjusted to their location if the precision requires that.

3. Aug 9, 2016

### FactChecker

Good question. Maybe they calibrate their scales using standard weights that have undergone the same weight reduction.

4. Aug 9, 2016

### Andreas C

Yes, I know, but I have never heard of scales adjusted to the location. Do you have any additional information on that?

5. Aug 9, 2016

### Andreas C

Yeah, maybe they have a standard set of weights that they use to calibrate them, but I can't find much information on that. It would also be very bad if someone took the scale and transported it somewhere else.

6. Aug 9, 2016

### Staff: Mentor

You should first read about the prototype kilogram and its copies: https://en.wikipedia.org/wiki/Kilogram#International_prototype_kilogram

In most countries, there is a standards institute responsible for certifying scales. That institute will have some standard masses that they use to calibrate their instruments and that will be used to certify that a scale is properly adjusted for use in trade.

7. Aug 9, 2016

### Andreas C

That makes sense, but what happens when a scale is transported?

8. Aug 9, 2016

### Bystander

It has to be re-calibrated.

9. Aug 9, 2016

### Merlin3189

So what would a standard weight look like?
A standard mass is a solid block, but could you make a standard weight like that? Wouldn't its weight vary from place to place and maybe from time to time as the earth rotated beneath the moon?
I guess a standard weight might be a spring stretched to a certain distance or maybe a pair of coils at a certain spacing with a certain current flowing. Are there any others in common use?

10. Aug 9, 2016

### DrGreg

Where people have said "standard weight" in this thread, they should really have said "standard mass".

If you use a "balance scale" instead of a spring-based scale, it doesn't need recalibration when you move it.

11. Aug 9, 2016

### Staff: Mentor

There is no standard weight, and no need for that. Devices that can measure accelerations can easily be transported, and masses can be transported as well, both combined allows to calibrate force measurements.

12. Aug 9, 2016

### Andreas C

Ah ok then.

13. Aug 9, 2016

### Andreas C

Sure, but balance scales are a bit old school, no longer used. When I'm saying "standard weight", I mean the object. Of course it's a standard mass, but for whatever reason, the objects are called weights.

14. Aug 9, 2016

### Andreas C

I know they CAN be transported, I just didn't know if they ARE transported in reality.

15. Aug 9, 2016

### epenguin

With the simplest of weight measurement devices - old fashioned two-arm balance - you don't need to recalibrate a thing. You just need to take the box of weights authorised directly or indirectly at the original Standards institute and it works for all commercial purposes and most others without any worries or adjustment just the same anywhere on earth, also on Mars and many other places.

16. Aug 10, 2016

### Andreas C

Yes, but as I've said, balance scales are no longer widely used.

17. Aug 10, 2016

### Staff: Mentor

That doesn't really matter, putting test mass 1 on the scale and then putting test mass 2 on the scale to compare has the same effect.

18. Aug 10, 2016

### sophiecentaur

An ideal standard of measurement should be specified in such a way that anyone (planet Zog) should be able to produce it with no more than a piece of paper which tells them how to produce it. Time and length are both referred to atomic standards - spectral lines and frequencies. Unfortunately, the inhabitants of planet Zog would have no access to standard (or sub standard) kg masses in Paris. There is a lot of work (See this wiki link) on finding a reliable way to assemble a known number of atoms of an element to produce a universal standard of mass. That process seems to be a lot more fraught than looking at a simple Hydrogen line. The Zoggians are still waiting for information how to get their own personal 1kg for laboratory use.

19. Aug 10, 2016

### Andreas C

I don't disagree, it's just that I didn't know to what extent that was done in trade.

20. Aug 10, 2016

### Andreas C

I remember hearing about an experiment involving a near perfect pure silicon sphere that weighs exactly 1kg, where there was effort trying to measure the silicon atoms contained inside the sphere so that the kilogram will no longer have to be defined based on a set of standard weights. Instead, we would say "1kg is defined to be the mass of n atoms of silicon" to the Zoggians, and their problem would be solved! Provided of course that they can weigh a precise number of silicon atoms...