Difference between mass and weight [duplicate]

I already know that when I standing on weighing machine it's measuring my mass not my weight, but when I take the same machine to moon it's will read different value. According to my information that the mass is constant! How does it change? You will answer me because it depends on gravity. I said to you so we must consider it weight not mass because mass is constant and not change with gravity force!

• Hi Waleed. Your question has been addressed many times already. Note that a weighing machine measures weight not mass. Aug 28 '15 at 6:28
• Hi Mr.John I'm so sorry for my bad pattern in question. my language is very week, I'm from sudan. Sorry again. Aug 28 '15 at 6:55
• Hi Waleed. You don't need to apologise. Your question is a duplicate, but searching this site to see if your question has been asked before isn't easy. I didn't downvote your question and I think the downvotes are unfair. Have a look at the previous question and see if it does give you the answer you want. Aug 28 '15 at 7:01
• Hi Mr. John. Yes it's clear now when I standing on weighing machine it's measuring my weight??? Aug 28 '15 at 7:21

It depends on the scale.

Some scales (called balance scales) balance your weight against the weight of a known mass. Your weight is a force, calculated as mass x acceleration. The weight of the known mass is calculated the same way. Generally, there's some kind of adjustable leverage for the known mass. The scale is calculated by measured weight of known mass in the lab x multiplier used for leverage = weight of person on scale. So if the mass weighs 3 lbs, and there's a 50x multiplier at a specific notch on the scale, then the mass is putting 150 lbs of force on the lever in one direction. If the lever is balanced at that setting, that means you have to weight 150 lbs.

But there's an "error" here. The weight of the known mass depends on the acceleration in your reference frame. If you're on the moon, the mass weighs only ½ lb, so it's only putting 25 lbs of force on the scale, so you only weigh 25 lbs. But the painting on the scale doesn't change, so it's wrong. Technically speaking then, the scale measures mass, not weight, and the numbers should reflect mass. So instead of 150 lbs, it should read 68 kg, or 10 stones, etc. Of course, the scale isn't intended to be used outside of Earth, so the "error" is just a simplification. Because you're actually on Earth, and the scale knows you mass 10 stones, you do, in fact, weight 150 lbs.

Now, spring scales operate differently. In this case, your mass is accelerated by gravity to put a force against a spring of some type. The spring's compression/expansion rate is independent of acceleration, so the length of displacement is directly proportional to the force you're putting on it. If the spring has a rate of 100 lbs per inch, and you're displacing it 1.5 inches, that means you're putting 150 lbs of force on it.

If you move this to the Moon, gravity is only accelerating you at 1/6 g, so you put 25 lbs of force on the spring, and it will only displace 1/4 inch. This 1/4 inch displacement will be correctly measured as 25 lbs.

Addendum: Technically, pounds are a measurement of force, and kilograms are a measurement of mass. In the real world, however, we typically use them a bit interchangeably. If I say "this has 200 lbs mass", I mean it's mass is enough that it would weigh 200 lbs on Earth. If I say "this weighs 68 kg", I mean it is putting the same force on the scale as a 68 kg object would on Earth.

So a spring scale, measuring weight, might use kilograms as the unit. Technically, that's wrong, because the scale measures weight, not mass. But as long as you're on Earth, the mass is correct. Just like the mass scale can correctly state your weight on Earth.