## a kilogram is a measure of an object’s

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## Kilogram

The kilogram (also kilogramme^{}) is the base unit of mass in the International System of Units (SI), the metric system, having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially.

The kilogram was originally defined in 1795 as the mass of one litre of water. Modern superseding definitions of a kilogram agree with this original definition to within 30 parts per million. In 1799, the platinum *Kilogramme des Archives* replaced it as the standard of mass.

### 1889

In 1889, a cylinder of platinum-iridium, the International Prototype of the Kilogram (IPK) became the standard of the unit of mass for the metric system and remained so until 2019.^{} The kilogram was the last of the SI units to be defined by a physical artefact.

The kilogram is now defined in terms of the second and the metre, based on fixed fundamental constants of nature.^{} This allows a properly-equipped metrology laboratory to calibrate a mass measurement instrument such as a Kibble balance as the primary standard to determine an exact kilogram mass, although precision kilogram masses remain in use as secondary standards for ordinary purposes.

The kilogram is the SI base unit for measuring the mass of an object, which when related to the pull of gravity on the object will

be recorded as its weight.

## Explanation:

The kilogram is defined by a mass equal to that of a platinum-iridium prototype that defines the SI base unit of measure.

The weight of an object is actually a measure of the pull of gravity on the mass of the object. Still, most of our records will describe the weight of objects or substances in kilograms.

Everyone agrees that it’s not a good idea, and they are quite close to redefining the kilogram using silicon spheres and watt balances. These are based on accurately measuring the Planck constant. This couldn’t be done before because it is such a tiny number that the technology didn’t exist to measure it to a satisfactory degree.

### The last holdout

Historically, all units were base on some arbitrary physical object. We have always been trying to get away from that. The kilogram is just the last holdout.

Side story: My dad told me a story about my grandmother. She would go shopping at the old markets in Hong Kong for her groceries. At the time the most common way to measure your goods at the market was using the simplest scale possible, like this:

Wherever the counterweight (a catty?) was positioned on the stick would indicate the weight of your goods. She never trusted the market vendors, so she always carried her own counterweight with her to the markets.

So why was the unit of mass based on a physical object? Because what else could you base it on when going to the markets?

### What items are measured in grams or kilograms?”

In the current S.I. system of measurements, the gram (or kilogram) is a unit of mass (not weight or force) applicable to literally any material. Note that it is often used as unit of weight or force whereas the correct unit for these quantities is the newton (N).

Weight is the attraction of an object to another mass and varies with distance and mass. Mass is constant, regardless of the proximity of massive objects. Thus, for example, a mass of a kilogram weighs six times more on the surface of the Earth than the same mass on the surface of the Moon.

## Grams and Kilograms

Imagine being at the beach and running you fingers through the sand. Have you ever tried to pick up a grain or even a handful of sand? It’s pretty light! Now, think about a large rock sitting on the beach. Would you be able to lift this on your own? Probably not. It’s much heavier than sand. We could measure that heavy rock in kilograms and that much lighter handful of sand in grams.

While the United States and a few other locations measure weight in pounds and ounces, most of the world uses the metric system. Grams and kilograms are used to measure mass in the metric system. Let’s learn more about these two units of measurement.

### Grams

Grams is a unit of measurement used to measure very light objects. For example, a small metal paperclip has a mass of around 1 gram. Other objects with a mass of about 1 gram are a stick of gum and a dollar bill. We can abbreviate the unit gram with the letter *g*.

### Kilograms

Kilogram is a unit of measurement used to measure much heavier objects. For example, a one-liter bottle of soda has a mass of about 1 kilogram. Fruits such as small watermelons and pineapples also have a mass of around 1 kilogram. We can abbreviate the unit kilogram with the letters *kg*.

True or False: If we measure the mass of lighter objects, it is better to use kilograms than grams.

When we measure the mass of an object, we’re trying to find out how heavy it is. And in our question, we can see two units of measurement that we can use to weigh objects. Can you see them? There’s kilograms, which we often write kg, and grams, for which we often write g.

#### which one? Kilograms or grams?

Now, we could use these units of measurement to weigh anything, but it’s not always a good idea. Sometimes it’s better to use one unit of measurement than the other. And this question is asking us about lighter objects. If we’re measuring the mass of lighter objects, is it better to use kilograms or grams?

What do you think we mean by lighter objects? A feather is a light object. This pen is a light object. You could probably say a carrot is a light object too. It’s not heavy, is it? If an object is light, it’s not heavy. Now, if we wanted to measure the mass of these light objects, is it better to use kilograms or grams?

To help us understand, we need to remind ourselves how heavy one kilogram and one gram are. One kilogram weighs about the same as a bag of rice. And one gram weighs about the same as the lid of a ballpoint pen.

### Now, which unit of measurement do you think it makes sense to measure these light objects with?

Would you weigh how heavy they are in bags of rice or in pen lids? If we had a set of scales, we put a bag of rice on one end and one of our light objects on the other, it would come crashing down.

A kilogram is much heavier than our light objects. It would be much easier to see how many pen lids weighed the same as a carrot. Grams are generally better for weighing lighter objects and kilograms for weighing heavier objects.

Now, our sentence says, if we measure the mass of lighter objects, it’s better to use kilograms than grams. Well, we’ve just said this isn’t true, is it? The statement is false. Let’s say it the correct way round just to end with. If we measure the mass of lighter objects, it is better to use grams than kilograms.

**The difference between an object’s mass and its weight is that**

**Mass**

*See Note at*weight.

**weight**

**1.** The force with which an object near the Earth or another celestial body is attracted toward the center of the body by gravity. An object’s weight depends on its mass (the amount of matter it consists of) and the strength of the gravitational pull. On Earth, for example, an object weighs less at the top of a very high mountain than it does at sea level, simply because the gravitational pull at the top of the mountain is lower than it is at sea level.

**2.** A unit used as a measure of graitational force: *a table of weights and measures.*

**3.** A system of such measures: *avoirdupois weight; troy weight.*

**Weight/Mass**

It’s easy to convert pounds into kilograms: just multiply the number of pounds by .45 (or if you want greater precision, use .4536). But no matter how many times you do this conversion, you will always be making a mistake, because a pound is a unit of *force,* while a kilogram is a unit of *mass.*

A pound is a measure of the force that a gravitational field exerts on an object. As such, a pound is a unit of *weight,* not of mass. An object’s mass is its ability to resist changes in the speed or direction of its motion, and it is always the same regardless of what forces are acting upon it.

If you were walking on the moon, for example, your mass would be the same as it is on the Earth, but your weight would be one sixth of what it is on the Earth because of the lower gravitational pull of the moon. If this is so, you might ask, how can my science teacher ask me to convert pounds into kilograms? When we make such conversions, we are assuming that the object in question is on the Earth, at sea level, where the conversion factor works for all practical purposes.

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