K - Factary

kelvin (K)

The kelvin (abbreviation K) is the unit of absolute temperature, where zero is the point at which all atomic and molecular matter ceases to move. Zero degrees Celsius is equal to +273 kelvin, whereas zero kelvin is the lowest temperature possible and is know as "absolute zero". See Temperature for more details about temperature scales and William Thompson for more about Lord Kelvin.

Kelvin, Lord (1824-1907)

See Thomson, William for more details.

Kilogram (kg)

The unit of mass in the SI system.

The kilogram is the only SI unit with a prefix as part of its name and symbol. When other prefixes are required they are referred back to the 'gram'. So, for instance, one millionth of a kilogram (10^-6 kg) is called 1 mg (one milligram) NOT 1 µkg (one microkilogram)!

For 106 years, a single cylinder of precious platinum-iridium (kept in Sèvres, France) was used as the kilogram standard. Using a standard mass to calibrate other ones has its disadvantages. "Le Gran K", as the kilogram standard is sometimes called, is very imprecise, when compared to other SI standards. It has the potentially to be damaged or even destroyed and its mass can change slightly when it collects stray dust particles or when it is cleaned. Having a standard that is only available in one laboratory is also a disadvantage.

For some time, scientists have considered replacing the kilogram mass standard with a natural constant. Other SI base units are defined this way. The metre, for example, is now defined in terms of the speed of light, not by comparison with some standard object. Scientists at the National Institute of Standards and Technology are exploring two possible ways of replacing the kilogram mass standard. Both methods would base the kilogram on the Avogadro constant, which is defined as the number of atoms in 12 grams of pure carbon-12 (¹²C) (see isotopes for more information on this). Either of these methods would tie the kilogram to a natural constant and the standard would, therefore, be accessible to researchers worldwide.

In science some words and terms can be confusing because they are used differently from the way they are used in everyday life.

Mass is one of those terms. When 'talking science', it shouldn't be confused with weight even though we normally use the words as if they mean the same thing. In everyday life they do mean the same thing - that's not wrong, it’s just different from what is required in science!

Why is science so awkward then? One of the features of science is its need to be clear and accurate and in science there is a need for two different words; one to describe how much matter there is in an object and another to describe the force with which gravity pulls on that object. When described in that way, those two things sound very different indeed, so it sounds like there really ought to be two different words! Physicists have decided that the two words should be MASS and WEIGHT.

In our normal lives, we have decided that WEIGHT will do for both of them! And the reason that works just fine is that not many people travel to places where gravity (the thing that links the scientists’ mass and weight) changes. Only if gravity changes do non-scientists need to worry about the difference between mass and weight. So we happily weigh out and buy 2 kg of grapes, even though what we are actually buying is a bunch of grapes with a weight of 20 newtons (how many scales have you seen marked in newtons?!).

Now take that bunch of grapes to a shop on the Moon and try and sell them while using the same set of scales. According to the scales (think about which kind of scales this does not apply to) you will only be selling about 320 gm of grapes so your customer will only pay about one sixth the price they paid on Earth. Then you'll wish you had thought more about the difference between mass and weight!

Mass is a measure of the amount of matter there is in something and is measured in kilograms.

Weight is the force with which gravity pulls on objects and is measured in newtons.

Your own mass doesn't change, regardless of where you are in the universe (unless of course you deliberately lose or gain mass by slimming or fattening up). Travelling around the solar system though, your weight certainly would change.

The more massive the planet you are standing on, the greater your weight would be. On the Moon you will weigh about one sixth of what you do on the Earth. However, if you could stand on the surface of Jupiter you would weigh nearly 2½ times as much as you do on the Earth. So if you want to lose weight, go to the Moon, if you want to lose mass, eat less and do some exercise.

Kinetic Energy

This is the energy an object has because it is moving. It comes from the Greek word for motion 'kinema'.

If a car with a mass of M kg is moving at a velocity or speed 'v' metres per second then its kinetic energy (KE), in joules, can be calculated by the formula:

KE = M v ² / 2

If a car with a mass of 1500 kg is moving with a speed of 20 m/s (45 mph) how much kinetic energy does it have?

KE = (mass x speed x speed) / 2
  = 1500 * 20 * 20 / 2
  = 300,000 joules

Now suppose the car crashes into a wall.

There is a big crashing sound and maybe some of the bricks will get knocked out of the wall. How and why did that happen? The energy of the car's motion, its kinetic energy, was transferred to other forms of energy such as sound, heat, energy to break the bricks apart, kinetic energy of the bricks that move, energy to dent the metal of the car and so on.

Notice from the formula for kinetic energy that it's the speed of the object that's more important in energy terms. If you double the mass of the object you double the amount of kinetic energy it has and therefore the amount of damage it could possibly do. However, if you double the speed of the object then you QUADRUPLE (multiply by four) the amount of kinetic energy it has! By trebling the speed the kinetic energy is increased by nine times. Can you see why speed limits are put on some roads?

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