# Mass

Mass is a scientific measure of the amount of matter an object is made up of. No matter where you are at given moment in time, mass is constant. So, whether you're walking to the shop to stock up on groceries or bouncing around on the moon, your mass is the same. Obviously, if you're on a diet that's not the answer you want to hear but don't panic - we'll get to the all-important definition of weight shortly.

### Some other key points about mass

• Mass is indestructible. No matter where you are in the universe your mass will never change
• Mass can never be zero. What we mean by this is that everything in the universe has mass. If it didn't it simply wouldn't exist
• Mass is not related to gravity, centrifugal force, etc and these forces have no effect whatsoever on your mass
• Mass is commonly measured in kilograms and grams. Fig. no.1:
Fact file on the prototype one kilogram measure, a platinum-iridium alloy block housed since the 1880s under lock-and-key at the International Bureau of Weights and Measures in Sevres, France.

### Standard of Mass:

The basic SI unit of mass, the kilogram (kg), is defined as the mass of a specific platinum iridium alloy cylinder kept at the International Bureau of Weights and Measures at Sevres, France. This mass standard was established in 1887 and has not been changed since that time because platinum iridium is an unusually stable alloy. A duplicate of the Sevres cylinder is kept at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland.

#### Some key points about weight

• The weight of an object changes based on where it is. If you've decided to visit the moon to test out this theory then you'll find that, in a matter of hours, you will have slashed your weight by two thirds.
• Weight is a vector and its direction of pull is towards the centre of the earth planet. What? Sorry, I did promise layman's terms: gravity, which is created by the mass of an object, moves towards the centre of the object and it is gravity that determines your, or any other object's, weight.
• The weight of any given object can go up or down depending on the amount of gravity acting on it. More gravity - the heavier the object. Less gravity - the lighter the object.
• Unlike mass, weight can be zero. An example of this is an astronaut floating in space - there's no gravity acting on his body and, therefore, he has no weight.
• Weight is commonly measured in Newtons (N). Figure No.2

#### What Is The Difference Between Mass And Weight?

Weight is a form of measurement that is dependent on gravity and, unlike mass, your weight can vary depending on where you are in the universe. But how can weight vary whilst mass has to remain constant? It's fairly simple: weight is a variable i.e. it can change based on the amount of gravitational pull an object exerts on a body. Scientists have defined weight using this equation: $$w=mg$$ where $w$ is weight, $m$ is Mass and $g$ is Gravitational Acceleration.

### Mass and weight example - the moon

In the below example, we've featured an astronaut on earth and on the moon. The Moon's gravity is much less than the Earth's gravity - approximately one sixth. So, a 100 kg astronaut weighs 980N on Earth. On the Moon, the astronaut would weigh only 162.2N. However, the astronaut's mass is 100kg where-ever they are.

Weight on Earth: $100kg \times 9.8m/s^2 = 980N.$

Weight on Moon: $100kg \times 1.622 m/s^2 = 162.2N.$

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