The concepts of weight and mass can often be confused due to their apparent interchangeability in modern vernacular.
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When somebody is asked their weight, almost always the response will be in kilograms or pounds, however, this is technically incorrect and is an expression of their mass, not weight.
In scientific terms, an object’s mass is an expression of the amount of matter in any given object, and an object’s weight is the sum of gravity acting on that mass.
At sea level on earth’s surface, weight and mass of objects are very similar, however, the weight will vary (0.5-1%) at sea level compared to on top of a mountain, or at the equator compared to the earth’s poles, due to the ovular shape of our planet.
These two terms are indeed closely related, however, the common assumption that they are identical is incorrect. They are in fact, quite distinct from one another.
Mass vs Weight
The difference between Mass and Weight is that Mass is a Scalar quantity which is defined as the amount of matter present in a physical body. Weight is a Vector quantity which is defined as a force exerted on a physical body by gravity with which it is attracted towards the centre of the Earth.
|Parameter of Comparison||Mass||Weight|
|It is an expression of…||The amount of matter in an object||The sum of gravity acting on a mass|
|Can it vary?||No, it’s always the same||Yes, it can even be zero|
|Scalar or vector quantity||Scalar. Only has a magnitude||Vector. Has a direction and magnitude|
|Unit of measurement||Gram, kilogram, tonne||Newton|
|Mathematical formula||Force / acceleration (M=F/a)||Mass x gravity (W=mg)|
|Tool of measurement||Ordinary balance||Spring balance/scales|
What is Mass?
The mass of an object is an unchanged quantity and is an expression of the amount of matter that comprises a given object. A mountain has a larger mass than a football, for example.
As mass is an expression of the intrinsic matter or an object, even if you moved the mountain and football into a zero-gravity environment, their respective masses would remain the same. This applies to chemical processes that may happen inside an object too.
Since the mid-eighteenth century, due to the works of Antoine Lavoisier, we have known that the total masses of any chemicals undergoing interactions inside an object will remain the same before and after the reactions take place.
Mass can also be described as an object’s resistance to acceleration (also known as inertia), meaning that the larger the mass of an object, the more force will be required to move it.
An interesting way to observe this is to hang an object of significant weight (10-20kg) above the ground from a rope several meters long. If you try to lift it directly upwards you will feel its weight, but if you push it from the side slightly you will be feeling the mass of the object
Mass is measured in grams, kilograms, and tonnes, which is how we commonly express weight here on the earth’s surface, and so adds to the confusion between weight and mass.
What is Weight?
Weight expresses the gravitational forces acting on an object, however, despite all objects having a gravitational pull, the only one of significance on earth is that of the planet itself.
On earth, all objects will be drawn towards the center of the planet with a force of nine point eight meters per second, whereas the moon is affected by the gravitational pull of earth as well as other extraterrestrial bodies. The weight of an object can vary slightly on earth as well.
Atop a tall mountain, an object will have a slightly lower weight than at sea level, due to the slightly decreased strength of the pull from the center of the planet (approximately 0.5%).
Due to the ovular shape of the earth, any given object will also have a slightly lower weight at the equator than at the north or south pole as well, for the same reason.
This also applies when astronauts land on the moon. Due to the moon’s gravitational pull being approximately a sixth of the earth’s, the astronaut will weigh much less than on earth, despite their mass remaining the same.
An object is also able to have zero weight. In a situation where gravitational forces are zero, like outer space, where an object will essentially be weightless.
Contrary to common phrasing, weight is expressed in newtons, which are the universally recognized unit of force.
Main Differences Between Mass and Weight
- An object’s mass is an expression of the total of the matter that makes up the object, whereas weight measures the sum of gravity acting on that mass.
- Mass is a constant variable and will not change due to location or chemical reactions, however, the weight will vary depending on the strength of gravitational pull that acts upon it.
- Mass is a scalar quantity which means it measures only a magnitude, whereas weight measures the magnitude and direction of a gravitational pull and is a vector quantity.
- When speaking in general terms, a point of confusion is the fact that grams, kilograms, and tonnes are all units of mass. Newtons are the unit of measurement for weight.
- To accurately measure weight you will need a spring balance, however, common scales give an accurate estimate as well. To measure mass you require a triple beam or electronic balance.
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