What is the difference between body mass and weight? We keep getting it wrong

In everyday life, we use the term “weight” to describe the amount of matter contained in an object. This popular confusion is reinforced by the fact that the weight of an object is used to measure its mass.

In this confusion, two terms coexist in our vocabulary: mass and weight. We tend to use one or the other as we please, when in fact they are two very different concepts. Therefore, sometimes we not only get confused, but we can also confuse the other. In fact, to be honest, we are sure that more than once, when using them, you have remembered that you were not quite about them, so so that you do not forget anymore, we will explain here what each of them is.

On the one hand, mass is a unit of measurement that refers to the amount of matter present in an object. It is usually expressed in kilograms (kg) or grams (g). It is a “fixed” quantity, that is to say, it depends neither on the position nor the speed of the object.

The weight, on the other hand, measures the force that gravity exerts on an object. In this case, it is directly related to the position of the object in space. It is usually expressed in newtons (N) or kilograms-force (kgf) and can be calculated by multiplying the mass of the object by the force of gravity.

A constant error

In everyday life, however, the term “weight” is most commonly used to describe the amount of matter in an object. This is, as you have seen, a cultural misunderstanding, a popular confusion reinforced by the fact that the weight of an object is often used to measure its mass.

Indeed, when we use a scale to measure the weight of an object, we are actually measuring the force exerted on the object, and this depends on gravity. If we take the example of a space traveler visiting the different planets of the solar system, the question would be:

Suppose that on Earth the person weighs 70 kilograms. Whatever happens, the quantity of matter contained in this person (i.e. his mass) will remain unchanged. On the other hand, his weight will vary according to the celestial objects he visits.

A simple example

Thus, on Mercury, your weight would be 27.3 kg, because on this planet the force of gravity is different from that on Earth, namely about 0.38 g. If you were on Venus, you would weigh 68 kg (the force of gravity is 0.9 g).

On the outer planets, in fact, the weight would be much higher: on Jupiter, which has a gravity of 2.5 g, it would weigh 243 kg. On Saturn, it would weigh about 94 kg, with a gravitational force of about 1 g. On Uranus, it would reach 87 kg (in this case, the gravity is 0.9 g).

Note, however, that these examples do not take into account the environmental conditions inherent to each planet, such as atmospheric pressure, temperature, and weather conditions, which can also affect the weight measurement.

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