Weight and mass

In physics, there are substantial differences between the two terms, mass and weight , which are often used incorrectly. These are two completely different physical quantities.

The difference between mass and weight is found in the concept of gravity. … If two identical bodies have the same mass on planet Earth and on the moon, their weight will change. On the moon, since the force of gravity is six times less than on Earth, an object will weigh six times less.

 It definició n  Weight is the force of attraction of a body towards the center of our planet (or any other in which it is)Mass is the amount of matter in an object.
 Unit of Measure Newton (N) -Also Kilos (Kg) Kilos (Kg.)

What is mass:

By “mass”, in fact, we mean the amount of matter that characterizes a body: the intrinsic property of a body does not vary with where it is located. The mass value is defined as constant because it does not depend on the position of the object in space and is not influenced by external factors, such as the gravitational field.

What is Weight:

With regard to “weight”, it is a physical quantity that measures the force with which a body is attracted to another reference body. Its value, unlike that of mass, is influenced by the gravitational field (a body with the same mass will undoubtedly have a different weight on Earth than on the Moon, thanks to a different force of gravity). The weight is evaluated in newtons (N).

Units of measurement for mass and weight

At the same mass, a body is found to weigh a little more at the North Pole than at the Equator, since, since the Earth is flattened at the poles, the distance between the Earth’s surface and the center of the Earth is less than the poles. Another substantial difference between the two terms is given by the different unit of measurement of the two physical quantities. The same unit of measurement was used by mistake: the kilogram (kg ). But actually, when measuring the mass of a body, it is correct to use the kilogram of mass (kgm) as the unit of measurement, while for weight we must use the newton (N). The newton is, in fact, the unit of measure of force, which in the case of weight is called “weight-force” (Fp).

In conclusion , weight is a force, the force of gravity that acts on mass. We can see this in the following relationship: P (weight) = m (mass) xg (acceleration of gravity, which at the surface of the Earth is about 9.8 m / s ^ 2). To find the mass in kg, we must divide the weight of the body (in newtons) by 9.81 (gravitational acceleration in meters / seconds squared).

The latter does not change with the change of the place where it is present, but the former does, since it depends on the value of the acceleration of gravity : this changes if we move from one point to another on Earth.
Unit of measure for mass and weight.

Also with respect to the unit of measurement there are differences: mass is evaluated in kilograms (whose symbol is kg), while weight is in newtons (N) .

Relationship between mass and weight

And yet there is a relationship that joins mass and weight: P = mg

P, m and g are respectively the weight, the mass and the acceleration of gravity to which a body is subjected. From this relationship, it is evident that mass is a fixed quantity that does not change and weight varies according to the acceleration of gravity and that, in turn, changes depending on where we are.

Furthermore, we can confirm how this formula shows that mass and weight are directly proportional to each other and that the constant of proportionality is the acceleration of gravity, despite being two different sizes.

Confusion between unit of measure between mass and weight

We hope that we have clarified and explained in the best possible way the difference between mass and weight, which are two very different things that we often confuse.

This dilemma has always instilled doubts and torments in the fervent minds of all students in the world, more or less of all ages.
If mass is in kg and weight is in kg, how important is this big difference?

The origin of the confusion lies precisely in the unit of measurement . So this reflection arises spontaneously: if the misunderstanding is in the unit of measurement, it means that one of the two entities is represented by an inappropriate unit of measurement.

This reflection, which in some way or another has made (kg here, kg there, however, are different things.
Well, is the unit of measurement for mass or weight inaccurate? The unit of measurement for weight is inaccurate! !

The actual, original, unit of measurement for weight is the Newton (N) . In fact, Newton is the unit of measure for force and weight is nothing more than a force.
Instead, the kg (which is conventionally, but inadequately, the unit of measurement for weight) is the unit of measurement for mass .

In a few lines, I’ll explain everything, but in the meantime, take the first step in solving the dilemma:

  • Mass is measured in kg
  • Weight (which is a force) is measured in Newton.

Of course, today the use of the kg symbol for weight is commonly accepted and therefore it is correct to use it, but it is important to know that it is only a convention.

Let’s make it all clear:


Clear your mind and start from scratch: originally it was mass and its name was kg. Mass is the quantity of matter and this quantity is measured in kg. It’s always the same, on Earth, on the Moon, or in empty space outside the galaxy. It can be seen that the unit of measurement kg has nothing to do with weight (which is a force), but with mass, which is the amount of matter that makes up a body.


Weight is the force that a body expresses when it is stimulated by the acceleration of gravity. A quantity of matter placed on our planet undergoes an acceleration imposed by the earth’s gravity. The interaction of mass (amount of matter) – acceleration of gravity, produces a force. This force is called weight.

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