What are the differences between mass and weight?

The terms mass and weight are often used as interchangeable concepts in colloquial conversations, however, they are terms that refer to very different physical concepts. The mass expresses amount of matter while the weight represents the resultant force of the gravity effect on that amount of matter.

definition of mass

mass is one scalar physical quantity which quantifies the amount of substance, which is related to the number, types and density of atoms. Mass is an intrinsic property of matter. does not depend on external factors. The unit of measure for mass in the International System is the kilogram (kg).

The mass of an object is usually measured from the resistance to acceleration or inertia offered by the object when subjected to a force. One of the most commonly used forces to calculate mass is gravity. The resulting mass is known as the gravitational mass and is usually calculated by applying the Newton's law of gravitation:

force, mass and gravity

This formula is the one used by most balances, where F is the applied force, g is Earth's gravity M the mass to be measured. For example, a common mechanism in scales is the use of a spring whose deformation is proportional to the applied force, a relationship that is calibrated to take into account the earth's gravity and thus the mass can be read.

weight definition

In classical physics, weight is defined as the force exerted by gravity on mass of an object. Is a vector magnitude which is calculated as the mass due to gravity. Gravity is a natural phenomenon of attraction between objects with mass, observable above all in very massive objects such as planets, stars and galaxies.

Gravity causes acceleration in attracted objects. On Earth, the average gravity is approximately 9.80665 m/s², although it varies slightly depending on latitude and altitude. At the poles the intensity of the gravitational field is greater than at the equator, since at the poles it is closer to the earth's center due to the flattened shape of the planet (gravity at the poles is 9,832 m/s², at the equator it is 9,789 m/s²).

Weight, or the force that gravity exerts on an object, is related to mass and gravity equally through Newon's gravitation:

weight formula

Where P is the weight, m the mass of the object and g the acceleration of gravity.

In the international system of units, weight is measured in newtons (N), which is the unit used by this system to measure any type of force. One newton is defined as the force required for an object of mass 1 kg to undergo an acceleration of 1 m/s.twonamely:

1N = 1kg 1m/stwo

Thus, an object with a mass of 1 kg will have a weight on planet Earth of:

P = 1 kg 9.80665 m/s2 = 9.80665 N

In other words, a force of 9.80665 N would be required to lift an object of mass 1 kg from the earth's surface.

Most important differences

  • The amount of matter defines the mass, the effect of gravity on the mass defines the weight.
  • Mass is an intrinsic or fundamental property of matter. It does not depend on the gravitational field or any other external factor.
  • The weight depends on the gravitational field.
  • mass is one scalar magnitude and the weight is vector magnitudesince weight has a direction in its effect while mass does not express any direction.
  • The mass of the object is the same at any location but the weight is different if the gravitational field changes; For example, the mass of an object is the same on Earth and on the Moon, but the weight is less on the Moon, since the lunar gravitational field is less (1.622 m/stwo). A 100-kg object would have a weight of 980.65 N on Earth and only 162.2 N on the Moon.
  • An object cannot have zero mass but can have zero weight if gravity is zero.
  • In the International System, mass is measured in Kilograms (kg), a basic unit; weight is measured in newtons (N)a unit of force derived from base units (kg m/stwo).

Therefore, it can be concluded that there is a direct relationship between weight and mass, the greater the mass, the greater the weight, but the mass of an object is always the same, while the weight depends on the gravitational field by which the object is affected.

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