Weight
The weight of an object (or the weight of an amount of matter) is the measure of the intensity of the force imposed on this object by the local gravitational field. Weight should not be confused with the related but quite different concept of mass.
In common language, or in the absence of other information, the weight of something is typically understood to be the value measured at or near the Earth's surface. For small objects on Earth, this force is directed towards the center of mass of the planet. For larger objects, such as the Moon orbiting around the Earth, the force is directed towards the barycenter or center of mass of the combined system.
[change] Units of weight
The unit of weight in the International System of Units is the Newton, which is represented by the symbol 'N'.
Other units have been in use in the past but have been abandoned, such as the dyne (the unit of force in the old CGS system) or the kilogram-force, which is the force exerted on a kg of matter by a 'standard' Earth: A mass of 1 kg has a weight of about 9.81 N at sea level.
[change] Measuring weight
The weight of an object, or of an amount of matter, is typically measured with an instrument such as a spring scale that provides a reading on the amount of another force required to balance the gravitational pull on the object of interest.
The weighing scale is a device that compares the weights of two masses in the same gravitational field: it determines whether one mass is heavier or lighter than the other. If one disposes of a series of reference (calibrated) masses, one can determine the mass of any object by adding those of the reference masses needed to balance the scale.
[change] Weight is variable
Weight is not an intrinsic property of matter because the local gravitational field that generates the force called weight is variable in space and time:
- Since the Earth's attraction decreases as the square of the distance to its center, the weight of an object is slightly smaller at high altitude (e.g., at the top of a mountain) than at sea level, or at the Equator than at the poles (because the Earth is slightly bulging).
- An arbitrary object on Earth is also attracted by all other celestial bodies, such as the Moon, for instance. Hence, its weight will be less with the Moon overhead than with the Moon on the other side of the Earth. This differential pull is the cause of tides, for example.
- Weight is not defined exclusively to Earth: An astronaut weighs 6 times less on the surface of the Moon than on the surface of the Earth.
- Weightlessness is an apparent condition experienced by astronauts or satellites in orbit around a planet, for instance, where their weight (gravitational pull) is the centripetal force that allows them to remain on orbit. This word is misleading in the sense that these bodies are not weightless: they actually keep falling towards the planet. However, if such an astronaut were stepping on a scale within the satellite, the reading would be null because the scale is itself falling at the same rate.
