A spring scale measures the weight of an object

In the physical sciences, weight is a measurement of the gravitational force acting on an object. Near the surface of the Earth, the acceleration due to gravity is approximately constant; this means that an object's weight is roughly proportional to its mass. The words "weight" and "mass" are therefore often used interchangeably, even though they do not describe the same concept. Look up weight in Wiktionary, the free dictionary. ... Image File history File links Size of this preview: 169 × 598 pixel Image in higher resolution (705 × 2495 pixel, file size: 173 KB, MIME type: image/jpeg) measuring spring (Hookes law) File links The following pages on the English Wikipedia link to this file (pages on other projects are... Image File history File links Size of this preview: 169 × 598 pixel Image in higher resolution (705 × 2495 pixel, file size: 173 KB, MIME type: image/jpeg) measuring spring (Hookes law) File links The following pages on the English Wikipedia link to this file (pages on other projects are... Spring scale. ... Physical science is a encompassing term for the branches of natural science, and science, that study non-living systems, in contrast to the biological sciences. ... Various meters Measurement is an observation that reduces an uncertainty expressed as a quantity. ... In physics, force is anything that can cause a massive body to accelerate. ... This article is about Earth as a planet. ... The nominal acceleration due to gravity at sea level on the Earths surface, also known as standard gravity, is defined as exactly 9. ... This article or section is in need of attention from an expert on the subject. ...

Weight and mass

In modern usage in the field of mechanics, weight and mass are fundamentally different quantities: mass is an intrinsic property of matter, whereas weight is a force that results from the action of gravity on matter. For other uses, see Mechanic (disambiguation). ... This article is about matter in physics and chemistry. ... Gravity is a force of attraction that acts between bodies that have mass. ...

However, the recognition of this difference is, historically, a relatively recent development – and in many everyday situations the word "weight" continues to be used when strictly speaking "mass" is meant. For example, we say that an object "weighs one kilogram", even though the kilogram is actually a unit of mass. This common parlance usage is due to the early proliferation of force-based measurement systems widely being displaced with the more scientific and mass-based SI system. This transition has led to the common and even legal intertwining of "weight" and "mass" as equivalents.

The distinction between mass and weight is unimportant for many practical purposes because, to a reasonable approximation, the strength of gravity is the same everywhere on the surface of the Earth. In such a constant gravitational field, the gravitational force exerted on an object (its weight) is directly proportional to its mass. So, if object A weighs, say, 10 times as much as object B, then object A's mass is 10 times that of object B. This means that an object's mass can be measured indirectly by its weight (for conversion formulas see below). For example, when we buy a bag of sugar we can measure its weight (how hard it presses down on the scales) and be sure that this will give a good indication of the quantity that we are actually interested in, which is the mass of sugar in the bag. Nevertheless, slight variations in the Earth's gravitational field do exist (see Earth's gravity), and these must be taken into account in high precision weight measurements. In mathematics, two quantities are called proportional if they vary in such a way that one of the quantities is a constant multiple of the other, or equivalently if they have a constant ratio. ... Precise values of g vary depending on the location on the Earths surface. ...

The use of "weight" for "mass" also persists in some scientific terminology – for example, in the chemical terms "atomic weight", "molecular weight", and "formula weight", rather than the preferred "atomic mass" etc. For other uses, see Chemistry (disambiguation). ... The atomic mass (ma) is the mass of an atom at rest, most often expressed in unified atomic mass units. ...

The difference between mass and force becomes obvious when

• objects are compared in different gravitational fields, such as away from the Earth's surface. For example, on the surface of the Moon, gravity is only about one-sixth as strong as on the surface of the Earth. A one-kilogram mass is still a one-kilogram mass (as mass is an intrinsic property of the object) but the downwards force due to gravity is only one-sixth of what the object would experience on Earth. Weight is relative to the local gravitional field, mass is not.

• masses are considered in the context of a lever, such as a cantilever structure.

• locating the center of gravity of an object.

• See More on this topic in physics book

This article is about Earths moon. ... Levers can be used to exert a large force over a small distance at one end by exerting only a small force over a greater distance at the other. ... The cantilevered beam (green) projects from its supports (blue), balanced by the structure (red block), which supports the load (red arrow). ... This article or section may contain original research or unverified claims. ...

Units of weight (force)

Systems of units of weight (force) and mass have a tangled history, partly because the distinction was not properly understood when many of the units first came into use. In physics, force is anything that can cause a massive body to accelerate. ...

SI units

In most modern scientific work, physical quantities are measured in SI units. The SI unit of mass is the kilogram. The SI unit of force (and hence weight) is the newton (N) – which can also be expressed in SI base units as kg·m/s² (kilograms times meters per second squared). Cover of brochure The International System of Units. ... Shown above is a computer-generated image of the International Prototype Kilogram (“IPK”). The IPK is the kilogram. ... In physics, the newton (symbol: N) is the SI unit of force, named after Sir Isaac Newton in recognition of his work on classical mechanics. ... The SI system of units defines seven SI base units: physical units defined by an operational definition. ...

The kilogram-force is a non-SI unit of force, defined as the force exerted by a one-kilogram mass in standard Earth gravity (equal to about 9.8 newtons). The deprecated unit kilogram-force (kgf) or kilopond (kp) is the force exerted by one kilogram of mass in standard Earth gravity (defined as exactly 9. ...

The gravitational force exerted on an object is proportional to the mass of the object, so it is reasonable to think of the strength of gravity as measured in terms of force per unit mass, that is, newtons per kilogram (N/kg). However, the unit N/kg resolves to m/s²; (metres per second per second), which is the SI unit of acceleration, and in practice gravitational strength is usually quoted as an acceleration.

The pound and related units

In United States customary units, the pound can be either a unit of force or a unit of mass. Related units used in some distinct, separate subsystems of units include the poundal and the slug. The poundal is defined as the force necessary to accelerate a one-pound object at 1 ft/s², and is equivalent to about 1/32 of a pound (force). The slug is defined as the amount of mass that accelerates at 1 ft/s² when a pound of force is exerted on it, and is equivalent to about 32 pounds (mass). U.S. customary units, also known in the United States as English units[1] (but see English unit) or standard units, are units of measurement that are currently used in the USA, in some cases alongside units from SI (the International System of Units — the modern metric system). ... The poundal is a non-SI unit of force. ... The slug is an English unit of mass. ... The slug is an English unit of mass. ...

Conversion between weight (force) and mass

To convert between weight (force) and mass we use Newton's second law, F = ma (force = mass × acceleration). Here, F is the force due to gravity (i.e. the weight force), m is the mass of the object in question, and a is the acceleration due to gravity, on Earth approximately 9.8 m/s² or 32 ft/s²). In this context the same equation is often written as W = mg, with W standing for weight, and g for the acceleration due to gravity.

Sensation of weight

See also: apparent weight

The weight force that we actually sense is not the downward force of gravity, but the normal force (an upward contact force) exerted by the surface we stand on, which opposes gravity and prevents us falling to the center of the Earth. This normal force, called the apparent weight, is the one that is measured by a spring scale. An objects weight, henceforth called actual weight, is the downward force exerted upon it by the earths gravity. ... Fn represents the normal force. ... In physics, a contact force is a force between two objects (or an object and a surface) that are in contact with each other. ... An objects weight, henceforth called actual weight, is the downward force exerted upon it by the earths gravity. ...

For a body supported in a stationary position, the normal force balances the earth's gravitational force, and so apparent weight has the same magnitude as actual weight. (Technically, things are slightly more complicated. For example, an object immersed in water weighs less, according to a spring scale, than the same object in air; this is due to buoyancy, which opposes the weight force and therefore generates a smaller normal. These and other factors are explained further under apparent weight.) In physics, buoyancy is the upward force on an object produced by the surrounding fluid (i. ... An objects weight, henceforth called actual weight, is the downward force exerted upon it by the earths gravity. ...

If there is no contact with any surface to provide such an opposing force then there is no sensation of weight (no apparent weight). This happens in free-fall, as experienced by sky-divers (until they approach terminal velocity) and astronauts in orbit, who feel "weightless" even though their bodies are still subject to the force of gravity: they're just no longer resisting it. The experience of having no apparent weight is also known as microgravity. Free Fall opens with one of the most stunning first paragraphs I have ever, or am ever likely to, read. ... An object reaches terminal velocity when the downward force of gravity equals the upward force of drag. ... Astronauts on the International Space Station display an example of weightlessness. ... Astronauts on the International Space Station display an example of weightlessness Weightlessness is the experience (by people and objects) during freefall, of having no weight. ...

A degree of reduction of apparent weight occurs, for example, in elevators. In an elevator, a spring scale will register a decrease in a person's (apparent) weight as the elevator starts to accelerate downwards. This is because the opposing force of the elevator's floor decreases as it accelerates away underneath one's feet.

Measuring weight

Main article: Weighing scale

Weight is commonly measured using one of two methods. A spring scale or hydraulic or pneumatic scale measures weight force (strictly apparent weight force) directly. If the intention is to measure mass rather than weight, then this force must be converted to mass. As explained above, this calculation depends on the strength of gravity. Household and other low precision scales that are calibrated in units of mass (such as kilograms) assume roughly that standard gravity will apply. However, although nearly constant, the apparent or actual strength of gravity does in fact vary very slightly in different places on the earth (see standard gravity, physical geodesy, gravity anomaly and gravity). This means that same object (the same mass) will exert a slightly different weight force in different places. High precision spring scales intended to measure mass must therefore be calibrated specifically according their location on earth. Digital kitchen scales. ... Digital kitchen scales. ... Digital kitchen scales. ... An objects weight, henceforth called actual weight, is the downward force exerted upon it by the earths gravity. ... g (also gee, g-force or g-load) is a non-SI unit of acceleration defined as exactly 9. ... g (also gee, g-force or g-load) is a non-SI unit of acceleration defined as exactly 9. ... Definition Physical geodesy is the study of the physical properties of the gravity field of the Earth, the geopotential, with a view to their application in geodesy. ... Gravity anomalies are widely used in geodesy and geophysics. ... Gravity is a force of attraction that acts between bodies that have mass. ...

Mass may also be measured with a balance, which compares the item in question to others of known mass. This comparison remains valid whatever the local strength of gravity. If weight force, rather than mass, is required, then this can be calculated by multiplying mass by the acceleration due to gravity – either standard gravity (for everyday work) or the precise local gravity (for precision work). Digital kitchen scales. ...

Gross weight is a term that generally is found in commerce or trade applications, and refers to the gross or total weight of a product and its packaging. Conversely, net weight refers to the intrinsic weight of the product itself, discounting the weight of packaging or other materials.

Relative weights on the Earth, other planets and the Moon

The following is a list of the weights of a mass on the surface of some of the bodies in the solar system, relative to its weight on Earth:

This article is about the planet. ... (*min temperature refers to cloud tops only) Atmospheric characteristics Atmospheric pressure 9. ... This article is about Earth as a planet. ... This article is about Earths moon. ... Mars is the fourth planet from the Sun in the solar system, named after the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color as viewed in the night sky. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... Atmospheric characteristics Atmospheric pressure 120 kPa Hydrogen 83% Helium 15% Methane 1. ... Atmospheric characteristics Surface pressure ≫100 MPa Hydrogen - H2 80% ±3. ...

References

See also

• Weights and measures
• Ancient weights and measures
• Medieval weights and measures
• Atomic weight
• Human weight
• Body Mass Index
• Curb weight

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