page_title: Mass versus weight
section_title: Types of scales and what they measure
hierarchical_section_title: Mass versus weight / Types of scales and what they measure
caption_attribution_description: Soehnle bathroom scales photographed by me in January 2007.
context_page_description: In common usage, the mass of an object is often referred to as its weight, though these are in fact different concepts and quantities. In scientific contexts, mass is the amount of "matter" in an object, whereas weight is the force exerted on an object by gravity. In other words, an object with a mass of 1.0 kilogram weighs approximately 9.81 newtons on the surface of the Earth, which is its mass multiplied by the gravitational field strength. The object's weight is less on Mars, where gravity is weaker, and more on Saturn, and very small in space when far from any significant source of gravity, but it always has the same mass.
  Objects on the surface of the Earth have weight, although sometimes the weight is difficult to measure. An example is a small object floating in water, which does not appear to have weight since it is buoyed by the water; but it is found to have its usual weight when it is added to water in a container which is entirely supported by and weighed on a scale. Thus, the "weightless object" floating in water actually transfers its weight to the bottom of the container.
context_section_description: When one stands on a balance-beam-type scale at a doctor’s office, they are having their mass measured directly. This is because balances ("dual-pan" mass comparators) compare the gravitational force exerted on the person on the platform with that on the sliding counterweights on the beams; gravity is the force-generating mechanism that allows the needle to diverge from the "balanced" (null) point. These balances could be moved from Earth's equator to the poles and give exactly the same measurement, i.e. they would not spuriously indicate that the doctor's patient became 0.3% heavier; they are immune to the gravity-countering centrifugal force due to Earth's rotation about its axis. But if you step onto spring-based or digital load cell-based scales (single-pan devices), you are having your weight (gravitational force) measured; and variations in the strength of the gravitational field affect the reading. In practice, when such scales are used in commerce or hospitals, they are often adjusted on-site and certified on that basis, so that the mass they measure, expressed in pounds or kilograms, is at the desired level of accuracy.
caption: Load-cell based bathroom scale: Affected by the strength of gravity.
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