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# Mass vs. Weight

Although often used interchangeably, weight and mass are two distinctly different units of measurement.

Mass measures the resistance of an object to changes in the speed or direction of its movement. The mass of an object does not depend on gravity and is therefore different from, but is proportional to its weight. Weight is a measure of the force generated by the mass of an object on which gravity acts. In other words, mass measures the amount of matter an object comprises, while weight measures gravitational attraction. Because of this, the weight varies by location and the mass is constant.

For example, a test weight of 100 kilograms on Earth will have the same mass property on Mars. A calibration weight of 100 kilograms will weigh 100 kilograms on Earth, but only 37.7 kilograms on Mars. The weight issue did not change; its gravitational pull yes. Also, the way weight and mass are measured is often used interchangeably. Because weight and mass do not have a significant difference in common use on Earth, people often use the same unit of measurement to express them. However, since weight is a force, it must be measured in Newtons, and mass must be measured in pounds or kilograms.

Weight (w) = Mass (m) x the acceleration of gravity (g)

The numerical weight on a scale would have to be converted to pounds or kilograms. To avoid confusion in most sectors of the Earth (in addition to scientists), weight is commonly expressed in pounds or kilograms.