Difference between Mass and Weight in Physics

January 14, 2017 by Editorial Team

In everyday talk, people use the terms “mass” and “weight” interchangeably to indicate how light or heavy an object is. In physics, however, these two concepts are very different. This article will explain what mass and weight are, and how they differ in physics.

Definitions

A replica of the standard kilogram, used to denote mass

Loosely speaking, mass is the amount of matter in an object. It also measures the resistance of the said object to acceleration when a force is applied, and determines how much gravitational force it exerts on other objects. Assuming that all other factors are equal or absent, objects of the same mass will exert the same force on their surface. An object of a given mass will retain its mass wherever it may be. The International System of Units (SI) uses kilograms to denote mass.

weight
An astronaut has zero weight in space

Strictly speaking, weight is the force exerted by gravity on an object. It is computed as the product of mass and gravitational acceleration. Because gravity varies according to location, weight also varies. For example, an object of a particular weight on earth might weigh less on a surface with lower gravity, such as the moon. Conversely, a surface with higher gravity, such as Jupiter, will make the same object weigh more. Barring other forces such as air resistance, objects of the same weight will fall down at the same speed. The International System of Units (SI) uses newtons to denote weight.

Comparison

Their main differences lie in their definitions, variability, their relationship to each other, and their units of measurement.

Definitions

Physicists loosely use the term “mass” to refer to the amount of matter in an object. Meanwhile, they use the word “weight” to refer to the amount of force exerted by gravity on an object.

Relationship

Mass is also defined as the measure of an object’s resistance to movement when a force is applied. Weight, on the other hand, is the product of mass and acceleration as a result of gravity.

Variability

An object of a particular mass will have the same mass anywhere. However, because gravity varies across locations, the same object’s weight will go either up or down, depending on the strength of the place’s gravitational pull.

Unit of Measurement

The International System of Units (SI) uses the kilogram as the standard unit of mass. In contrast, the same system uses the unit of force, the newton, to refer to a certain object’s weight.

Comparison Chart

MassWeight
Refers to the amount of matter in an objectRefers to the force exerted by gravity on an object
Mass is an object’s resistance to accelerationWeight is the product of mass and gravitational acceleration
Mass is constantWeight varies according to the strength of gravity
Measured in kilogramsMeasured in newtons