## Main Difference – Mass vs. Weight

In everyday usage, we use the terms *mass* and *weight* interchangeably. However, there is a technical difference between the two. The **main difference** between mass and weight is that **mass is a measurement of the amount of matter in a body**, whereas **weight** **is a measurement of force felt by a body in a gravitational field**.

## What is Mass

Mass is a measurement of the amount of matter in a body. There are two separate contexts in which mass becomes important. Firstly, the mass of a body attempts to resist acceleration whenever a force is applied on a body. This reluctance by an object to resist acceleration is called **inertia** and so in this context, the mass is referred to as **inertial mass (****)**. If a resultant force acts on the body, the acceleration that can be produced is inversely proportional to the object’s mass, such that:

Secondly, mass is *also* the property that determines the gravitational force between two objects. In this context, mass is referred to as **gravitational mass (****)**. The gravitational force of attraction between two objects with masses and , separated by a distance can be given by,

where is called the universal gravitational constant, with 6.67×10^{-11} m^{3} kg^{-1} s^{-2}. (Note: gravitational mass can be further subdivided into *active* gravitational mass and *passive* gravitational mass, but that is a distinction we will not go into).

Inertia and gravitational attraction are two separate things, and there is no reason for the *inertial* mass of an object to be equal to the *gravitational* mass. However, no experiment has demonstrated that the two are unequal. Mass is one of the base physical quantities, and in the SI system of units mass is expressed in kilograms. A set of traditional balancing scales measures the *mass*. Since objects are added to both sides of the scale, the gravitational force gets “canceled out” and the measurement you make ends up becoming a comparison of masses.

## What is Weight

Weight is the gravitational force experienced by a body in a gravitational field. If the weight of the body is given by , then it is given by

where is the gravitational field strength. Comparing with the equation we had for gravitational mass, we then have where is the gravitational mass of the body that forms the gravitational field that the mass is in.

Weight is a *force*, and it is measured in newtons (N). Since weight depends not only on an object’s own mass but also on the masses of bodies surrounding it and distances to these other bodies, the weight can change from place to place. For instance, the gravitational field strength on the surface of the Moon is about a sixth of the gravitational field strength at the Earth’s surface. Therefore, two objects having same *masses* will still experience different *weights* if one of them is on the Earth and the other is in the Moon. The object in the Moon will “weigh” about a six times less! A bathroom scale measures your *weight* because it records how much force you are being pulled down.

## Difference Between Mass and Weight

### What it Measures

**Mass** is a measurement of the amount of matter in a body. Mass resists acceleration, and mass also produces gravitational attraction.

**Weight** is the *force* experienced by a body due to gravity.

### Variability

**Mass** of a body does not change, depending on its position.

**Weight** of a body depends on masses of objects surrounding it and the separations between them.

### Nature of Quantity

**Mass** is a scalar quantity.

**Weight** is a vector quantity.

### Units

**Mass** is measured in kilograms.

**Weight** is measured in newtons.

**Image Courtesy**

“How to Weigh a Dog: part 1” by Stephanie Sicore (Own work) [CC BY-SA 2.0], via flickr