What Is The Difference Between Mass and Weight?

In layman terms, mass and weight are often used to mean the same thing. But in reality, they are quite different from each other. In scientific parlance, we can define mass as the amount of matter found in an object or particle. While the matter is anything that is tangible.

For example, everything that we see around us has mass, including tables, chairs, books, bottles, balls, pencils, rulers, sharpeners, bags, air, water, etc. To put it more simply, the heaviness or lightness of an object is directly proportional to its mass. But weight, on the other hand, is the gravity-induced force that works on these objects.

Let us delve deeper into these two concepts to understand them better.

Mass

We can refer to mass as an inertial property or a quantitative measure of inertia. Inertia is the most basic characteristic of all objects.

Knowing More About Mass

A body of matter is inclined to remain at a constant velocity unless influenced by an external force. Or in other words, inertia is an object’s resistance to change its motion or position, when acted upon by an outside force.

Explaining with Examples

A prominent example of inertia would be when we slide a bowling ball down the bowling alley, it continues to move unless met by some force. This could be friction generated by the ground or any other obstruction that comes in its path. So mass is the measurement of the force that is required to change the motion of an object, in this case, it is the bowling ball.

So now we can safely say that the larger the mass of a body, the greater is the force needed for the object to change its motion or position. Since mass is not dependent on gravity, no matter wherever you go, the mass of an object will always remain the same. Mass will never be zero. This is why a satellite in orbit, though weightless, will still have its mass and inertia intact.

The principle of mass conservation also states that no matter how the constituent parts of an object or collection of objects realign themselves, the mass always remains unchanged.

The Change in Notion of Mass with the Advent of the Theory of Relativity

In 1905, this notion underwent a drastic change. It was the year in which Einstein introduced the world to his special theory of relativity. With this, mass came to be regarded as equivalent to energy and was seen as interconvertible with energy. It was also observed that at extremely high levels of speed, the mass of an object increased significantly.

In Physics, mass is known as locked up energy. We call this mass ‘inertial mass’. According to the theory of relativity, both ‘inertial mass’ and ‘gravitational mass’ are the same. When we say that the mass of an object is increasing as a result of acceleration, we are referring to an increase in this inertial mass. Because the mass of an object is the same as the energy stored inside the object.

Some Other Facts About Mass

According to the International System of Units (SI), the unit of mass is the kilogram. The formula of mass can be represented as Density X Volume. Mass only has magnitude and no direction. It is called a scalar quantity. Mass can be measured using any regular balance like a beam balance, pan balance, or lever balance.

Weight

Knowing More About Weight

Unlike mass, weight varies from place to place because weight is determined by the gravitational pull exerted by the Earth on an object.

In other words, weight can be defined as the force of gravity. The heavier an object weighs on earth, the lighter it would as it moves further away from the center of gravity.

Explaining with Examples

This also explains why your weight is comparatively lesser on mountain tops than at sea level. Similarly, an object would weigh slightly more at the South Pole than at the Equator due to its distance from any significant source of gravity.

So gravity or gravitation is the constant force that pulls all objects downwards towards the center of the earth. This is irrespective of which direction the objects are moving and what other external forces are involved in accelerating them.

An appropriate example in this regard would be the moon. Since the moon has no gravity the weight of an object on the moon would never be the same as that on the earth. In fact, because of the smaller mass and radius of the moon, any object on the moon would weigh one-sixth of its weight on the earth.

Based on the same principle, your weight might be 50 kilograms on Earth, but you might be weightless in outer space. In a nutshell, weight is the measure of how heavy or light an object is.

Other Factors that Influence Weight

But gravity is not the only force that can cause a change in the weight of an object. Other factors like buoyancy and downward or upward acceleration also play a role. When an object floats in water, it becomes weightless.

This is due to buoyancy exerted by the fluid. Buoyancy is nothing but an upward force that opposes the weight of the object wholly or partially submerged in the fluid. The point on the object where the buoyancy is applied is also the center of gravity of the liquid. Any object immersed in water will lose some weight in consequence of the buoyant force working vertically against gravity.

This is also true in the case of air balloons which have mass but are found to have no weight during the flight due to buoyancy exerted by the air.

The weight of a body inside a moving elevator can change. If the motion is upwards, the body becomes heavier. But if the motion is downwards, the body becomes lighter.

Some Other Facts About Weight

Unlike mass, weight is a vector quantity. A vector quantity is defined as something that has both magnitude and direction like velocity, acceleration, momentum, displacement, etc. Weight can be measured using a spring balance. According to SI, the unit of weight is Newton.

FAQ

How does inertia work?