What is the difference between mass and weight

Mass vs. Weight

Mass quantifies how much substance an object contains and does not change when the object moves to a different location. Weight is the force that gravity exerts on that mass, so it varies with the strength of the gravitational field. Because weight is a force, it follows Newton's second law (F = m g\).

The main distinctions are:

• Mass is intrinsic; weight is extrinsic
• Mass uses kilograms (kg); weight uses newtons (N)
• Mass stays the same on Earth, Moon, or Mars; weight changes with g
• Mass is a scalar; weight is a vector pointing toward the planet’s center

In the International System, mass is expressed in kilograms, grams, or tonnes, while weight is expressed in newtons. Some textbooks still list weight in kilograms‑force, but that mixes units and leads to confusion. Remember that (1\,$\text{N}$=1\,$\text{kg}$$\cdot$$\text{m/s}$^2\), so converting between mass and weight always requires the local acceleration due to gravity, (g\).

To calculate weight from mass on Earth:

1. 1Identify the mass in kilograms
2. 2Use Earth’s standard gravity (g = 9.81\,$\text{m/s}$^2\)
3. 3Apply (W = m g\)
4. 4Write the result in newtons

Example: A 12‑kg textbook sits on a desk. Using Earth’s gravity, its weight is $W = 12\,\text{kg}\times9.81\,\text{m/s}^2 = 117.7\,\text{N}$. On the Moon, where (g = 1.62\,$\text{m/s}$^2\), the same book weighs $12\times1.62 = 19.4\,\text{N}$. The mass stays 12 kg in both places, but the weight drops dramatically because lunar gravity is weaker.

Mass and weight on several celestial bodies for a 10‑kg object: