What is Weight?
Weight is the force exerted on an object by gravity. It is a vector quantity, meaning it has both magnitude (how much force) and direction (down, toward the center of the planet or large body).
This is a specific application of Newton's Second Law ($F = ma$), which is also related to our force calculator. In the case of weight, the force ($F$) is the object's Weight ($W$), and the acceleration ($a$) is the local gravitational acceleration ($g$). It is also directly related to the gravitational force between two objects.
The Weight Formula
The formula to calculate weight is simple:
- $W$: The Weight of the object, measured in Newtons (N).
- $m$: The Mass of the object, measured in Kilograms (kg).
- $g$: The local gravitational acceleration (or gravitational field strength), measured in meters per second squared (m/s²).
On the surface of Earth, $g$ is approximately $9.81 \text{ m/s}^2$. On the Moon, $g$ is much weaker, about $1.62 \text{ m/s}^2$. This calculator allows you to input any value for $g$ to find the weight of an object on different planets.
Weight vs. Mass
This is one of the most common points of confusion in introductory physics.
- Mass ($m$): This is an intrinsic property of matter. It measures an object's inertia, or "how much stuff" it's made of. Your mass is the same whether you are on Earth, on the Moon, or floating in space. It is measured in kilograms (kg). Mass is a key component in momentum ($p=mv$).
- Weight ($W$): This is a force. It is the measure of the pull of gravity on an object's mass. Because it depends on $g$, your weight changes depending on where you are. A 70 kg person weighs about 687 N on Earth, but only 113 N on the Moon.