Weight is the force with which gravity pulls an object toward the center of a planet or any large body. Unlike mass, weight is not constant — it changes depending on where the object is, because gravity itself changes from place to place.
Weight is a vector quantity (it has both magnitude and direction — always pointing downward, toward the center of the gravitational body), and it is measured in Newton (N), the SI unit of force.
Weight Formula
W=mg
Where:
- W = Weight of the object (in Newton, N)
- m = Mass of the object (in kilograms, kg)
- g = Acceleration due to gravity (in m/s²)
On Earth, the standard value of g is 9.8 m/s² (sometimes rounded to 10 m/s² for quick calculations).
Weight vs Mass — Why Students Get Confused
This is one of the most commonly confused topics in physics, so here’s a direct comparison:
| Point | Mass | Weight |
|---|---|---|
| What it measures | Amount of matter in an object | Force of gravity acting on the object |
| SI Unit | Kilogram (kg) | Newton (N) |
| Changes with location? | No — stays constant everywhere | Yes — changes with gravity |
| Type of quantity | Scalar | Vector |
| Measured using | Beam balance | Spring balance |
Simple example: A 60 kg person has the same mass (60 kg) whether they are on Earth or the Moon. But their weight changes — because gravity on the Moon is much weaker than on Earth.
Weight on Different Planets (using the same mass)
For an object with mass = 60 kg, here’s how its weight changes across the solar system:
| Planet/Body | g (m/s²) | Weight (N) |
|---|---|---|
| Mercury | 3.7 | 222 N |
| Venus | 8.87 | 532.2 N |
| Earth | 9.8 | 588 N |
| Moon | 1.625 | 97.5 N |
| Mars | 3.71 | 222.6 N |
| Jupiter | 24.79 | 1487.4 N |
This is why astronauts feel “lighter” on the Moon — their mass hasn’t changed, only the gravitational pull acting on them has.
Solved Examples
Example 1: Calculate the weight of a body on the Moon if its mass is 60 kg. (g on Moon = 1.625 m/s²)
Given: m = 60 kg, g = 1.625 m/s²
W = mg
W = 60 × 1.625
W = 97.5 N
Example 2: Calculate the weight of a body on Earth whose mass is 25 kg.
Given: m = 25 kg, g = 9.8 m/s²
W = mg
W = 25 × 9.8
W = 245 N
Example 3: An astronaut has a mass of 75 kg. Find their weight on Earth and on Mars, and compare.
On Earth: W = 75 × 9.8 = 735 N
On Mars: W = 75 × 3.71 = 278.25 N
The astronaut weighs less than half on Mars compared to Earth, even though their mass (75 kg) stays exactly the same on both planets.
Frequently Asked Questions
Q1. Is weight a scalar or vector quantity?
Weight is a vector quantity — it has magnitude and always acts in the direction toward the center of the gravitational body (i.e., downward).
Q2. What is the SI unit of weight?
The SI unit of weight is the Newton (N), same as the SI unit of force, since weight itself is a force.
Q3. Does weight change on the Moon?
Yes. Since the Moon’s gravity (1.625 m/s²) is about 1/6th of Earth’s gravity (9.8 m/s²), an object’s weight on the Moon is roughly 1/6th of its weight on Earth — even though its mass stays the same.
Q4. Can weight be zero?
Yes — in a state of zero gravity (like deep space, far from any planet), the value of g becomes 0, making weight zero, even though the object still has mass.