What is the difference between mass and weight in SI units?

In one line

The rule

Mass is the amount of matter in an object, measured in kilograms (kg). Weight is the force exerted by gravity on that mass, measured in newtons (N).

The formula for weight is: W = m × g

• W = weight (in newtons, N)
• m = mass (in kilograms, kg)
• g = acceleration due to gravity (about 9.8 m/s² on Earth)

Let's understand it simply

Imagine you are holding a 5 kg bag of rice. The mass is just 5 kg—this tells you how much rice is inside, no matter where you are (on Earth, in a train, or even on the Moon).

But when you try to lift it, you feel its weight. This is because Earth's gravity pulls the bag down. The heavier you feel it, the more weight it has. If you took the same bag to the Moon, it would still have 5 kg mass, but it would feel much lighter because the Moon's gravity is weaker.

How it works

1. Every object is made of matter. The total amount is its mass.
2. Mass does not change, no matter where you are.
3. Weight is the pull of gravity on that mass.
4. To find weight, multiply mass by gravity: W = m × g.
5. On Earth, gravity (g) is about 9.8 m/s². On the Moon, g is about 1/6th of that.

The story behind it

Scientists needed a way to measure how much stuff is in an object (mass) and how strongly gravity pulls on it (weight). The SI system (International System of Units) was made so everyone in the world could use the same units. Kilogram (kg) was chosen for mass, and newton (N) for weight, named after Isaac Newton, who explained gravity.

Where you see it in real life

• Buying vegetables: The shopkeeper weighs potatoes. The scale shows mass in kg, but the force you feel lifting the bag is its weight.
• Weighing yourself: The bathroom scale shows your mass in kg, but it works by measuring your weight (force) and converting it.
• Lifting a gas cylinder: The cylinder's mass is fixed, but it feels heavy because of its weight (gravity pulling it down).
• On the Moon: An astronaut's mass is the same as on Earth, but their weight is much less, so they can jump higher.
• In a lift (elevator): When the lift moves up or down, your weight changes for a moment, but your mass stays the same.

Why it matters

• Science and engineering: Machines, vehicles, and buildings are designed using mass and weight.
• Space travel: Astronauts need to know their mass and weight on different planets.
• Medicine: Doses of medicines are based on mass, not weight.
• Exams: Many physics questions ask you to calculate weight from mass.

Worked example

Given: m = 10 kg, g = 9.8 m/s²
Step 1: Write the rule        W = m × g
Step 2: Put in the numbers    W = 10 × 9.8
Step 3: Answer                W = 98 N

Common mistakes

• Mixing up units: Mass is in kilograms (kg), weight is in newtons (N).
• Thinking mass changes with location: Mass stays the same everywhere; weight changes if gravity changes.
• Saying "weight" when you mean "mass": For example, "My weight is 50 kg" is wrong. It should be "My mass is 50 kg".

Remember it

Mass is the amount of stuff (kg); weight is the pull of gravity on it (N).