1. What is Mass?
Mass is a measure of how much matter an object contains. It tells us how heavy or light something is in terms of the amount of material inside it. Mass does not change from place to place—it remains the same whether you are on Earth, the Moon, or in space.
Mass also determines how difficult it is to change the motion of an object. This resistance to motion is known as inertia.
1.1. Key Characteristics of Mass
- Mass is a scalar quantity.
- It is measured in kilograms (kg).
- It does not depend on gravity.
- It represents the amount of matter.
- Mass remains constant everywhere.
1.1.1. Simple Examples
- A 1 kg stone has the same mass on Earth and the Moon.
- A suitcase with mass 15 kg remains 15 kg no matter where you take it.
2. What is Weight?
Weight is the force with which Earth (or any other planet) pulls an object toward its centre. Unlike mass, weight is not the amount of matter—it is the gravitational force acting on that matter.
Weight depends on gravity. If gravity changes, weight also changes.
2.1. Formula for Weight
The weight of an object is given by:
\( W = mg \)
Where:
- \( W \) = weight
- \( m \) = mass
- \( g \) = acceleration due to gravity
2.2. Key Characteristics of Weight
- Weight is a vector quantity (it has direction—toward the Earth’s centre).
- Measured in newtons (N).
- Depends on local value of \( g \).
- Changes from place to place.
- Becomes zero in free fall.
3. Difference Between Mass and Weight
Mass and weight are often confused, but they are quite different. The table below shows a simple comparison:
3.1. Comparison Table
| Mass | Weight |
|---|---|
| Amount of matter | Force due to gravity |
| Measured in kg | Measured in newtons (N) |
| Same everywhere | Changes with gravity |
| Scalar quantity | Vector quantity |
| Never zero | Can be zero (in free fall) |
4. How Mass and Weight Change in Different Places
Even though mass stays constant, weight varies depending on the intensity of gravity at a location.
4.1. On Earth
The value of \( g \) changes slightly across the Earth:
- Higher at poles → more weight
- Lower at equator → slightly less weight
4.2. On the Moon
The Moon’s gravity is only about 1/6th of Earth’s. So, a 60 N weight on Earth would weigh only about 10 N on the Moon, but its mass remains unchanged.
4.3. In Space
Astronauts appear weightless because they are in free fall around Earth. Their mass remains the same, but their apparent weight becomes zero.
5. Why Distinguishing Mass and Weight Matters
Understanding the difference between mass and weight is important because:
- Mass is used to measure quantity of matter.
- Weight determines how strongly gravity pulls you.
- Engineers must consider weight while designing buildings, bridges, and vehicles.
- In space travel, mass determines fuel needs while weight affects launch conditions.
In the next topic, we explore acceleration due to gravity (g) and why objects fall with a particular acceleration.