5.5 Weight & Mass
Gravity pulls every object — but does it pull them all equally hard? The answer leads us to one of the most misused pairs of words in everyday life: weight and mass.
What Is Weight? The weight of an object is the force with which the Earth pulls it toward itself. Weight measures how strongly gravity tugs on the object. Since weight is a force, its SI unit is the newton (N) — the same as any other force.
Activity 5.9 — Different pulls, different stretches Hang a spring from a nail and hook objects of different masses onto it one by one — a pencil box, a tiffin box, a stone. The spring stretches by different amounts. A bigger stretch means a bigger pull. So the Earth pulls different objects with different forces — they have different weights.
The spring balance
That stretching spring is a measuring device. A spring balance is a spring fixed at the top with a hook below; hanging an object stretches the spring, and a scale marked in newtons reads off the weight. (Many spring balances also carry a second scale in grams, assuming they’re used on Earth.)
Worked Example
Reading a spring balance scale. A spring balance reads 0 to 10 N. Between the “1 N” and “2 N” marks there are 5 small divisions. What is the smallest weight it can measure?
The gap between two big marks is 1 N, split into 5 equal divisions. So one small division reads: \frac{1\ \text{N}}{5} = 0.2\ \text{N}. The balance can therefore measure weight to the nearest 0.2 N — its least count.
Weight is not mass
The Crucial Difference Mass is the amount of matter in an object, measured in grams or kilograms. It is the same everywhere — on Earth, the Moon, or in space. Weight is the gravitational force on the object, measured in newtons, and it changes with the strength of gravity. So: mass stays constant; weight can vary.
The same 1 kg object has very different weights across the Solar System:
| Earth | Moon | Mars | Venus | Jupiter | |
|---|---|---|---|---|---|
| Mass | 1 kg | 1 kg | 1 kg | 1 kg | 1 kg |
| Weight | 10 N | 1.6 N | 3.8 N | 9 N | 25.4 N |
The mass never budges; the weight follows the local gravity. On the Moon, gravity is about one-sixth of Earth’s, so the object weighs about one-sixth — yet it still contains exactly the same amount of matter.
Everyday Language vs Science We say “this bag of wheat weighs 10 kg” — but kilograms measure mass, not weight! In casual speech we mix them up; in science we keep them straight: mass in kg, weight in N. Because an object’s weight stays nearly the same all over the Earth, we can still use a weighing to find its mass for everyday purposes.