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๐ฌ Think about it
Hang a pencil box from a spring and it stretches a little; hang a stone and it stretches more. The Earth is pulling each one — but not equally. That stretch is a clue we can turn into a measurement. How do we measure how hard the Earth pulls? Let's find out.
What is the weight of an object?
- It is the force with which the Earth pulls an object towards itself.
- Weight measures how strongly the Earth pulls the object.
- Since weight is a force, its SI unit is also the newton (N).
Example:
A 1 kg object has a weight of about 10 N on the Earth.
Does the Earth pull every object with equal force?
- No — a spring stretches by different amounts for different objects.
- So the Earth pulls different objects with different forces.
- That means different objects have different weights.
How does a spring balance measure weight?
- A spring balance has a spring fixed at one end and a hook at the other.
- When an object is hung, the spring stretches.
- The amount of stretch shows the weight on a scale marked in newton.
- It often has a second scale showing mass in grams.
How do we find the smallest weight a spring balance can read?
- Find the weight between two big marks — here it is 1 N.
- Count the small divisions between them — here there are 5.
- Divide to get the value of one small division (the least count).
One small division \( = \dfrac{1\ \text{N}}{5} = 0.2\ \text{N} \)
Example:
For this balance the smallest weight that can be read is 0.2 N. Always check the range and least count of your own balance before using it.
In this Activity, we will hang different objects from a spring to see if the Earth pulls them equally.
Materials needed
A spring and a few objects of different masses, such as a pencil box, a tiffin box, and a small stone.
Procedure
1. Hang one end of the spring from a nail.
2. Hang an object from the other end and observe the stretch.
3. Hang the other objects one by one and note the stretch each time. Is it the same?
2. Hang an object from the other end and observe the stretch.
3. Hang the other objects one by one and note the stretch each time. Is it the same?
Observation
The stretch is different for different objects.
Explanation
The Earth pulls different objects with different forces, so their weights differ. The stretch in the spring can be used to measure weight.
โ Summary
- Stretch differs by object
- Earth's pull differs
- Spring can measure weight
What is the difference between mass and weight?
- Mass is the amount of matter in an object, measured in grams or kilograms.
- Mass stays the same everywhere.
- Weight is the gravitational pull on the object, measured in newton.
- Weight can change from place to place, but mass does not.
Mass
Weight
Amount of
matter
in an object.
The
gravitational pull
on the object.
Measured in
grams or kilograms
.
Measured in
newton (N)
.
Stays the same
everywhere.
Can change
from place to place.
How does weight change on different planets?
- The same object has the same mass everywhere.
- But its weight is different on the Earth, Moon and other planets.
- This is because the gravitational pull is different in each place.
| Earth | Moon | Mars | Venus | Jupiter | |
|---|---|---|---|---|---|
| Mass of the object | 1 kg | 1 kg | 1 kg | 1 kg | 1 kg |
| Weight of the object | 10 N | 1.6 N | 3.8 N | 9 N | 25.4 N |
โก A step further
- The mass of an object can be found indirectly using a spring balance or a beam balance.
- Since weight stays almost the same everywhere on the Earth, weighing is used to find mass for practical purposes.
- In daily life we say "the wheat bag weighs 10 kg", but kilogram is a unit of mass.
- Scientifically, the correct term and unit should be used.
โ
Test Yourself
-
What is the weight of an object?
View Answer
The force with which the Earth pulls the object towards itself. -
What is the SI unit of weight?
View Answer
The newton (N). -
A spring balance has 5 divisions between marks 1 N apart. What is its least count?
View Answer
\( \dfrac{1\ \text{N}}{5} = 0.2\ \text{N} \). -
Which stays the same everywhere: mass or weight?
View Answer
Mass stays the same; weight can change from place to place. -
Why is the weight of an object less on the Moon?
View Answer
The Moon's gravitational pull is weaker than the Earth's.
Important Definitions
- Weight — The force with which the Earth pulls an object towards itself; measured in newton (N).
- Mass — The amount of matter in an object; measured in grams or kilograms and the same everywhere.
- Spring balance — A device that measures weight from the stretch of a spring.
- Least count — The smallest value an instrument can measure, here \( 0.2\ \text{N} \) for the spring balance.
๐ NCERT Question 9 — The weight of an object on the Moon
The weight of an object on the Moon is one-sixth of its weight on the Earth. What causes this? Does its mass also become one-sixth?
View Answer →
