Difference Between Mass And Weight | Kg vs Kgf

Mass and weight are two very common terms used in science. Kids often use them interchangeably without understanding that there is a difference between mass and weight. Yes, you read it right, mass and weight are not the same.

Mass is a measure of the amount of matter in an object, whereas weight is a force exerted by an object of fixed mass due to gravity.

Before we proceed with the differences between the two, let’s learn more about mass and weight.

Skip to >> The difference between mass and weight.

Topics covered:

• Mass
  • Definition of mass
  • Standard units for measurement of mass
  • Multiple and sub-multiple of a kilogram
• Weight and its SI unit
• Difference between mass and weight

MASS

All things around us consist of atoms or molecules. In certain substances, these molecules or atoms are more tightly packed, as compared to other substances. Mass is the measure of the total amount of matter of these molecules present in a given substance. Thus, the more the number of atoms or molecules present in a given substance, the more mass it has.

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A definition of mass for kids-

Mass is a measure of the amount of matter present in an object.

Let us understand it with this simple exercise –

Exercise

Get two empty glasses of the same size. Fill one of them to the top with sand and the other with water. Now lift each of them one by one. Which one of the two glasses do you find is heavier?

You will notice that the glass containing sand is heavier than the glass containing water in it. 

The reason behind it is that sand is a solid and most solids contain more atoms than liquids. So although the size of both glasses is the same, glass containing sand in it has more atoms than glass containing water in it. In other words, we can say glass with sand has more mass in comparison to one with water.

So, mass is a measure of the amount of matter present in an object. If we compare two objects of the same volume, then one with more atoms or molecules in it will have more mass.

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STANDARD UNIT (SI) FOR MEASUREMENT OF MASS

We measure mass in kilograms. The kilogram is the Standard International (SI) unit of mass. In short form, we write it as (kg).

When you visit a shopkeeper and ask for 1 kg of sugar, you specify the quantity of sugar required by stating its mass i.e. 1 kg. The shopkeeper uses beam balance to measure the quantity of sugar equivalent to 1 kg of mass before handing over it to you. So, in this case, mass is a basic measurement of the quantity, the beam balance is the instrument that measures the same, and kilogram (kg) is the unit in which it is measured.

To know how big is 1 kilogram or 1 kg, visit the shop of the nearest grocer and request him to show you a 1-kilogram mass. Kindly note this mass is commonly mistaken as weight.

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MULTIPLES AND SUB-MULTIPLES OF KILOGRAM (kg)

The unit kilogram (kg) is too small for measuring some mass and too large for others. So, we increase it in multiple of 10 for large measures and decrease it by sub-multiples of 10 for small measures. These multiples and sub-multiples of the kilogram are:

Multiples of kilogram

Quintal, 1 quintal = 100 kilograms = 100 kg

Metric Tonne, 1 metric Tonne = 1000 kilograms = 1000 kg

To measure the mass of extremely large objects like stars and galaxies we use Solar mass as the unit. The solar mass is defined as the mass of the Sun.

Sub-multiples of kilogram

Gram (g), 1 g = 0.001 kilograms = 1.0 x 10^-3 g or 1 kg = 1000 g

Milligram (mg), 1 mg = 0.000001 kilograms = 1.0 x 10^-6 g or 1 kg = 1000,000 mg

To measure the mass of extremely small objects like atoms or molecules, we use the atomic mass unit (amu or u) as the unit. The atomic mass unit (u) is 1/12 of the mass of a carbon-12 atom, approximately 1.66×10⁻²⁷ kg.

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WEIGHT

Force exerted by an object of fixed mass under the influence of gravity is called weight.

WEIGHT = MASS     X     GRAVITY

The weight being a force, its SI unit is ‘Newton (N)’. The gravity of the earth exerts a force of about 10 N on the mass of 1 kg. In other words, a body of mass 1 kg will weigh 10 N on the Earth. However, people often use kilograms (kg) as the unit for the weight which is not correct. This confusion arises due to another unit of weight which is ‘kilogram-force (kgf)’, where 1 kgf = 10 N (nearly). So, a body having a mass of 1 kg will weigh nearly 1 kgf or 10 N on the earth.

A definition of weight for kids –

Weight is a force exerted by object of fixed mass under influence of gravity.

As the value of gravity changes around the earth, thus the weight of the object varies at different places, but its mass remains the same. Usually, variation in gravity at different places on the earth is very less, so the change in the weight of objects goes unnoticed. But if we go to any place outside the earth, such as the moon, then we will notice the difference.

Difference between the Mass and Weight on Moon and the Earth’s Surface

The gravity of the Earth exerts a force of about 10 N of 1 kgf on the mass of 1 kg. If your mass is 36 kg then your weight on the Earth will be 360 N or 36 kgf.  Whereas on the moon, the force due to gravity is 1/6th that of the Earth. Thus, you will weigh only 360 x 1/6 = 60 N or 6 kgf on the moon. However, as your body consists of the same number of atoms on the moon as on the earth, thus your mass will remain the same at both places.

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DIFFERENCE BETWEEN MASS AND WEIGHT

1. Mass is a measure of the number of atoms or amount of matter in an object, whereas weight is a force exerted by an object of fixed mass due to gravity.
2. Mass remains constant for a body and does not change with place, whereas weight changes from place to place.
3. To measure mass we use beam balance, whereas for the weight we use spring balance.
4. Mass is a scalar quantity, whereas weight is a vector quantity.
5. S.I. unit of mass is the kilogram (kg) and that of weight is Newton (N). Another unit of weight is kilogram-force (kgf) where 1 kgf = 10 N (nearly)

Difference between mass and weight in the tabular form:

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