What is mass? Differences between mass and weight

What is mass?

We can define mass as the measure of the amount of matter that a body contains. In physics, it is a measure of inertia, the most fundamental property of all matter. Where inertia is an inherent property of each body by virtue of which it has a tendency to resist the change in its state of rest or state of motion.

The property of inertia is because of the mass of the body. The greater the mass, the greater the inertia of the body. In other words, the more the mass of the body, the more difficult it is to move the body from rest or to stop the body if it is initially moving.

According to the principle of conservation of mass, the mass of a body does not change. If a body divided into two or more parts, then the mass also divides with the parts. The sum of the masses of every individual part is equal to the mass of the original one.

What is mass?

Another way if two or more body combines together, the mass of the composite body is equal to the sum of the masses of every constituent bodies. So it is notable that the mass of a body does not change at any time.

But the principle is not always correct. It changes when some extreme situation happens where a very large amount of energy is exchanged (given or taken) from a body.

According to Albert Einstein's special theory of relativity, the mass of an object is equivalent to energy, the mass and energy can be interchangeable. If the mass of a substance is lost, then the amount of mass lost in the substance is converted into energy.

For example - the mass of an atomic nucleus is smaller than the sum of the rest masses of its constituent neutrons and protons. It's happened because some energy is removed when the nucleus is formed. This energy has mass, which is removed from the total mass of the neutrons and protons. 

So the mass of a substance was no longer considered constant or unchangeable. In both the cases of chemical and nuclear reactions, some conversion between mass and energy happened, where the products generally have smaller or greater mass than the reactants.

Where does mass come from?

The weight of an object is determined by its mass (m) and gravitational acceleration (g). But the question is where the mass of the object comes from?

Researches in Large Hadron Collider (LHC) in CERN might now provide a solution for this problem. In this experiment, scientists find a new particle called  'Higgs boson' which may provide mass to other particles.

The Standard Model of the Elementary particles says that this Higgs boson has a field called Higgs field and the mass comes due to the interaction between particles and the Higgs field. Theoretically, without the Higgs field, there would be no mass of fundamental particles.

We know that photon has no mass. It is because photons do not interact with the Higgs field and so they are massless. All the subatomic particles like quarks, leptons ( the building block of all matter ) gain their masses by interacting with the Higgs field.

Watch the video to better understand - Watch here

What are rest mass and relativistic mass?

In special relativity, the mass has two meanings - one is the rest mass and another is the relativistic mass. The rest mass of a body is the mass of a body when measured in the reference frame of an observer. Where the relativistic mass of a body, is the mass when the body is observed traveling at a velocity relative to an observer.

The rest mass is the same for all observers in all frames of reference, while the relativistic mass is always dependent on the velocity of the observer.

According to the principle of mass-energy equivalence, rest mass is equivalent to rest energy, while relativistic mass is equivalent to relativistic energy (also called total energy). This relativistic energy of an object was understood to form its rest mass as well as its increase of mass caused by high speed.

Relation between rest mass and relativistic mass

According to special relativity, the mass of an object in a frame of reference at rest is called its rest mass m0. When the object comes in motion then a relative change in mass is also perceived, which is the relativistic mass of the object.

This means if the object travels at a constant speed v then the mass will not remain constant. This mass variation can be measured by this formula - 

m =  mass of the object in motion
m0 =  rest mass
v =  speed of the object
c = speed of light in vacuum

case (1):

When the velocity of the object v is very small compared to the velocity of light c (i.e v2/c2 is negligible compared to one), then m = m0.

case (2):

When the velocity of object v is comparable to the velocity of light c (i.e √(1-v2/c2) is less than one), then the mass of the moving object appears greater than its rest mass, (i.e m>m0).

case (3):

When the velocity of the object v is equal to the velocity of light c, then the mass of the moving object is possessed infinite mass.

How to measure mass?

Mass can be measured in kilogram (the SI unit of mass) and gram (the CGS unit of mass). Where one kilogram is equal to 1000 grams. Actually, one kilogram is a measure of a block of Platinum-Iridium alloy in the shape of a cylinder 39.17 millimeters in both height and diameter.

The instrument for measuring mass such as weighing scale, mass balance, etc. In physics, mass and weight are two different physical quantities. To measure the mass of an object, at first, we actually measure its weight and then divide it by the gravitational acceleration (g).

So mass is often measured by measuring the weight of the objects using a weighing scale, rather than a balance scale comparing it directly with known masses.

What is weight?

According to physics, the weight of an object is a force acting on the object due to gravity. It is a vector quantity. Its direction is downwards towards the center of the earth. Since weight is a force, its SI unit is also Newton.

Now if we talk about the relationship between weight and mass, it is as follows

Weight = mass × acceleration due to gravity

Since the value of g varies from place to place, the weight of a given body also varies from place to place.

For example - If an object of mass 10 kg is measured on the earth where g = 9.8 m/s2 then its weight becomes 98N. If it is measure on the moon where g = 1.62 m/s2 then its weight becomes 16.2N and if it is measure in space where g = 0 then its weight becomes 0.

What are the differences between mass and weight?

1. Mass of an object can not be zero. But weight can be zero, such as in space where no gravity acts upon an object, the weight of the object becomes zero.

2. Mass has only magnitude and no direction so mass is a scalar quantity. But the weight has magnitude and direction (which is directed toward the center of the Earth or other gravity well), so weight is a vector quantity.

3. The mass of an object doesn't change with location. However, the weight of an object varies from place to place.

4. Mass is measured in kilograms (SI unit) and gram (CGS unit). Where weight is measured in Newtons (SI unit) and dyne (CGS unit).

5. Mass can be measured with an ordinary balance. Where weight is measured using a spring balance.

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