Mass: Definition, Unit, Types, Mass vs Weight

Have you ever been to a vegetable market? Or have you ever gone to buy fruits or vegetables from a local vendor? Now, do you recall seeing a beam balance like the one shown in the figure below:

So, we have to choose the amount and type of vegetables we want and put them separately. The vendor would one by one put all vegetables of a given type on one side of the balance, and on the other side, he would put some weights. He would either add a few vegetables or remove some until both sides are on the same level. Once the two sides of the beam balance are in balance, he would pack those vegetables for us. Ever wondered what exactly is the vendor doing with the beam balance? The answer to this question is pretty easy. He is comparing the two sides of the balance. But why? To calculate the mass! He compares the quantity of the vegetables we selected with a known quantity. Suppose the side with vegetables is lower than the known weight. In that case, we will have to remove some vegetables, or if the side with vegetables is higher than the known weight, we will have to add some vegetables till both the sides of the beam are balanced, i.e. the mass on both sides of the beam balance is equal. So what exactly is mass? Let us learn about it in detail.

Learn to Convert Lbs to Kg here

What is Mass?

Mass is the fundamental property of a given body. It is the inherent attribute of any physical system. We know that anything that occupies space and has mass is matter. So mass is the measurement of matter in a body. The total mass of a given object is assumed to be constant, or it is believed that the mass remains relatively unaffected by the strength of the gravitational field, irrespective of the object’s location. The SI unit of mass is Kilogram. Mass is a scalar quantity, i.e. it has only magnitude but no direction.

What is One Kilogram?

In the metric or international system of units (SI), Kilogram is defined as the basic unit of mass. It is almost equivalent to the mass of \(1000\;\rm{cm}^3\) of water. In the late \(18^{\rm{th}}\) century, a Kilogram was represented by a solid cylinder of platinum. In 1889, a standard kilogram was represented by a solid cylinder made from a platinum-iridium alloy of diameter same as its height. But it was found to be incorrect. So in 2011, at the General Conference on Weights and Measures (CGPM), a proposal was passed to redefine mass using a fundamental physical constant. Plank’s constant was chosen to be that constant. We know that Planck’s constant can be given as:
\(h = 6.62607015 \times {10^{ – 34}}\,{\text{joule}}\,{\text{second}}\)
Also, \(1\,{\text{joule}} = 1\,{\text{kg}}{{\text{m}}^2} / {{\text{s}}^2}\)
The meter and the second have already been defined in terms of the speed of light and the frequency of the spectral line of caesium, respectively. Thus, using these measurements, the Kilogram was successfully defined in terms of Planck’s constant. This proposal was accepted, and from May 20, 2019, the Kilogram is now defined by Planck’s constant.
Other units of mass are:

1. Pound: \(1\,{\text{pound}} = 0.45359237\,{\text{kg}}\)
2. Slug, the unit of mass in English system: \(1\,{\text{Slug}} = 32.17\,{\text{pound}}\)
3. Atomic Mass Unit: \(1\,{\text{a}}.{\text{m}}.{\text{u}} = 1.67 \times {10^{ – 27}}\,{\text{kg}}\)
4. Gram: \(1\,{\text{g}} = {10^{ – 3}}\,{\text{kg}}\)

Inertial Mass

Mass is one of the basic characteristics of a given object; it remains independent by the temperature, pressures, or position of an object in space. In physics, we apprehend mass as a quantitative measure of the inertia of a body. It can be defined as the resistance that a body offers to the change in its speed or position when an external force is applied to it. Thus, the change due to an applied force will be smaller on a body of greater mass, but it will be huge on a body with lesser mass. Inertial mass can be defined using Newton’s second law of motion. Thus, the resistance to acceleration offered against an applied external force is the object’s inertial mass.
Let’s understand the concept of mass by understanding Newton’s second law. Suppose a force \(F\) applied to a body produces an acceleration \(a\) in the body, then its mass \(m\) can be given as:
\(m = \frac {F}{a}\)
Thus, an object with a small inertial mass will accelerate more than an object with a large inertial mass when the same force is applied to it. Therefore, it can be concluded that a body of greater mass will possess greater inertia.

Gravitational Mass

Mass is often stated as the amount of matter enclosed in a body. The gravitational mass of an object is defined using the universal law of gravitation. According to this law, the gravitational force between two bodies can be given as:
​\(F = G\frac{{{m_1}{m_2}}}{{{r^2}}}\)
Here, \(G\) is the universal gravitation constant, \(m_1\) is the mass of the first body, \(m_2\) is the mass of the second body, \(r\) is the distance between the two bodies. Suppose we have to determine the value of \(m_1\), then, from the above relation:
\({m_1} = \frac{{F{m_2}}}{{G{r^2}}}\)
Thus, by comparing the force of gravity acting on a known mass with the force of gravity on a body whose mass we have to determine, we can obtain the gravitational mass of a body. We generally use a balance scale to measure the gravitational mass of an object.

What is Weight?

‘Weight’ is essentially the measure of how much an object weighs. The weight of an object is described as the force acting on it at all times, near the surface of the Earth, or the force with which the Earth pulls any object towards itself. It determines the heaviness of an object. Since it is a force, weight is measured in terms of newton\((\rm{N})\). It is a vector quantity, i.e. it has both direction and magnitude. The weight of an object varies from place to place.

Are Weight and Mass the Same?

Often the terms weight and mass are used carelessly with no regard to science. The two terms may be related yet are completely different.
Weight is the force of gravity with which the Earth or any other heavenly body pulls an object kept close to its surface towards itself. The equation to determine the weight of an object:
\(W = mg\)
Where \(W\) is the weight, \(m\) is the mass, and \(g\) is the acceleration due to gravity.

This equation stands correct under all situations. In fact, weight is the only force acting on a body in fall. The weight of an object will vary with the acceleration due to gravity. Since the acceleration due to gravity is \({\frac {1}{6}} ^{\rm{th}}\) the acceleration due to gravity at the Earth, we will weight \({\frac {1}{6}}\) times lighter on the moon.

Mass is the amount of matter in a given body, and that will be the same everywhere. Mass of an object is a fundamental property, and it is constant at all places, at all times. So even on the moon, where you weigh less, your mass is still the same.

Does Mass Always Remain Constant?

Mass, as we have discussed above, is the intrinsic property of matter. Technically, this means that the mass of an object is constant everywhere. In the case of nuclear reaction, mass does not remain conserved. A small amount of mass is converted into a large amount of energy. Einstein’s mass-energy equivalence gives the relation between mass and energy: \(E = m{c^2}.\)
Where \(E\) is the energy, \(m\) is the mass, and \(c\) is the speed of light in a vacuum.

How to Measure Mass?

Balances are used to measure the mass of an object. These are the instruments that measure the amount of matter contained in an object. In a balance, the mass of an object is measured by comparing its mass with the mass of a known object, a process that remains unaffected by gravity. A balance gives an accurate measure of the unknown mass everywhere because the force of gravity acts equally on both sides of the balance -for example, a Beam Balance.

Scales like weighing scales measure the weight of an object. The weight of an object is the force experienced by it equal to the product of its mass and the acceleration due to gravity at that place.
The gravitational acceleration on the Earth varies around \(0.5\%\) as the distance varies from the core of the Earth along longitude or latitude.  But is it possible to measure mass using a scale? Well, it is. But a scale cannot measure the mass of an object directly since its weighing mechanism is dependent on the local gravity at a given place. To measure the mass, we will have to calibrate the given scale for the local gravity at that place. That is why, when we measure ourselves on a weighing scale, we report it in terms of Kilograms and not Newton.

Mass vs Weight

Summary

Mass is the measurement of matter in a body. The total mass of a given object is assumed to be constant, or it is believed that the mass remains relatively unaffected by the strength of the gravitational field, irrespective of the object’s location. The SI unit of mass is Kilogram. Mass is a scalar quantity, i.e. it has only magnitude but no direction.
Often the terms’ weight’ and ‘mass’ are used carelessly with no regard to science. The two terms may be related yet are completely different.
Weight is the force of gravity with which the Earth or any other heavenly body pulls an object kept close to its surface towards itself. The equation to determine the weight of an object: \(W = mg.\)
Where \(W\) is the weight, \(m\) is the mass, and \(g\) is the acceleration due to gravity.

Frequently Asked Questions

Q.1. Are mass and weight the same?
Ans: No mass and weight are two different quantities. Mass is the quantity of matter in an object, while weight is the force due to gravity acting on an object at a place.

Q.2. Is mass always conserved?
Ans: No, mass is converted into energy in the case of nuclear reactions.

Q.3. Is mass a scalar or vector?
Ans: The mass has no magnitude and no direction; thus, it is a scalar quantity.

Q.4. Can the mass of an object be zero?
Ans: The mass of an object can never be zero, although weight can be zero.

Q.5. What is inertial mass?
Ans: Inertial mass is the resistance to an externally applied force offered by an object. It is governed by newton’s second law of motion using the equation, \(F = ma\), where \(F\) is the force, \(m\) is the mass, and \(a\) is the object’s acceleration.

We hope you find this article on ‘Mass helpful. In case of any queries, you can reach back to us in the comments section, and we will try to solve them.