Basic Physics lesson-4 : Mass and Weight.
There are many concepts in basic Physics which make the rudimentary understanding of the science to the students. In our earlier articles on Distance and displacement, Speed and velocity, and Acceleration, we learned about displacement, velocity and acceleration which were related to the motion of the bodies. Now in this lesson we would like to understand the most basic concepts regarding the physical entities known as mass and weight.
Do you remember studying mass and weight during your school days?
What do we understand by mass?
Mass is the amount of matter in the body. It has a unit of gram or kilogram (1 kilogram=1000 gram) or ounce or pound (1 pound=16 ounce). If you go to market and buy 300 grams of chocolate and find it consisting of only 10 piece as against your requirement of 20 then you would have to buy 300 grams more. So, here the amount of material is doubled. So, mass is an inherent characteristics of the matter. So when we specify a quantity as 100 grams it signifies a fixed mass of 100 grams. One interesting thing about mass is that one can not tell the mass just by appearance of an item. 100 gram of cotton would look much bigger when compared to the 100 gram of an iron piece.
What do we understand by weight?
Many people get confused between mass and weight. They use both interchangeably. For understanding the difference between the two we have to understand the acceleration due to gravity on our planet Earth. What happens is that when we drop a thing from a height it gains velocity because Earth attracts it by its gravitational force. It then strikes the ground with increased velocity and comes to a rest. The acceleration during this downward journey is known as acceleration due to gravity and is denoted by g. Let us see what happens when we throw a thing upwards. The Earth attracts it downwards and it gets deaccelerated and stops after reaching a height and comes backwards. This deacceleration is also caused by Earth's gravitational field. The acceleration due to gravity at the surface of the Earth is about 9.8 metre per second2 and is same for each body whether having less mass or more mass.
So, weight is defined as the multiplication of mass with this acceleration. Hence, we can say that 1 kg weight = 1 kg mass x 9.8 meter per second2. For sake of simplicity 1 kg meter per second2 is represented by 1 Newton. So, finally we get 1 kg weight = 9.8 Newton. So, remember that weight is a force and 1 kg weight is nothing but 9.8 Newton. In our practical life we do not use Newton in day to day conversation and simply say so much kg of weight.
Mass and weight are different and we can understand it by an interesting example. Let us consider Moon where the acceleration due to its gravity is about one sixth than that of Earth. What would happen to the weight of a body on Moon. It would be one sixth of than that on Earth. What will happen to the mass? It would remain same as the material in the body is same. Different planets in our solar system have different gravitational force and accordingly the same mass would have different weights there. Just imagine that we would be 25 times heavier on Jupiter!
Weightlessness is an interesting thing to know. What happens when a person goes to a place where there is no gravity? For example a place far from Earth in the void of space. What would be our weight there? It would be zero. So we would be in a state of weightlessness there and would freely float in the space. Astronauts feel it when they go in space. Even on Earth in some of the laboratories artificial weightlessness environment is created to train the Astronauts.
How do we move when we are in a state of weightlessness? We have to use a propellent firing system so that when we fire it in one direction the recoil or reaction would move us in opposite direction. Have you seen in movies persons flying in air opposite to some firing exhaust mechanism at their back? That is what is meant by reaction of that firing on him.
Was this lesson useful?
In essence mass is the amount of matter in a body while weight is the force with which the Earth attracts it. As laymen, we use both the words interchangeably but in Physics they have entirely different meaning.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2020 Umesh Chandra Bhatt