# Free Tutorial in Basic Physics - Understanding Gravity

Physics is a vast subject and there are many interesting things in it to learn and understand. In this article, we would try to conceptualise the basic understanding of the property called gravity and will learn what is gravity and what does it mean to us.

### Introduction

In some earlier articles in this series related to basic Physics concepts we had covered some topics in Physics and learned a good deal about density, acceleration, electric voltage, electric current etc. Now in the present article, we would like to learn and understand various interesting things about the entity called gravity which is the invisible force between the bodies of all sorts and sizes in this universe. This force is known as gravitational force and the property is known as gravity.

### Basic understanding of gravity

When we are standing on Earth we feel our weight in the downward direction and if we try to raise ourselves or any part of our body then we have to do some effort. What is this weight and why it is working on us? Who is pulling us towards Earth? These questions naturally come to our mind and the answer is the gravitational power of Earth. Everybody or object in this universe has a gravitational power that depends on its mass. We are glued to Earth because of its massive gravitational power. Moon is also a massive body near us and in fact, encircling the Earth but its mass is much smaller than the Earth and consequently, its gravity is also less. So the astronauts who went to the Moon felt this thing and felt if as their weight was reduced significantly. They could do a bigger high jump there!

Small bodies also have their gravity or gravitational power but it is less in magnitude as their mass is smaller and the force they exert on other bodies would not be perceptible by us. On the other hand, Earth is a massive body, we feel this force day and night, pulling us downwards.

The great scientist Sir Issac Newton discovered this and it is said that he was inspired by an apple falling from a tree and was thinking as to why it came down and did not go up or fly away in the air. Many people might have observed such things happening in nature but it was Newton who had that curiosity in his mind and then he came out with a theory to explain it and it was a great contribution by him in the field of science.

### The Gravitational force

Earth has a strong gravitational field and that is why we feel glued to it. The Sun, the centre of our solar system is a very massive body as compared to the Earth. Sun would have a very high gravitational field as compared to the Earth and that is the reason why so many planets (Mercury, Venus, Mars, Earth, Saturn, Neptune, Uranus, and Plato) and some of the smaller bodies like some asteroids and comets are bound to it by that invisible gravitational force and are rotating around it. Sun is a star and like that there are billions of stars in this universe having their gravity and most of them have their planets encircling them. A cluster of so many solar systems constitutes a galaxy like the famous Milky Way galaxy inside which we are living on Earth. Everything is balanced in this universe under the gravitational fields of these bodies and in fact, the galaxies have their combined gravitational field which they impart on other galaxies or scattered heavenly bodies and even in the vast space of universe, there is a hidden order and discipline that these bodies follow under this mysterious but powerful gravitational force.

### Acceleration due to gravity

In one of our earlier basic Physics lessons on acceleration, we had learned that the rate of change of velocity is acceleration. Which in simple words means that if we push a body with some continuous force and try to increase its velocity then it will get acceleration. The gravitational field of the Earth creates a continuous force on anybody and is also capable of creating an acceleration in that. For example, if we drop a ball or a toy from the rooftop of a building to the ground then due to the gravitational field of Earth it would fall and reach the ground. As the force of gravity is continuously acting on this body it gets an acceleration and this acceleration is called acceleration due to gravity. The interesting thing is that this acceleration would be the same for a big body or small body irrespective of their masses because the force of gravity will be more on the bigger mass as compared to the smaller one. Here we have neglected the small drag force that is imparted by the air in the atmosphere on the falling bodies. So, precisely speaking, in a vacuum, the freely falling bodies will have the same acceleration irrespective of their masses.

The acceleration of a freely falling body is called the acceleration due to gravity and its value on the surface of the Earth is about 9.8 m/s^2 (9.8 meters per second squared). It is denoted by the letter 'g'. The acceleration due to gravity is also known as surface gravity. Earth is having a surface gravity equal to 1 g (1 g = 9.8 m/s^2) while the massive Sun is having surface gravity as large as 28 g. On the other hand, Moon is having a surface gravity of only about 0.167 g. It is interesting to note that for the same mass of a body its weight will be one-sixth on Moon. So an astronaut having a weight of 84 kg on Earth would feel his weight only about 14 kg on the Moon! The surface gravity of other planets of our solar system is: Mercury - 0.377 g, Venus - 0.904 g, Mars - 0.38 g, Saturn - 1.065 g, Uranus - 0.886 g, Neptune - 1.14 g, and Pluto - 0.063 g.

### Weight of a body

Every kind or type of matter is having some mass and it is measured in grams or pounds. An apple may have a mass of say 170 grams while a watermelon may have about said 3200 grams (3.2 Kg). So the material inside a body makes up its mass. Some bodies will have a small mass while some others might have a very large mass. For example, Earth is a massive body and it will have a very large mass. Incidentally, the mass of Earth is 5.972 × 10^27 gram which is a mind-boggling number. Stars like Sun are still much bigger in their sizes and have a much larger mass. The mass of the Sun is about three million times that of the Earth!

So mass is the amount of material in a body. What is weight then? Weight is the force with which this body is attracted and pulled by the Earth. We all have some individual weights. The unit of weight is the gram weight but in practice, it is also called gram and there could be some confusion sometimes in differentiating mass with weight but we have to remember that the basic thing is that mass is quantity or amount of material while weight is a force.

For a body, its weight (W), mass (m), and acceleration due to gravity (g) are related by the equation:
W = mg. In the MKS system, the mass is in kilogram (kg) and the value of g is in m/s^2. So W will be in Kgm/s^2 which is known as Newton. So 1 Newton = 1- kilogram meter per second squared. Please remember that m is the mass but W is the force with which Earth is attracting the body towards it.

### The Gravitational force between two bodies

In a general way, we can summarise that there is a force of gravity between two bodies but it is only felt when bodies are big or at least one of them should be big enough. The great scientist Issac Newton in the year 1687 formulated the law of gravitation and told us that the force (F) between two bodies having the mass of m1 and m2 respectively and situated at a distance of r from each other is given by -
F = Gm1m2/r^2

Here the unit of F is in Newton, unit of mass is in kg, and unit of r is in meter. G is the constant of proportionality known as gravitational constant and is having a unit as Newton meter^2 / kg^2.
The above law says that the force of gravitational attraction between two bodies is proportional to the masses of the bodies and is inversely proportional to the square of the distance between them.
It is interesting to note that the experimental verification of the newtons law of gravitation was done only in the year 1798 (almost about 111 years later) by the scientist Henry Cavendish. He found the value of G to be 6.67 x 10-11 Newton meter^2 / kg^2.

### Conclusion

There is a universal force of gravity acting between the bodies having various masses and Earth being a massive body keeps us attached to it due to its large mass and the force with which it attracts us is known as the gravitational force of Earth. In common terms, we call it our weight. The heavenly bodies in the vast universe are balanced by the gravitational forces between them.

 If a body is sent out of the Earth's gravitational field then would it come back?No, it will not come back until there are rockets and fuel in it to fire remotely from Earth or automatically for giving it propulsion energy for coming back. Is gravitational force a strong force?No it is not a strong force but the heavenly bodies are so massive that the total force becomes sufficient to be felt by the people or animals on it. What happens when there is no gravity?That is a hypothetical situation. But yes in that case nothing remains bound to each other and the result would be difficult to explain. Can heavenly bodies collide with each other due to gravitational attraction?No, it would be very rare because bodies are moving and there is a type of equilibrium and nothing of that sort is going to happen. I am a big person having my own gravitational field but a small pin does not attach to me?Because that force is so small that it cannot glue that pin to you and at the same time the force with which the Earth is pulling it is much more and the pin falls down.