# Basic Physics lesson-5 : Gravity

## Introduction

In the earlier lessons in this series we tried to understand and learn the concepts of displacement, velocity, acceleration, and mass & weight. Now we move on to something which is fundamentally present in the nature and in the universe around us and that is called Gravity.

Gravity is a natural propery of the bodies to exert a force of gravitational attraction on other bodies. More bigger the body is more profound the attraction. The credit of discovering the gravity and formalising it with the physical equations goes to Sir Issac Newton who was inspired by an apple falling straight from the tree to the ground and then Newton started enquiring himself as why the apple came downwards and did not go in any other direction. A layman would say it is natural but Newton went a step further and came out with the famous gravitstional attraction formula which is the basis for all our calculations related to the gravity or gravitation. That is the scientific attitude some preople possess when they ask why and how and search the answers with a rational and logical mind. From that point of view Issac Newton was a great scientist.

## Reader poll

### Do you remember studying gravity in your school?

## Understanding gravity

The easiest example of gravity is we ourselves. Earth is such a large body and it attracts us to it and we all are glued to Earth. Like a powerful magnet which attracts a small nail of iron to it, we all are attached to Earth. This is the Earth's gravity which is doing this. That is the force that made that famous Newton's apple to fall straight in the lap of mother Earth.

## The gravitational field

Earth has its own gravitational field. Likewise other planets also have their gravitational field. Sun, the centre of our solar system is having its own field and that is why so many planets including Earth which are much smaller than Sun are rotating around it bound by that invisible gravitational force. Moon which is revolving around us is also having its own gravitational field. Moon is very small compared to Earth and is actually a natural satellite of it.

These bodies depending upon their mass have different gravitational fields. For example Sun having a very large mass is having a much greater gravitational field as compared to relatively smaller planets revolving around it.

## Acceleration due to gravity

In our earlier lesson on acceleration we had learned that rate of change of velocity is acceleration. Let us consider a free falling body on the Earth surface like a toy falling from rooftop to ground or a fruit falling from a tree to ground. Due to Earth's gravitational attraction force it gets an acceleration which is about 9.8 m/s^{2}. This value is irrespective of the mass of the body falling down. Readers should note here that the air in the atmosphere has a small drag on the falling objects like a thin paper piece would start floating in the air and reach the ground later in comparison to a steel ball. So, precisely speaking, only in a vaccum condition, the statement that same acceleration of gravity would be there for all objects, would hold true. In practice, we neglect the drag of air and proclaim this happening for the bodies falling down through the atmosphere.

It is interesting to note that though acceleration is same but force of gravitational attraction would be more for bodies having larger mass and in fact this force is nothing but is the weight of the body. To know the difference between mass and weight one can refer the earlier article in this series on mass and weight.

## Suface gravity

The acceleration due to gravity is also known as the suface gravity. Interestingly as different planets, Moon and Sun have different masses, their surface gravities are different. For example Moon being so smaller than Earth has a surface gravity only one sixth than that of Earth. On the other hand the Sun (which is in fact a star) has a surface gravity about 28 times than that of Earth. Surface gravity is generally represented as a multiple of Earth's gravity (g) which is 1g = 9.8 m/s^{2}. From this perspective the gravities of some of the planets are -

Mercury : 0.377 g

Venus : 0.904 g

Mars : 0.38 g

Saturn : 1.065 g

Uranus : 0.886 g

Neptune : 1.14 g

Pluto : 0.063 g

Accordingly, to obtain the weight of a body on these places, one has to multiply the mass of the body with the respective gravity value.

## The force of gravity

It is now clear that there is a gravitational force being imparted by one body on another. The question is how can we find the force between two bodies. Which are the factors on which it depends. Sir Issac Newton based on his intuition and observing the Earth-Moon relationship gave a law in the year 1687 based on two things. First is that the gravitational force between two bodies is proportional to their masses. More the mass more is the force. It was a simple assumption because we have already understood that gravity is due to the mass only. Another thing is distance between the two bodies. Here Newton proposed a inverse square relationship that force will reduce proportional to inverse of the square of the distance. This means that if distance increases five times the force would decrease to one twenty fifth of its value. Please note that square of five is twenty five.

Once we understand these two things then in scientific notations, we can write the force F, between two bodies having masses m_{1} and m_{2} respectively and situated at a distance of r from each other as -

F = G m_{1} m_{2 }/ r^{2}

Where G is the universal gravitational constant required to make the proportionality into an numerical equality. The units of these elements are -

F in Newton

m_{1 }and m_{2 }in kg

r in meter

One can also derive the unit of G by the same formula and it would be Newton meter^{2 }/ kg^{2}

One of the most amazing thing about the Newton's deduction of this equation is that it was experimentally verified later in the year 1798 by the scientist Henry Cavindish. The value of G was found to be 6.67 x 10^{-11 }Newton meter^{2 }/ kg^{2}.

## Verdict

### Do you find this lesson useful?

## Conclusion

Gravity is a natural entity and the various bodies in the universe are possessing this characteristic and following the law of gravitation. In nutshell we can conclude that the force of gravity is keeping the heavenly bodies bound with each other in this vast universe.

## References

1. https://spaceplace.nasa.gov/what-is-gravity/en/

2. https://en.m.wikipedia.org/wiki/Surface_gravity

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**

## Comments

interesting facts

Nicely depicted!!!

Memory refresher article. You have tried to imbibe the basics pertaining to the laws of gravity to a good extent!!

Keep it up!!

You remind me of high school....thanks

I recall the problem solving questions on this in high school. They were always so odd. I always liked chemistry more. It made more sense to me.

While we can describe gravity and its effects. I don't think we really understand it. After all, what CAUSES gravity? Mass does not cause gravity, even if the greater the mass, the greater the gravitational pull. I THINK (don't know) that some people are proposing a gravitational unit, rather like a photon but I don't know enough about that area.

I knew you would get to this after references to weight being established by gravity.

I would suggest that there are other types of gravity -- not so scientific. Like auras.

Gravity is such an interesting phenomenon. You explained it well.

Nice work!

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