Chapter 2
Understanding Gravity
You are always acted upon by a force called gravity. Gravitation is a universal force and every object in the universe attracts every other body with a force proportional to the product of their masses. The force falls off as the distance increases. How do you know that you are under earth’s gravitational pull? Even though you cannot directly see gravity, you can still feel it. Jump up and you will get down. You should not try to fly because you cannot. Gravity pulls you down.
But nature has some boundaries. Earth has a limited gravity. We have learned to overcome gravitational pull and there is a special speed with which if we travel, we could fly into space. If you throw a stone straight up, gravity will eventually bring it down. But if you have enough power to throw this stone at a special speed, and let us assume that the stone continues to travel at the same speed, the stone would fly out into space. This special speed is called escape speed, because it can be used to escape earth – its value is 11.2 km / s directed upward.
We are here to understand gravity. What is gravity? How did we get to know about this force and how did we understand its behavior? It turns out that there are 5 heroes of gravity. They are Copernicus, Galileo, Kepler, Newton and Einstein. They have led to our understanding of the universe as well as gravity. So let us begin the story of gravity.
It was Nicholas Copernicus, a Polish priest who predicted that sun was the centre and not the earth. Before him, everybody had believed on the geo-centric model of solar system, i.e. earth was the centre of the universe. This earth-centred model was supported and firmly established by Aristotle and Ptolemy. Thus, it was a daring move to challenge the old theory. Copernicus was punished for his bold proposition.
Copernicus was unlucky because he did not have enough data to explain what he wanted to say. The next hero was Johannes Kepler. He was a German astronomer and mathematician who was keenly interested in geometry and wanted to use geometry to explain celestial motion. He used Tycho Brahe’s data – another astronomer of his day and deduced three laws of planetary motion which are called Kepler’s laws of planetary motion. Kepler had always supported Copernican theory – every object revolved around the sun. The most important law, however, was that of the shape of the orbits. Copernicus had used circular orbits in his theoretical model but when matched with Brahe’s data, only elliptical orbits fit the best. An ellipse is an elongated circle. A circle has a unique centre – on the other hand, an ellipse has two foci. You can think that circle is an ellipse in which distance between the two foci is 0, while an ellipse is an elongated circle in which the two foci are separated by some distance. The figure below shows you an ellipse and a circle.
It was Galileo who was the first to realise the existence of a mutual force between heavenly bodies because he found tiny bodies circling the planet Jupiter. He therefore modified the Copernican theory by proposing that not every object had to directly revolve around the sun. Galileo also confirmed the Copernican theory because he had proof. He had a special instrument called telescope with which he became the first man to peep into space. He discovered Saturn’s rings and was astonished at the beauty of Saturn. Galileo paved the way for Newton to realise the existence of gravity.
Isaac Newton was an extraordinary kid. He was curious about nature, about plants and animals. He soon realised his interest in mathematics and physics. This story is very interesting. He was once sitting under a tree and was accompanied by an apple fall. An apple fell and a quick question popped up: Why did the apple fall down? Why didn’t it go up? These questions haunted Newton for several years until he came up with an answer. As he fought for his answers, he came up with a new branch of mathematics called calculus. He used his imagination and his knowledge of mathematics to unlock a secret force – the force of gravity. Gravity was born and Newton explained that every object attracted every other within the universe with a mutual force.
We have thus studied about the four heroes so far. One more is left. But before that, we must encounter special and amazing characteristics of gravity. So let us first examine the fall of an apple. When an apple falls, it is pulled by the earth. This force is F, it just so happens that the apple also pulls the earth with the same force F but directed oppositely. Thus, earth pulls the apple down, and the apple pulls the earth upward – very much in agreement with Newton’s third law of motion. We see only the apple moving and not the earth because we know that the same force is acting on the tiny apple and the same force is acting on the massive earth. If you try to move a bicycle with a force F and you use the same force to move a mountain, you would hardly succeed. In our example, we have an apple, which is less massive than a bicycle and we have earth, more massive than any mountain. So we won’t see the earth moving, we would see the apple fall down. This is obvious but now you also know why it should be obvious.
The second thing that we must know about is weight. Mass and weight are two different quantities. Your weight is the direct result of earth’s gravitation. If you were on the moon, you would weigh very less than what you weigh on earth but if you were on Jupiter, you would weigh more than the double than what you weigh on earth. Thus, weight is always perceived and it changes from place to place.
We must also know about rockets which are able to escape earth’s gravity. In the previous chapter, we looked at Newton’s three laws of motion. Actually, rockets work on Newton’s third law – the law of action and reaction. A rocket uses a mixture of gases as its fuel. These gases are highly compressed, i.e. they are under enormous pressure. When there is takeoff, all these pressurized gases escape the nozzle. They apply an enormous force on the launch pad, i.e. on earth and in return earth pushed the rocket upward. This is an excellent example of Newton’s laws and this allows it to escape earth’s gravity. Not every rocket can escape earth – only the powerful ones are capable to do so.
The last hero of gravity is Albert Einstein who managed to explain how gravity actually worked. He explained that the universe was composed of a special fabric. Let us suppose this to be made of rubber. A trampoline is a good example. If you put a massive object on this fabric, there is a depression. So if you stand on a trampoline, your feet produce a depression on the rubber sheet. Einstein stated that this was the case with masses in space. Planets, stars and all other bodies produce depression in the fabric of the universe. This depression would be greater if the mass would increase, thus, sun would produce the greatest depression in our solar system. So you stand on a trampoline, in our analogy, you are the sun. Now I come around, I take out a small iron ball from my pocket, just about the size of a tennis ball – but it is made of iron – it is very dense. I put it very carefully around your feet. What will happen? Clearly, this ball will try to fall toward your feet. In doing so, it will revolve around you. This is the truth – Einstein stated that all bodies in our solar system fell into the sun’s depression. All that we have discussed about Einstein’s theory has been carefully shown in the diagram below. It shows a planet creating its depression. You can now see where gravitation comes from – it comes directly from the curved depressions created by the masses. Thus, we have understood the five heroes of gravity and their stories.
Here, earth is creating a depression on the fabric of cosmos
Gravitation as we know it, is a force. If you want to know more about the stories of Galileo, Newton and much more about how Einstein corrected Newtonian theory of gravity, you should try this book below. It is called Gravitation: A crucial breakthrough in Physics – available on Amazon
Revision 1
1. What is motion? Give examples of a moving body. Also give examples of a body which is at rest. Always consider relativity of motion.
2. State and explain the three laws of motion as given by Newton.
3. Take a pencil and a paper. Hold them at the same height in your hands and then drop them simultaneously. What do you observe? What hits the floor before? Now crumble the paper and make it like a ball. Repeat the experiment. What do you observe this time?
4. A man is trying to push a car with a force of 1000 Newton but the car is still not moving.
5. Eventually, as he continues to push, the car finally moved. He noted that the force he had to apply to move the car was 1500 Newton. What was stopping the car?
6. A box is kept on a table. How many forces are acting on this box? If the box is at rest, what can you say about these forces?
7. Take a balloon and blow in air. Make it big. You have now filled air inside – the molecules of air are now moving around randomly. Now leave it to fly. The balloon will fly around. Why do you think it flies?
8. A tennis ball and a leather ball (both of the same size) are dropped from the same height on a muddy ground. Which would create a deeper depression on this ground?
9. Now you drop a tennis ball from height 20 m and drop a leather ball from 10m at the same time. They are of the same size. Which would create a greater depression on the muddy ground?
10. Why do planets revolve around the sun? Which force exists between all bodies?
11. Earth’s gravitational acceleration is close to 10 m/s2 and your mass is 30 kg. What will be your weight on earth? If moon’s gravity is only 1/6th times that of earth, what will be your weight on moon?
12. Who were the five heroes of gravity? Who is your favourite? Why?
13. Write different things that you see so that you can be sure that gravity exists.
14. A bus and a car are moving side by side. Both are moving at the same speed. You stand on earth, do you measure their speeds to be zero?
15. You get on the bus. The driver of the car decided to overtake the bus. He drives the car at twice the speed as that of the bus. What speed do you see?