Force
We observe some surprising and interesting events in our daily life. For instance, when a body is released from a certain height, it always falls towards the surface of the earth. The fruits from a tree fall towards the earth after they are ripened. Why do any objects or ripened fruits fall towards the earth? Why do they not move upwards? In the same way, planets revolve around the sun in their own oval orbits. The satellites of the planets also revolve around them in their own oval orbits like the planets round the sun. Why do they not go away?
We usually take the events mentioned above as simple ones and most of them go unnoticed. But there are some scientific reasons to cause these events. In this lesson, we are going to discuss gravitation, Newton's universal law of gravitation, gravity, acceleration due to gravity, etc.
Gravitation
Before 1542 AD, the earth was considered to
Moon be the center of the solar system. Other heavenly
bodies like the sun, the moon, the planets and the
satellites all were also regarded to revolve around the
Earth in with a uniform circular motion. This earth centered model of the solar system is called geocentric or ptolemaic model. In the 16" century,Nicholas
Copernicus, an astronomer put forward the heliocentric model about the solar system.
In this heliocentric model the sun is considered to be the center of the solar system and the other heavenly bodies such as planets and satellites are believed to revolve in the sun in their own orbits. Late Newton (1642-1727) tried to think and find out the reasons why planets revolve around the sun and began to investigate. In 1687 AD, Newton found out that the earth attracts every object towards its centre. So, when a body is released from height that al wards the earth because of the force of attraction of the earth. He also found the earth but also every object in this universe attracts other bodies towards them. He called this force of attraction between any two bodies in the universe the force of gravitation or simply gravitation. In this way, Newton propounded the law of gravitation. On this basis, the force of gravitation could be measured
According to Newton, the mutual force of attraction between any two bodies in the universe is called gravitation. It is also called gravitational force. For example, the earth tracts the moon and the moon also acts the earth. Similarly, the earth attracts the Jupiter and the Jupiter attracts the earth. The effect of force of attraction of a body with big mass on a body of small can be observed. But the effect of a body with small mass on a body with big one cannot be observed. Thus, all the planets revolve around the sun. The sun is so large in size that even the sum of the masses of all the planets and satellites is only about 0.0015 port of the sun. The effect of the gravitational force can be observed more on liquid than on solid Thus, the tides in seas and oceans are due to the gravitation of the sun and the moon.
Newton's universal law of gravitation
The universal law of gravitation between two masses was formulated by Newton in 1687 AD. Now, it is known as Newton's universal law of gravitation Newton's law of gravitation is called universal law because this law holds truth for all the bodies, whether microscopic or massive placed everywhere in the universe.
Newton's universal law of gravitation states that every object in the universe attracts each other with a force called gravitation which is directly proportional to the product of the masses and inversely proportional to the square of the distance between the centres. The direction of force is along the line joining the centre of two masses.
Gravity
We know that the earth is spherical like an orange. It may be a great surprise to think that how people can stay on the spherical earth. Nepal and America are located just nearly opposite on the surface of the earth. A person living in Nepal will have their head directed towards the sky while standing on their feet on the surface. Likewise, a person living in America, just opposite to Nepal, will have their head faced towards the sky too while standing on their feet on the surface. The reason behind this effect is the gravity of the earth. It pulls both of them towards the center of the earth.
If, out of the two involved bodies, in Newton's law of gravitation, one body is the earth (or planets/satellite) then the gravitational force is called force of gravity or simply gravity. Thus, gravity is merely a special case of gravitation in which there is involvement of a heavenly body. For example, the gravitational force between the earth and a body near or on its surface is called the earth's gravity. Similarly, the attraction of Jupiter and a body near or on its surface is called Jupiter's gravity.
Thus, the force of attraction with which the earth pulls a body towards its center is called gravity of the earth or simply gravity. The force of gravity on a body is its weight. The factors that affect gravity are mass and radius of the planets and satellites.
Free Fall
When a body is falling freely towards the centre of the earth under the influence of the gravitational pull of the earth only, the motion of the body is called free fall. In other words, if a body is falling with acceleration equal to the acceleration due to gravity (i.e. a = g), thus the body is said to have a free fall. In fact, a body falling in vacuum or in space is free fall. One of the examples of free fall could be a body falling on the surface of the moon. This is because the moon has no atmosphere around it. The acceleration of the body in the free fall is equal to the acceleration due to gravity 'g'. On the earth, there is no prefect free fall of any falling body due to air resistance. Free fall is a type of motion on a body which is moving under the influence of gravity only. In this condition, the body is in weightlessness. This is because the body feels zero reaction force.
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