Attractive gravity

Gravity or gravitational force is a force with which every object attracts every other object around it.

The essential words here are that “every” object attracts “every other” object with a force. That means, even a table, chair, tree, human being have gravitational force. Then why aren’t they felt. By definition, the gravitational force is proportional to the mass of the two concerned objects (F = GM1*M2/ R*R). Most of the objects do not have sufficient mass to be able to attract each other. The difference comes in when one or both the concerned objects are of planetary sizes or bigger. As in such cases, the mass of the objects attracting is huge leading to strong gravitational pull between the two.

Hence, we are attracted by the gravity of the earth. This is also a flawed statement. It is actually that the earth is also attracted by us and so are we to the earth with a common force which is the gravitational force. The only difference is that the acceleration of earth due to our gravitational pull is insignificant for the earth considering its mass, leading to almost zero displacement and acceleration whereas it is very significant for us and other objects similar to us leading to huge displacement and acceleration.

On similar lines, the earth is attracted by the gravity of the sun and sun is attracted by some other star and so on. Thus every interstellar object is attracted by every other object which leads to relative motion between them.

Now let us explore how gravitational force affects the launch of a space ship and also the orbit of it. In order to enable that, lets first delve into the understanding of how artificial and natural satellites are continuously orbiting the planet they orbit.

When a space shuttle is launched, it accelerates away from the earth and reaches a point at which it is supposed to leave the satellite into the orbit. When the satellite is launched in this orbit, it experiences a strong gravitational pull from the planet (Earth). Now to ensure that the satellite doesn’t fall on the earth and stays in the orbit, another force needs to be active which is equal and opposite to the force of gravity. To ensure this, the satellite is launched into the orbit with a velocity and starts moving in a circular motion. Due to its circular motion, it experiences a force called the centrifugal force which tends to push it away from the earth. This is the force which tends to balance the gravitational force. To ensure that the force is exactly equal, the satellite must orbit at a velocity called the critical velocity. If the velocity is lesser, it will soon be attracted to the earth. If it is greater, it will move away. If it exceeds a threshold called, escape velocity, it will escape the earth’s gravity and move into space to be attracted by the gravity of some other planet/ star. As this force is pretty weak, it generally tends to drift slowly in space with very little motion.

So the next time when you see a star, feel the force of attraction from it and feel how earth’s gravitational force (which is more powerful than that of the star) helps you on holding on to your ground!