Universal Law of Gravitation — Definition
Definition
Imagine you have two objects, say, a cricket ball and a football. Newton's Universal Law of Gravitation tells us that these two objects, simply by virtue of having mass, will attract each other. This attraction is what we call the gravitational force. It's not just limited to balls; it applies to everything that has mass – from tiny atoms to massive planets and stars. The law states two crucial things about this force:
Firstly, the strength of this gravitational pull depends on how much 'stuff' (mass) each object has. If you have two very heavy objects, the gravitational force between them will be stronger than if you have two very light objects. Specifically, the force is directly proportional to the product of their masses. So, if you double the mass of one object, the force doubles. If you double the mass of both objects, the force becomes four times stronger.
Secondly, the strength of this force also depends on how far apart the objects are. The closer they are, the stronger the pull. But here's the interesting part: it's not just a simple inverse relationship.
The force is inversely proportional to the *square* of the distance between their centers. This means if you double the distance between the objects, the force doesn't just halve; it becomes one-fourth ().
If you triple the distance, the force becomes one-ninth (). This 'inverse square law' is a very common and important pattern in physics, appearing in other fundamental forces as well.
Finally, there's a constant involved, 'G', known as the Universal Gravitational Constant. This 'G' is a tiny number, which tells us that gravity is a very weak force compared to other forces like the electromagnetic force.
That's why you don't feel the gravitational pull between yourself and your friend, but you certainly feel the Earth's gravity pulling you down. The Earth is incredibly massive, making its gravitational pull significant despite 'G' being small.
This law is universal because it applies everywhere in the universe, from apples falling on Earth to planets orbiting the Sun.