 Let's explore the universe of law of gravity. It says that any two masses, like say you and me, you and the sun, the sun and the earth, any two masses will attract each other, will pull on each other. And this force is called the force of gravity. And the universal law says that this force of gravity is equal to G times M1, which is the mass of the first object, times M2, the mass of the second object, divided by D square, where D is the distance between their centers. So what is G over here? G is called the universal constant, or the universal gravitational constant. And its value happens to be 6.67 times 10 to the power, minus 11 units. It's such a small number. And by the way, what is the unit of G? Well, to find the unit of G, what you do is you write G, isolate G from this equation. So that gives you F, D squared, divided by M1, M2. And so the unit of G now becomes the unit of force that is Newton's unit of distance, which is meters squared, and the unit of mass is kilograms, so kilograms times kilograms, kilograms squared. It's called a universal constant because the value is the same anywhere in the universe. And now if you're wondering, if everything attracts everything, why don't we feel this force? That's because of G, because this force of gravity is incredibly weak and therefore we don't feel it. And to give you some idea, let's plug in some numbers. So let's say the mass, my mass, or mass of the first person is 60 kilograms. Let's say the mass of the second person is also 60 kilograms. We'll just keep things simple. Let's assume that distance between them is about 60 meters. If you plug in these values now, what do we get? We get six times six seven times 10 to the power minus 11 times the mass of the first object, which is 60 kilograms, times mass of the second person, that is 60 kilograms, divided by D squared, D is 60, but you have to square it. And this cancels out, 60, 60, 60 squared. And notice what we left out was just this. So this will be the force, that many newtons. And look, that's such a small number. You have 10 to the power minus 11. And therefore, forces between everyday objects, you can't feel it. The gravitational force, you can't feel it. But what if you consider the gravitational force between huge objects, like the Earth and let's say you, our bodies. Now, the mass of the Earth is huge, six times 10 to the power 24 kilograms. And the distance between the Earth and me, remember you have to always consider the distance between the centers. So when you consider the distance from the center of the Earth to your center, since Earth is so big, that is basically the radius of the Earth. And now if you put in these numbers over here, now you'll find the force to be much bigger. And that is the force of gravity that we feel, which makes us stuck to Earth. A, and by the way, that is the same force which we call weight. Weight is basically the force of gravity. So we can now write down the formula for weight. Weight of any object is basically the force of gravity between Earth and that object. So that is going to be G times mass of the first object, which is the Earth times mass of the second object, which could be U, for example, your body, divided by the distance between the center square, which is basically just the radius squared. And if you can plug this in, you'll find what your weight is. But you'll do something else. We know that weight is MG, mass times G, where G is the acceleration due to gravity. We've studied this before. And so if I equate these two, look, M cancels out. And I end up with a formula for G. G happens to be capital G times M divided by R squared. This G is what we call the acceleration due to gravity. And if you plug in these numbers for G for Earth, you get the value to be 9.8 meters per second squared. That's where that 9.8 meters per seconds where it comes from. This value depends on the mass of the planet and the radius of the planet. So if you go to a different planet, you'll have different values for G. And by the way, you don't have to remember the formula for G. Look, it just comes from weight, which basically is coming from the universal law of gravity. And finally, you also understand the difference between the small G and the big G. I used to get confused here. Small G is the acceleration close to a planet, acceleration due to gravity. It depends on different planets. For different planets, you have different values. Big G is a universal constant, a very different number. Its value is the same everywhere in the universe. It has very different units altogether. So don't confuse between the two.