 astrophysics and cosmology. These are the prime topics. Let's have a look at the learning objective for this video. In this video, we will discuss gravitational field, its formal definition. Later on, we will discuss gravitational field strength and the formula, how to calculate it. And finally, we will conclude this video with Newton's law of universal gravitation. To understand the concept of gravitational field, let's create a scenario consisting two stellar objects, Object 1 and Object 2. So the region of space surrounding a body in which another body experiences a force of attraction is called gravitational field. Let's make it more simpler and easier. So what is gravitational field? Remember, it's a region. So it means it's a space, a region, an imaginary place where an object with mass. So this field is actually depend on the object where the mass, heavy mass means more gravitational field. Experience a non-contact force. And the third and very important point that this force is a non-contact force. So a region where an object with mass experience a non-contact force is called a gravitational field. So how can we calculate the gravitational field strength? In order to calculate gravitational field strength, we can use Newton's second law of motion. That's very simple and easy. You know that the force is directly proportional to acceleration where mass is constant. In case of Earth, we will replace a with g, which is gravitational acceleration. So the equation will be f is equal to mg. Or when we substitute the equation, g is equal to f over m. It's very simple. So for the gravitational strength, we will use g is equal to f over m, where f is force and m is the mass of an object. Newton's law of gravitation. Let's take an example of Mars and Earth. They are very massive objects. And the Mars side is about half of the Earth, more than half of the Earth size. You can see the mass of the Earth and the mass of the Mars on the screen. So the distance between them is 225 millions kilometers. According to Newton's law of gravitation, the force of gravitation is directly proportional to the product of their masses and inversely proportional to the square of the distance. Before I understand this law completely, we need to look what do you understand by the term directly proportional and inversely proportional. So I made a two simple graphics here to explain the relationship of directly proportional and inversely proportional. You can see the top one will represent the directly proportional. It means when mass is increasing, the force is also increasing. This is called directly proportional. Whereas the bottom one diagram shows the relationship of inversely proportional. It is between the force and the distance. So distance is increasing, force is decreasing, and the vice versa. So this is the relationship we have to understand. So the equation of law of gravitation is here now. This is f of equal to g m1 m2 over r square. If you remember the last example, m1, let's say, is a mass of earth. m2 is a mass of Mars. And f is the gravitational force. r is showing the distance between them. And g is a gravitational constant, which is 6.673 multiplied by the power minus 11 Newton meter square per kilogram square. So this is now finally the Newton's law of gravitation. I just want to repeat that the force is directly proportional to the mass, and the product of the mass means more heavy objects have more gravitational force. And if the objects are far away with each other, the force is going to be reduced. If they are very close to each other, the force is really great in size. Okay, so at the end of the video, I would like to solve one numerical problem related to Newton's law of gravitation. Let's consider you are seated in your physics class next to another student, and the distance between you and him is 0.60 meter away. So let's estimate the magnitude of gravitational force between you and him. Assumed that each have a mass of 65 kilogram. So we will apply the same formula. Here is the solution. Like you can see this f is equal to g m1 m2 over r square. Here m1 and m2 both are same because both are 65 kilogram as assumed. And g is the constant value of 6.673 multiplied by minus 11 Newton meter square per kilogram square. And the distance between them is also given 0.60 and we will take the square value here. When we substitute the value and solve it, we'll get 7.8 multiplied by minus 7 Newton. And you can see the value is very small because the force is quite small, roughly the weight of one here on your head. This seems reasonable but you don't normally sense the attractive force. So I hope you understand my whole lesson and please repeat the video if you have any confusion and thank you very much for watching us listening. Take care. Bye bye.