 channel for physics. Please subscribe my channel. Hello and welcome back to another episode of physics partner. Today's video is the third part of the space series newly injected in IGCSA physics curriculum for the code 0625 and 0972. In today's video, we will study about inner and outer planets of the solar system, gravitational field strength and factors depending on. Let's first discuss what are inner and outer planets? In their planets are the four closest planets to the sun. They are also called the terrestrial planets because they are made mostly of rocks and metals. Whereas the four farthest planets from the sun, they are also called gas giants because they are made mostly of gases. This difference is referred as accretion model for solar system formation. In planetary science, accretion is the process in which solids agglomerate means piled up to form larger and larger objects and eventually planets are produced. This model is the most commonly accepted mechanism. In this model, a rocky core forms through the concentration of a small planetary until it is sufficiently massive to accrete a gaseous envelope. This accretion is the certain result of gravitational forces operating on all scales on all types of material such as gas, dust, plasma, even dark matter. The region between the stars contain interstellar clouds composed primarily of dust and gas. Over 100 molecules, which include neutrals, ions and radicals, mostly carbon-containing compounds have been identified in interstellar molecules clouds. The presence of many elements in interstellar clouds of gas and dust has been confirmed. The rotation of material in the cloud creates the formation of an accretion disk. Accretion disks are flattened astronomical objects made of rapidly rotating gas, which slowly spirals onto a central gravitating body. The accretion disk forms when diffused material is attracted to a massive central body like a black hole. Let's move to the second part of the video, the gravitational field strength. So, what do you think the gravitational field strength is? The gravitational field strength at the point described how strong or weak a gravitational force is at that point. At the surface of the planet, the gravitational field strength depends on the mass of the planet. If the mass is greater, the gravitational field is also greater, whereas the gravitational field strength is weaker with the increasing distance. It means more distance means less gravitational field. The gravitational field strength phenomenon is also explained by the Newton's law of gravitation. For more details on this, you can refer my video on the Newton's law of gravitation. Don't worry, I will share the link in the description. Let's discuss how to calculate the time for light to travel a significant distance such as between objects in the solar system. Let's see that how long it takes to reach the light from sun to earth. You can see a simple calculation on your screen as well. The sun weighs about 333,000 times as much as earth. It is so large that about 1,300,000 planet earth can fit inside of it. Because of the heavy mass of the sun, sun contains most of the mass of the solar system and this explains why the planet orbit the sun. Anyway, the basic reason why the planets revolve around or orbit the sun is that the gravity of the sun. The gravity of the sun keeps them in their orbits. This gravity is because of heavy mass of the sun as compared to other planets. Just as the moon orbits the earth because the pull of earth's gravity, the earth orbits the sun because of the pull of the sun's gravity. A planet's orbit speed changes depending on how far it is from the sun. The closer a planet is to the sun, the stronger the sun gravitational pull on it. As we already discussed before, planetary orbits are an excellent sample of the principle of energy conservation. Because over time, a mass in an elliptical orbit converts energy from potential to kinetic and the reverse while maintaining the constant total energy. Like the case of circular motion, the total amount of mechanical energy of a satellite in elliptical motion also remain constant.