Earth and Space: Planet Earth

In physics, it's crucial to distinguish between mass and weight. Mass is the amount of 'stuff' or matter an object is made of, and it is measured in kilograms (kg). Your mass is the same whether you are on Earth, the Moon, or floating in space. Weight, however, is a force. It is the force of gravity pulling on an object's mass. It is measured in newtons (N). Your weight depends on where you are. On a planet with stronger gravity, you would weigh more. The link between mass and weight is an object's gravitational field strength, represented by 'g'. On Earth, g is approximately 9.8 N/kg, which we often round to 10 N/kg for simpler calculations. This means every kilogram of mass is pulled down by a force of about 10 newtons.

Gravity is a fundamental, non-contact force of attraction that exists between any two objects with mass. A gravitational field is the region of space around a massive object where this force can be felt by another mass. We can visualize this invisible field using 'field lines'. For a spherical object like a planet, these lines point radially inwards towards the centre, showing the direction of the gravitational pull. The spacing of the lines indicates the field's strength: where the lines are closer together, the field is stronger (i.e., closer to the planet's surface). As you move further away from the planet, the lines spread out, showing that the gravitational field strength decreases.

The Earth is composed of several distinct layers. From the surface inwards, they are the crust, mantle, outer core, and inner core. The crust is the thin, solid, outermost layer we live on. It's broken into tectonic plates. Beneath it lies the mantle, the thickest layer, made of hot, dense, semi-molten rock that flows very slowly over geological time. Deeper still is the outer core, a layer of liquid iron and nickel. The movement of this liquid metal generates the Earth's magnetic field. At the very centre is the inner core, a solid ball of iron and nickel. It's the hottest part of the Earth, but the immense pressure from the layers above keeps it solid.

The atmosphere is the blanket of gases that surrounds our planet, held in place by gravity. It is essential for life. It is composed of approximately 78% nitrogen, 21% oxygen, and small amounts of argon, carbon dioxide, and other gases. The atmosphere serves several vital functions: it provides the oxygen needed for respiration; the ozone layer within the stratosphere absorbs the majority of the Sun's harmful ultraviolet (UV) radiation; it regulates the planet's temperature through the greenhouse effect, trapping heat to keep the surface warm; and it protects the surface from impacts by burning up most meteors and space debris through friction.

The Earth travels in a nearly circular path, called an orbit, around the Sun. The force that keeps the Earth in this path is gravity. The Sun's immense gravitational pull provides the required centripetal force, constantly pulling the Earth towards it, preventing it from flying off in a straight line. The same principle applies to satellites. The Moon is Earth's natural satellite, held in orbit by Earth's gravity. Humans have launched thousands of artificial satellites for purposes like communication (TV, phones), observation (weather forecasting, spying), and navigation (GPS). For a satellite to maintain a stable circular orbit at a certain height, it must travel at a precise speed. If it moves too slowly, gravity will pull it back to Earth. If it moves too fast, it will overcome Earth's gravity and fly off into space.