GRAVITY
The force of gravity is a force of attraction that exists between all objects with mass, from microscopic atoms to stars and planets. On Earth, gravity can be seen when objects fall to the ground, pulled by an invisible force. In space, the force of gravity keeps the Moon in its orbit around Earth, keeps planets in orbit round the stars, and holds huge clusters of stars together as galaxies.
NEWTON’S DISCOVERY An apple falling from a tree is said to have inspired English scientist Isaac Newton (1642-1727) to explore the force of gravity. He developed a theory stating that every mass attracts every other mass by a force between both masses. The more massive an object is and the nearer it is, the greater its gravitational attraction.
BIRTH OF A STAR A star is born inside a cloud of dust and gas in space called a nebula. The dust and gas begin to clump together, forming a core. The clump’s gravitational attraction increases as its mass increases, dragging in more and more matter. The centre, or core, becomes so massive and dense that nuclear fusion begins, and the star begins to shine.
MOON AND TIDES As the Moon orbits Earth, its gravity tugs at the water in the oceans, making a mass of water bulge towards the Moon. The force of Earth’s spin creates a matching bulge on the other side of Earth. These bulges cause the regular rise and fall of the water level at the sea’s edge that we know as tides.
MASS AND WEIGHT These are not the same. An object’s mass is the amount of matter it contains. An object’s weight is the force exerted on its mass by gravity. This means that on the Moon, where gravity is just one-sixth of Earth’s gravity, an astronaut will weigh one-sixth as much as he weighs on Earth, although his mass is the same.
ZERO GRAVITY In orbit around Earth, astronauts become weightless and float about their spacecraft as if gravity did not exist. In fact, gravity is still pulling the astronauts and their spacecraft towards Earth, but as the spacecraft travels forwards, it is also continually “falling” as it follows the curve of Earth. The craft and astronauts are in a state of free-fall, but falling without ever reaching the ground.
CENTRE OF GRAVITY An object’s centre of gravity is the point at which it balances. An object with a low centre of gravity is more stable — so a sports car is more stable than a double-decker bus. The secret to driving a car on two wheels is to ensure that the centre of gravity remains above the wheels – any further over and the car will tip over.
EINSTEIN’S THEORY German-born scientist Albert Einstein (1879-1955) developed a theory of relativity to explain how gravity works in space. He compared space and time to a sheet of stretchy rubber, which everything in the Universe rests on. Massive objects like stars make a big dip in the rubber. Less massive objects like planets fall into these large dips and so are trapped orbiting stars. The dips create the effect we call gravity.
AIR RESISTANCE In a vacuum, gravity causes everything to fall at the same speed. However, if an apple and a feather are dropped from the same height in Earth’s atmosphere, the apple will fall faster. As they fall, objects are slowed down by air resistance, created by friction between the air and the object. The speed an object falls depends on the balance between gravity’s pull and the air’s resistance.
BLACK HOLES When a massive star dies, its core may collapse. As it shrinks, the core becomes ever denser and forms a region of space called a black hole. The force of gravity in a black hole is so strong that anything entering it is swallowed up, including light. Although invisible, black holes can be identified by the effect their gravity has on everything around them. Material being sucked into the hole heats up, emitting X-rays that can be detected by X-ray telescopes.
Picture Credit : Google
