Copyright © 2007 Dorling Kindersley
Gravity is the force that makes things fall to the ground on Earth and holds the planets in their orbits (paths) around the Sun. The force of gravity acts over immense distances between objects in the Universe and holds them all together. The gravitational force between objects increases with their MASS. It also increases the closer they are. The gravity between objects on Earth is usually too small to notice.
On Earth, objects have a point, often near their centre, which is called their centre of gravity. The lower it is, the more stable they are. Cars are designed with their heavy engines near to the ground, to keep their centre of gravity low. This means they can corner at speed without tipping over.
Which falls faster, a ball or a feather? In Earth’s atmosphere, the ball reaches the ground first because air resistance slows the feather down. In a vacuum, there is no air and therefore no air resistance. The feather and the pool ball fall at the same rate because gravity pulls them with exactly the same amount of force.
The mass of an object is the amount of matter it contains. The greater the mass of an object, the more matter it contains, and the more it pulls on other objects with the force of gravity. The mass of an object does not vary unless the amount of matter inside it changes for some reason. Mass is measured in kilograms (kg).