Gravity

When the Earth spins on its axis, people on the equator hurtle round at about 1,000 mph (1,600 kph), but they don't fly off into the sky. Gravity is the force that pins us to the planet, and it also keeps the stars spinning endlessly in space. Gravity is the pull of every object on everything else and it holds the Universe together like a giant, invisible spiderweb.

Earth's gravitational pull is lower the further you go from the center of the planet, so you weigh slightly less at the top of a mountain than you do down a mine. But, no matter how high up you go, even if you take a rocket to the stars, there is no escape from gravity altogether because it extends to an infinite distance.

The only way to fight gravity is to balance it with another force. Planes and helicopters fight gravity by using lift to shoot into the sky. Not everyone wants to fight gravity, however. Skydivers embrace gravity by jumping out of planes and using the force to hurtle at great speeds toward the ground.

Force of attraction

Gravity is a force of attraction - always a pull and never a push. This is different from magnetism and other forces, which can either pull (attract) or push (repel). The gravitational pull between two things depends on their mass and the distance between them. The bigger the mass and shorter the distance, the greater the pull.

Falling apple: Gravity makes an apple and the Earth pull each other with the same force. The apple falls to the ground because the Earth has a greater mass and accelerates far less than the apple.

Mass and weight

The words mass and weight are often used to mean the same thing, but they are different. Mass is the amount of matter an object has, and weight is the force pulling on matter because of gravity. Your mass is always the same, but your weight varies from place to place because gravity varies across Earth.

Earth versus Moon: People weigh about a sixth as much on the Moon as they do on Earth. Astronauts can leap further on the Moon because the gravitational force is much weaker there than on Earth.

Pull of the tides

The Moon is smaller and lighter than Earth, and about 240,000 miles (384,000 km) away. Even so, it is big enough to pull on the Earth's oceans as it spins around our planet, and this is what causes tides.

The Sun, which is further away, also affects the tides. Twice a month, when the Sun and Moon line up, we get higher and lower tides than usual because of their combined pull.

Neap tide: When the Sun and Moon are at right angles, they pull in different directions, canceling each other’s effects, so both the high and low tides are smaller than usual.

Spring tide: When the Sun and Moon line up, their gravitational pulls add together. This makes high tides higher, and low tides lower, than usual.