P1.2 Space · KS3 Science

Gravity, mass and weight

Gravity is a non-contact force that pulls masses towards each other. Bigger masses have stronger gravity, and gravity gets weaker further away.
Mass is the amount of matter in an object, measured in kilograms (kg). It is the same everywhere.
Weight is the force of gravity on an object, measured in newtons (N): weight = mass × gravitational field strength.
On Earth, the gravitational field strength is about 10 N/kg; on the Moon it is 1.6 N/kg. A 50 kg person weighs 500 N on Earth but only 80 N on the Moon.

The Sun is a star at the centre of our Solar System. Eight planets orbit it: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The inner four are rocky; the outer four are gas and ice giants.
Dwarf planets (such as Pluto), asteroids and comets also orbit the Sun. Moons orbit planets.
The Sun is one of billions of stars in the Milky Way galaxy, and there are billions of galaxies in the universe.
A light year is the distance light travels in one year. It is used to measure the huge distances to stars.

Gravity keeps the planets in orbit around the Sun and the Moon in orbit around the Earth.
A satellite is an object that orbits a planet. The Moon is a natural satellite. Artificial satellites are used for communication, satellite navigation (GPS), weather forecasting and observing the Earth.

A day is the time for the Earth to spin once on its axis (24 hours). A year is the time for the Earth to orbit the Sun (about 365¼ days).
Seasons are caused by the tilt of the Earth's axis. In the UK summer, the northern hemisphere is tilted towards the Sun, so days are longer and the Sun is higher in the sky, making its rays more concentrated. They are not caused by the Earth being closer to the Sun.
The Moon doesn't give out light; we see it because it reflects sunlight.
In a solar eclipse, the Moon passes between the Sun and the Earth, and its shadow falls on the Earth. In a lunar eclipse, the Earth is between the Sun and the Moon, and the Earth's shadow falls on the Moon.

Gravity: A force of attraction between masses.
Mass: The amount of matter in an object, measured in kilograms.
Weight: The force of gravity on an object, measured in newtons.
Gravitational field strength: The force of gravity on each kilogram, in N/kg.
Orbit: The path of an object around a star, planet or moon.
Satellite: An object that orbits a planet.
Star: A huge ball of gas that gives out light, such as the Sun.
Planet: A large object that orbits a star.
Galaxy: A huge group of billions of stars.
Light year: The distance light travels in one year.
Axis: The imaginary line the Earth spins around.
Solar eclipse: When the Moon blocks the Sun's light from reaching part of the Earth.
Lunar eclipse: When the Earth's shadow falls on the Moon.