The planets travel round the Sun in nearly circular paths. So they are constantly changing direction. A force called ‘gravity’ makes the planets turn. You will have felt the force of the Earth’s gravity – without it you would float up into space! Gravity is a force which pulls. All the planets exert their gravity. The more massive they are, the stronger is their pull of gravity. The Sun is so massive that its gravity is strong enough to keep all the planets turning around it. Without this force, they would each fly off in a straight line.
The Moon also has gravity. Because it is less massive than the Earth, its pull of gravity is weaker – as the astronauts that landed on the Moon found out!
Force
Like the planets, this ball will keep moving in a circle as long as it travels fast enough. The force that acts along the string pulls the ball and makes it change direction. Although a different kind of force, its effect can be compared to the gravity exerted by the Sun. if the child let go of the string, there would be no pulling force to keep the ball turning. It would fly away in a straight line.
An astronaut leaps from the Moon’s surface while saluting the US flag.