1. © Oxford University Press 2009 12.3 Circles in space The planets follow circular paths around the Sun. The centripetal force needed to keep the planets in orbit is provided by gravitational attraction.

2. © Oxford University Press 2009 Centripetal force The gravitational attraction is greatest between the Sun and planets closest to it. The centripetal force is largest on the planets closest to the Sun. These planets travel fastest and have the smallest orbit times. 12.3 Circles in space

3. © Oxford University Press 2009 The centripetal force needed to keep the Moon in orbit around Earth is provided by gravitational attraction between the Earth and the Moon. 12.3 Circles in space

4. © Oxford University Press 2009 Back to the rubber bung What happened when you whirled a bigger or smaller bung? The bigger the mass the bigger the tension in the string. This meant the centripetal force was bigger. 12.3 Circles in space

5. © Oxford University Press 2009 In space… The bigger the mass of a planet the stronger the centripetal force on it as it orbits the Sun. But… The farther away the planet is from the Sun the weaker the gravitational attraction of the Sun. 12.3 Circles in space

6. © Oxford University Press 2009 Artificial satellites There are many artificial satellites in orbit around Earth. They, like the Moon, stay in orbit because there is a gravitational force of attraction between the satellite and the Earth. This force provides the centripetal force needed for circular motion. How does the size of this force depend on: their mass? their distance from Earth? 12.3 Circles in space