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P3 1.7 Planetary orbits Learning objectives Be able to explain that; 1. The force of gravity provides the centripetal force that keeps planets and satellites moving in their orbits. 2. To stay at a particular distance, a planet must move at a particular speed around the sun. 3. The larger the orbit the slower the speed and the longer it takes. If the Earth’s orbit were more elliptical, how would we be affected? P3 1.7 Planetary orbits Speed = dist time R T Radius Time for each orbit (AU) (years) Mercury 0.39 0.24 Venus Earth Mars 0.72 1.00 1.52 0.61 1.00 1.88 Jupiter Saturn 5.20 9.53 11.9 29.5 Important: The orbits on this slide are not shown to scale! What does this data show? P3 1.7 Planetary orbits To stay at a particular distance, a planet must move at a particular speed around the sun. Velocity of planet Earth R = 1 AU T = 1 Yr sun Force of gravity on planet P3 1.7 Planetary orbits To stay at a particular distance, a planet must move at a particular speed around the sun. Velocity of planet Earth R = 1 AU T = 1 Yr sun Force of gravity on planet If its speed is too slow, it will spiral into the sun If its speed is too high, it will move away from the sun P3 1.7 Planetary orbits To stay at a particular distance, a planet must move at a particular speed around the sun. Velocity of planet Earth R = 1 AU T = 1 Yr sun Force of gravity on planet speed = dist (2πR) time (T) speed = 2 π1 1 If its speed is too slow, it will spiral into the sun If its speed is too high, it will move away from the sun AU/yr P3 1.7 Planetary orbits The larger the orbit the slower the speed and the longer it takes Velocity of planet sun Force of gravity on planet Saturn R = 9.53 AU T = 29.5 yr P3 1.7 Planetary orbits The larger the orbit the slower the speed and the longer it takes Velocity of planet Saturn R = 9.53 AU T = 29.5 yr sun Force of gravity on planet The speed is less because gravity is weaker when further away. P3 1.7 Planetary orbits The larger the orbit the slower the speed and the longer it takes Orbit time is longer because the planet has further to go and it is moving slower. Velocity of planet sun Force of gravity on planet Saturn R = 9.53 AU T = 29.5 yr P3 1.7 Planetary orbits The larger the orbit the slower the speed and the longer it takes Orbit time is longer because the planet has further to go and it is moving slower. Velocity of planet Saturn R = 9.53 AU T = 29.5 yr sun Force of gravity on planet speed = dist time P3 1.7 Planetary orbits The larger the orbit the slower the speed and the longer it takes Orbit time is longer because the planet has further to go and it is moving slower. Velocity of planet Saturn R = 9.53 AU T = 29.5 yr sun Force of gravity on planet speed = dist time speed = 2 π 9.59 29.5 speed = 0.65 π AU/yr P3 1.7 Planetary orbits Elliptical orbits: The squashed circle orbit has two ‘foci’ F1 & F2 Distances OF1 + OF2 = constant The sun is at one focus of the ellipse P3 1.7 Planetary orbits Q P3 1.7 Planetary orbits Learning objectives Be able to explain that; 1. The force of gravity provides the centripetal force that keeps planets and satellites moving in their orbits. 2. To stay at a particular distance, a planet must move at a particular speed around the sun. 3. The larger the orbit the slower the speed and the longer it takes.
