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?

Time for each orbit Radius Earth1.00 Mars Jupiter Saturn Important: The orbits on this slide are not shown to scale! (AU)(years) Mercury Venus P3 1.7 Planetary orbits What does this data show? Speed = dist R time T

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 Force of gravity on planet sun Earth R = 1 AU T = 1 Yr

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 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 sun Earth R = 1 AU T = 1 Yr

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 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 sun speed = dist (2πR) time (T) speed = 2 π1 1 AU/yr Earth R = 1 AU T = 1 Yr

P3 1.7 Planetary orbits The larger the orbit the slower the speed and the longer it takes Velocity of planet Force of gravity on planet sun 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 Force of gravity on planet sun The speed is less because gravity is weaker when further away. Saturn R = 9.53 AU T = 29.5 yr

P3 1.7 Planetary orbits Velocity of planet Force of gravity on planet sun Orbit time is longer because the planet has further to go and it is moving slower. The larger the orbit the slower the speed and the longer it takes Saturn R = 9.53 AU T = 29.5 yr

P3 1.7 Planetary orbits Velocity of planet Force of gravity on planet sun Orbit time is longer because the planet has further to go and it is moving slower. The larger the orbit the slower the speed and the longer it takes speed = dist time Saturn R = 9.53 AU T = 29.5 yr

P3 1.7 Planetary orbits Velocity of planet Force of gravity on planet sun Orbit time is longer because the planet has further to go and it is moving slower. The larger the orbit the slower the speed and the longer it takes speed = 2 π speed = dist time speed = 0.65 π AU/yr Saturn R = 9.53 AU T = 29.5 yr

P3 1.7 Planetary orbits Elliptical orbits: The squashed circle orbit has two ‘foci’ F 1 & F 2 Distances OF 1 + OF 2 = constant The sun is at one focus of the ellipse

P3 1.7 Planetary orbits Q

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.