Newtons Universal Law of Gravitation Questions If the planets are orbiting the sun, what force is keeping them in orbit? What force keeps the moon in its orbit? Could the force of gravity be universal? Gravitation Every object with mass attracts every other object with mass. Newton realized that the force of attraction between two massive objects: Increases as the mass of the objects increases. Decreases as the distance between the objects increases. Newtons Law of Universal Gravitation Newtons big idea The force of gravity gets weaker as distance squared The moon is 60x further from the Earths center than objects on (near) the Earths surface The strength of Earths gravity near the Moon is (1/60) 2 t=1/3600 times weaker Gravity gets weaker as 1/dist 2 Very fast horizontal toss t = 0s t = 1s x= 8km 5m t = 2s x=16km t = 3s x=24km 20m 45m Orbital motion is free fall V = 4 km/sV = 6 km/sV = 8 km/sV = 10 km/s Circular Orbit!Eliptical Orbit Artificial satellite a v Moon-earth v a=v 2 /r