Apart from any acceleration that might occur in the direction of the path, the lower panel of the image above indicates the forces on the ball. There are two forces; one is the force of gravity vertically downward through the center of mass of the ball mg, where m is the mass of the ball and g is the gravitational acceleration; the second is the upward normal force exerted by the road at a right angle to the road surface man. The centripetal force demanded by the curved motion is also shown above. This centripetal force is not a third force applied to the ball, but rather must be provided by the net force on the ball resulting from vector addition of the normal force and the force of gravity. The resultant or net force on the ball found by vector addition of the normal force exerted by the road and vertical force due to gravity must equal the centripetal force dictated by the need to travel a circular path. The curved motion is maintained so long as this net force provides the centripetal force requisite to the motion.
Launched as GRAIL A and GRAIL B in September 2011, the probes were renamed Ebb and Flow by schoolchildren in Montana. The probes operated in almost circular orbit near the lunar poles at an altitude of approximately 34 miles, until their mission came to an end in December 2012. The distance between the twin probes altered a bit as they soared over areas of lesser and greater gravity that were caused by visible topological features on the Moon's surface, such as impact craters and mountains--as well as by masses that were secreted beneath the lunar surface. But small moons like Methone are usually geologically inactive and bereft of an atmosphere. Therefore, they are usually unable to smooth away the scars. Dr. Peter Thomas of Cornell University in Ithaca, New York, explained it this way in the May 17, 2013 New Scientist: "When we look at objects less than 200 kilometers in radius, they are all like potatoes. They have lumps, grooves, craters." This makes Methone's smooth surface a mystery. Dr. Thomas is a Cassini team member. Saturn, the smaller of the two gas-giant planets inhabiting our Solar System, is an enchanting world. It is dwarfed only by Jupiter, the larger gas-giant planet, and it is probably the most beautiful planet in our Solar System. Magical and mysterious, Saturn's lovely rings and tumbling moonlets of ice, evoke wonder in the eye of the beholder.