Nora tied a string around a tennis ball, and then she swung it in a circle in front of her to demonstrate a planet orbiting the sun. she explained that her hand represented the sun, the ball represented a planet, and the string kept the ball from leaving the "orbit" around her hand. why doesn't the string work in the same way as gravity? a. the string holding the ball in orbit is a contact force; gravity is a noncontact force. b. the string only pulls one way; gravity pulls both ways. c. the string keeps the distance between the bodies the same; gravity makes the distance vary. d. there is no difference between the two systems.