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?
Responses:
The string only pulls one way; gravity pulls both ways.
The string holding the ball in orbit is a contact force; gravity is a noncontact force.
The string keeps the distance between the bodies the same; gravity makes the distance vary.
There is no difference between the two systems.