Which model component illustrates the effect of gravity in the following case?
A model of the earth going around the sun is made from a ball (the earth) attached to a string attached to a hollow tube (representing the sun).  Holding the center of the handle, she swings the ball in a circle to show how a planet orbits a star.  She knows that the orbit of a planet depends upon the gravitational pull of the star.  

Gravity is the force by which a planet or other body draws objects toward its center. The force of gravity keeps all of the planets in orbit around the sun. Gravity is very important to us. We could not live on Earth without it. The sun's gravity keeps Earth in orbit around it, keeping us at a comfortable distance to enjoy the sun's light and warmth. It holds down our atmosphere and the air we need to breath.

Gravity is what holds our world together. Gravity is what holds the planets in orbit around the sun and what keeps the moon in orbit around Earth. The gravitational pull of the moon pulls the seas towards it, causing the ocean tides. Gravity creates stars and planets by pulling together the material from which they are made.