Gravity and Its Effect on Planetary Orbits

1. Introduction:

  • Review the definition of gravity
  • Drop a ball and explain why it falls downward
  • Explain that the strength of a gravitational pull is determined by the masses of the objects involved and the distance between the objects
  • Hold up a scale model of Earth and a scale model of the Sun. Inquire as to which object would exert a stronger pull
  • 2. Materials:

  • Large Rubber or plastic sheet
  • Heavy round object, such as an 8-lb medicine ball or round watermelon
  • Smaller, lighter objects, such as tennis balls
  • 3. Activity:

  • Explain that the sheet represents spacetime, the large object represents the Sun and the smaller object represents a planet
  • Have the students pull the edges of the sheet outward. Make sure the sheet is taut.
  • Roll the lighter balls across the sheet. Note how they travel in a straight line.
  • drawing showing students holding sheet taut around rim while ball rolls straight across it

  • Place the heavy object in the center of the sheet. Keep the sheet pulled taut.
  • Roll the lighter balls across the sheet once again. Note how they now curve. This illustrates Kepler's First Law that states a planet orbits the Sun in an elliptical orbit (Ideally there would be no friction and the lighter balls would make a complete elliptical orbit.)
  • drawing showing students holding sheet taut around rim while heavy object in center causes path of balls to curve

    4. Closure:

  • Discuss how large massive objects such as the Sun curve spacetime toward them (i.e., the ball follows a curved path)
  • Discuss how smaller objects such as the ball, or planets, follow the curve of spacetime created by the more massive object
  • 5. Extensions:

  • Demonstrate or let the students demonstrate Kepler's Three Laws on an orbit simulator at
  • Demonstrate different types of orbit by manipulating data at


    Artwork by Aurore Simmonet


    This activity is part of the StarChild site.