Physlet Illustration: Energy of Orbital Motion
 

 

 

Interactive Help
on      off
A planet (mass 1025 kg) orbits a star (mass 2 x 1030 kg) under the influence of gravity. The distance grid is in Astronomical Units (AU), and the time is shown in years.  The graph shows the planet's kinetic energy, gravitational potential energy, and total mechanical energy in Joules as functions of time. What can you conclude?

Hints:

  1. Are the signs (positive or negative) of the various energies reasonable?
  2. How does the planet's potential energy seem to vary as it moves around its orbit? What about its kinetic energy?
  3. Pause the animation at some time. "Measure" the energies from the graph. Can you verify that the potential energy is calculated correctly? (Recall that G = 6.67 x 10-11 Nm2/kg2 and that 1 AU = 1.5 x 1011 m.)
  4. Turn on the Interactive Help. How does the planet's speed (now shown in m/s) vary as it executes its orbit? What about the planet's kinetic energy? Can you verify that it is calculated correctly? 
  5. Is total mechanical energy conserved?

Reference

See Walker, Section 12-5

Illustration written by Steve Mellema