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Coupled orbits

Balance two masses so that they can spin around each other. Experiment with relative mass size to see how it changes orbit sizes and speeds.
Science content (2016 curriculum): 
Physics: Motion and Forces, Newton’s Laws, Gravity (K, 2, 6)
Earth/Space: Sun, Moon, Solar System, Universe (1, 4, 6)
Science topic (2005 curriculum connection): 
Earth and Space Science: Stars and Planets (grade 3)
Lessons activity is in: 
  • bamboo skewers
  • string
  • playdough (see recipe)

When two stars orbit each other, or a star orbits a black hole, or a planet orbits a star, the sizes of the orbits and the relative speeds of the orbits depend on the relative masses of the two objects.
When astronomers find a star in orbit with an invisible companion, they can measure the size and speed of the star's orbit to figure out the size of the companion. If the companion is larger it might be a black hole. If the companion is smaller is could be a planet.

Experiment with two masses orbiting each other to see how their relative sizes change their orbits and speeds:
Tie a string around a bamboo skewer so that it is tight, but can slide along the skewer.
Make a loop in the other end of the string, so that it can be help securely.

First make two equal-sized masses of play dough and mould them around each end of the skewer.
Move the string along the skewer until it is balanced (it will be near the middle).
Gently push one of the masses so that they spin around each other.
What orbits do they trace out? How fast are they moving relative to each other?
They should rotate around each other, following the same orbit path. They will also be moving at the same speed.

Now make the masses very unequal. Move the string until the skewer is balanced again. You may need to adjust the masses slightly so that balancing is possible with the string pushed right up to one mass.
Now gently push to make the masses orbit one another.
What orbits do they trace out? How fast are they moving relative to each other?
You should find that the large mass has a small orbit, while the small mass has a larger orbit around it.
Although it might be hard to see the relative speeds, you might notice that the small mass moves much faster than the large mass.

For black hole discoveries:
The large mass represents a black hole and the smaller mass a star.
The more massive the black hole, the faster the star moves around it.
See this webpage for more information and images:
Astronomers look for the presence of black holes by looking for stars that appear to be orbiting around “nothing” - they "wobble” in the sky.
The more massive the black hole, the faster the stars move, so the speed of orbiting star(s) can be used to calculate the mass of the black hole.

For planet discoveries:
The large mass represents a far away star.
The small mass represents a planet, too small to see itself.
A tiny planet will make the star "wobble" a little bit (move back and forth in its orbit), and a larger planet will make the star wobble more.
Astronomers measure the wobble in stars to discover exoplanets and calculate their size and orbit around the star being observed.
The wobble of a far away star is measured by reading its Doppler shift as it moves away from and towards us.


Search "barycentre" for more information on this phenomenon:

Grades tested: 
Gr 6
Gr 7
Phyllis Daly
Reid McInnes
Teaching site: 
Strathcona Elementary
Activity originally developed and delivered: 

Strathcona Elementary with the Vancouver School Board's Scientist in Residence Program