Barycentric balls – orbits and the centre of mass

Brief description:

In this activity, the principle of moments is applied to rotating systems to demonstrate the concept of a barycentre, or centre of mass, and how objects in orbit around each other move.

Students then consolidate this concept by calculating the centre of mass in a number of astronomical contexts.

It is recommended that students should already have some background knowledge on the concept of the principle of moments and torque, as well as the concept of the Doppler effect applied to the electromagnetic spectrum.

Science, Physics, Mathematics
Learning Objectives:
  • Students will learn about the centre of mass and understand that for a gravitationally bound system with two or more bodies, all objects orbit about a common centre of mass.
  • Students will learn how to apply the principle of moments in order to calculate the centre of mass of a two body system.
  • Students will apply the physical concepts to several astronomical situations, learning about binary star systems, planet-moon systems and extrasolar planets.

Age range: 14 – 18 years old


Preparation: 1 hour
Lesson: 10 to 30 minutes

Resource available in:
English, French, German, and Spanish
Activity 1: Barycentric balls

In this demonstration, two pre-assembled pairs of tennis balls are used to show how the position of the barycentre of a two-body system changes with the mass of the two bodies.

The balls in the first pair have the same mass. In the second pair, one of the tennis balls is filled with coins or ball bearings to increase its mass.

  • Pair of equal-mass tennis balls, connected with string
  • Pair of unequal-mass tennis balls, connected with string

Did you know?

The Hubble Space Telescope (HST) is a joint ESA/NASA project. It was launched into an orbit 600 kilometres above the Earth in 1990, and is one of the largest and most successful space observatories ever.

From its vantage point outside the Earth’s constantly moving atmosphere, which distorts the light reaching the ground from space, HST has provided stunning high resolution images of thousands of space objects such as planets, binary star systems, galaxies, nebulae and star-forming regions. HST has dramatically improved our view of the Universe.

The Hubble Space Telescope in orbit of the Earth