Centre of Mass

The point that moves simply

An interactive lesson with 7 steps, about 6 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• Centre of mass: $x_{\text{cm}} = \sum m_i x_i / \sum m_i$.
• With no external forces, it moves at constant velocity, whatever the parts do.
• The Earth–Moon barycentre lies 4,680 km from Earth’s centre.

Steps

1. The balance point
2. Balance the rod
3. Earth and Moon
4. A firework
5. The burst and the balance point
6. Find the balance point
7. Walking in a boat