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Isolated system
Beginner
No net external force acts on it.
A system is isolated when external forces sum to zero (or are negligible over the short interaction). Internal forces of any size don't change total momentum.
Two ice skaters pushing off each other on frictionless ice.
Gravity and normal forces cancel on a level surface, so horizontal momentum is conserved.
In more detail
An isolated system has no net external force acting on it, so its total momentum does not change. Internal forces between parts can redistribute momentum but cannot change the total.
Real-world uses
- A spaceship and its fuel form an isolated system when coasting in space.
- Two skaters pushing off each other form an isolated system on frictionless ice.
- A spacecraft far from any planet is nearly isolated, so its momentum changes only when it fires its thrusters.
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Frequently asked questions
Is the Earth isolated?
No. Gravity and friction from the Sun, Moon and the air act on it.
What if friction acts?
Then the system is not isolated, and its momentum changes.
How do I know if a system is isolated?
Check that the net external force is zero or negligible compared with the forces inside the system.
Related topics in Momentum & Collisions
Impulse–momentum theorem · Why airbags work · Conservation of momentum · Recoil of a gun · Elastic collision · Inelastic collision