Fill in the blanksQuestion 1 · Recall
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By the end you can state the principle of conservation of momentum in the words examiners reward, explain why it's true using Newton's third law, and use it on a simple collision.
The main symbols used in this lesson:
| Symbol | Meaning |
|---|---|
| momentum of object B |
Snooker ball hits snooker ball; car rear-ends car; skater shoves skater. Each time, the objects pass momentum between them: whatever one gains, the other loses, to the last .
Think of a bank where money only moves between accounts and never leaves the building: the total in the bank never changes.
Where the analogy breaks: momentum is a vector, so it can be negative. Two objects can both be moving and still have a total momentum of zero.
Principle of conservation of momentum
The total momentum of a system of interacting objects remains constant, provided no resultant external force acts on the system.
Equivalently, for a closed (isolated) system: total momentum before an interaction = total momentum after it.
The system is whichever objects you choose to include. Then sort the forces:
So "no resultant external force" is the condition, not "no forces". During a collision there are huge forces; they're just internal.