alleryStart learning →Under what condition does the total momentum of a system of interacting objects stay constant?
ANo internal force acts on it
BAll the objects have equal mass
CNo external force acts on it
DThe objects do not touch at all
Answer & Solution
Correct answer: C. No external force acts on it
1. Inside a system the objects push on each other with equal and opposite forces.
2. Those internal pushes cancel out, so the changes in momentum they cause add up to zero.
3. A force from outside the system has nothing to cancel it, so it can change the total.
4. The total momentum is therefore constant only while no external force acts.
5. Internal forces are exactly the ones that must be present for a collision to happen at all, so the second option is wrong.
_Source: Siyavula Physical Sciences Grade 12 (Everything Science, CC BY 4.0), Chapter 2: Momentum and impulse_
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