What is it
Momentum is the product of an object's mass and velocity, and the principle of conservation of momentum states that in a collision, total momentum before equals total momentum after, as long as no external forces act.
Why it matters
Conservation of momentum is one of the most powerful tools in mechanics precisely because it holds true even when the details of a collision (like the exact forces involved) are unknown or too complex to calculate directly.
Exam tip
Define a clear positive direction before starting any momentum calculation, and stay consistent with it throughout — sign errors from an inconsistent direction convention are the single most common mistake in momentum and collision questions.
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