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When a mass is rotating in a plane about a fixed point, its angular momentum is directed along

Asked in CBSE AIPMT 2012 · Angular momentum of a particle

Answer: (1) a line perpendicular to the plane of rotation

Step-by-step solution

Idea: angular momentum is itself a cross product, ⃗L=⃗r×⃗p.

For motion in a plane, both ⃗r and ⃗p lie in that plane.

A cross product is perpendicular to both of its factors.

So ⃗L is perpendicular to every direction in the plane, which means it lies along the normal to the plane of rotation.

Why the other options are wrong

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