A particle of mass m moves along the line PC with velocity v, where P is a fixed point on that line. What is the angular momentum of the particle about P?

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(4) zero. Angular momentum about a point is ⃗L=m(⃗r×⃗v), where ⃗r runs from that point to the particle. The particle moves along the line PC, and P lies on that line, so ⃗r always points along PC as well. ⃗r and ⃗v are therefore parallel at every instant. The cross product of two parallel vectors is zero. ⃗L=⃗0. Said another way: the perpendicular distance from P to the line of motion is zero, and L=mv×(that distance).

A particle of mass m moves along the line PC with velocity v, where P is a fixed point on that line. What is the angular momentum of the particle about P?

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