For three bodies

PHYSICS · Single correct · A Level · Physics · Mechanics and materials · Mechanics and materials

Problem

For three bodies PP, QQ and RR interacting only with one another in an isolated system, which one of the following equations correctly expresses the result obtained after combining the time-rate forms of Newton’s second and third laws? (a) ddt(p⃗P+p⃗Q)=0\displaystyle \frac{d}{dt}\left(\vec p_{P}+\vec p_{Q}\right)=0 (b) ddt(p⃗P+p⃗Q+p⃗R)=0\displaystyle \frac{d}{dt}\left(\vec p_{P}+\vec p_{Q}+\vec p_{R}\right)=0 (c) ddt(p⃗P+p⃗Q−p⃗R)=0\displaystyle \frac{d}{dt}\left(\vec p_{P}+\vec p_{Q}-\vec p_{R}\right)=0 (d) ddt∣p⃗P+p⃗Q+p⃗R∣=0\displaystyle \frac{d}{dt}\left|\vec p_{P}+\vec p_{Q}+\vec p_{R}\right|=0

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