External force and total momentum change for a system of objects (7-13)

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Question

Duration of a time interval over which the external forces act

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Review

The quantity in parentheses on the left-hand side of Equation 7-12 is the sum of all of the \(\textit{external forces}\) that act on the \(\textit{system}\) of objects A and B. We’ll call this \(\sum{\vec{F}_\mathrm{external\ on\ system}}\) for short. The right-hand side of the equation is the difference between the total momentum of the system after the time interval \(\Delta{t}\), \(\vec{P}_f = \vec{p}_{Af} + \vec{p}_{Bf}\), and the total momentum of the system before the time interval, \(\vec{P}_i = \vec{p}_{Ai} + \vec{p}_{Bi}\). So we can rewrite Equation 7-12 as: