A lab cart with a mass of 15 kg is moving with constant velocity, v, along a straight horizontal track. A student drops a 2 kg mass into it from directly above, and the cart continues moving.

Conserve momentum along the horizontal track. Before the mass lands, the cart has horizontal momentum 15v_i, while the dropped mass has zero horizontal momentum because it falls straight down. Afterward, the cart and mass move together at v_f, so their total momentum is (15 + 2)v_f. This gives 15v_i + 2(0) = (15 + 2)v_f, choice C.

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