# How fast are the two moving if she has a mass of 45kg?

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a raft in a lake is initially moving towards the shore with a speed of 1.0m/s. Sally runs to the water and jumps onto the raft with a speed of 4.0m/s

First set up a coordinate system, make the boat traveling to the left initially and make Sally run to the right initially. Set right to be the positive x direct. $$P_i=45kg\times 4.0m/s-m_{boat}\times (1.0)m/s=(45kg+m_{boat})\times v_f$$
$v_f$ is the velocity of the two moving together after Sally jumps on the boat. The issue is that we don't know the boat's mass -- is it given in the question?