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Homework Help: Physics 12 momentum question

  1. Jun 28, 2013 #1
    A stationary lift raft of mass 160 kg is carrying two survivors with masses of 55 kg and
    72 kg. They dive off the raft at the same instant, the 55kg person East at 4.4 m/s and the 72
    kg person North at 4.2 m/s. AT what speed and in what direction does the raft start to move?

    So i started off by finding the impluses of both the people. P(north)=mv=72(4.2)=302.4 then p(east)=4.4(55)=242 and then i said p(north) +p(east)=p(raft). so 302.4+2424=160v then solving for v 3.4m/s @15degrees S of W (my teacher said the direction is correct)
    Last edited by a moderator: Jun 28, 2013
  2. jcsd
  3. Jun 28, 2013 #2


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    Staff: Mentor

    Please follow the posting template for each new thread.

    Can you elaborate on your calculations/reasoning? Why are you setting the raft's momentum equal to the total impulse of the divers? Won't the raft be propelled in the opposite directions to the divers?

    How did 242 become 2424? Don't these momenta have directions associated with them, so that you'll have to use vector addition not algebraic addition?

    Neither the magnitude nor the angle you've found look correct to me; I suspect that your teacher may have been mistaken about the angle of the raft's motion.
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