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Homework Help: Change of Momentum

  1. Mar 3, 2005 #1
    A 140 kg astronaut (including space suit) acquires a speed of 2.56 m/s by pushing off with his legs from a 1730 kg space capsule. The change in speed of the space capsule is .207 m/s.

    How would I find the change of momentum in the motion from above?

    m1 = 140 kg astronaut
    m2 = 1730 kg space capsule

    v1 initial = astronaut 0
    v2 initial = space capsule 0

    v1 final = astronaut 2.56 m/s
    v2 final = space capsule .207 m/s

    would the change of momentum equal the following?

    J (impulse)= pfinal - pinital

    Momentum of Final...---------------------------------------------------

    p1 = astronaut m1 x v1 final
    140 x 2.56

    p2 = space capsule m2 x v2 final
    1730 x .207

    pfinal about = 358

    Momentum Initial --------------------------------------------------------

    If i solved for the momentum using v1 inital and v2 inital then the momentum of both would be 0.

    Pinitial = 0

    Pf = 358
    Pi = 0

    J = 358 - 0
    J = 358

    Would 358 be the change of momentum in the motion above????
    Does momentum have units?

    Momentum is a vector quantity so therefore no units?
    Last edited: Mar 3, 2005
  2. jcsd
  3. Mar 3, 2005 #2
    I haven't looked through your working, but of course momentum has units! Why would being a vector quantity mean no units? Force is a vector quantity and has units of kgms^-2. What's the definition of momentum? You get the units of it from that.
  4. Mar 3, 2005 #3

    Andrew Mason

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    Science Advisor
    Homework Helper

    First of all, the question is incomplete. The question has to specify a change in momentum of a mass. There is no such thing as a change in momentum of motion.

    If it is asking what is the change in momentum of the system consisting of the astronaut and capsule, the answer has to be 0. Momentum of an isolated system cannot change in the absence of external forces.

    If it is asking about the change in momentum of the astronaut, the answer has to be the same as the change in momentum of the capsule but in the opposite direction. So the change in momentum of the astronaut is equal and opposite to the change in momentum of the capsule.

    The units of momentum are that of mass x velocity or force x time - kg.m/sec or N.sec.

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