Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Dog vs Raft relativity momentum Question (PLEASE HELP)

  1. Apr 13, 2010 #1
    1. The problem statement, all variables and given/known data
    A dog of mass 10 kg is standing on a raft so that he is 20 m from shore. He walks 8 m along the raft towards shore and then halts. The raft has a mass of 40 kg, and we can assume that there is no friction between the raft and the water. How far is the dog from shore when he stops? [Answer: 13.6m

    2. Relevant equations

    3. The attempt at a solution
    i can work out the solution by utilizing the momentum of the dog raft and then implement in into the vector addition no problem however according to my teacher no matter which way i explain how i've done it i am wrong. I know that the momentum of the dog / raft scenario works out to mass of the dog times the displacement divided by the mass of the raft is the same as the displacement of the raft backwards. you then take the two displacements, find their sum, minus that from the original distance. this results in 13.6m. not a problem, however there is apparently a flaw in my logic somewhere. apparently i am supposed to take the 20m and place it into the dog raft displacement equation before i solve for the momentum peice. PLEASE ANYONE WHO CAN HELP, IT WOULD BE GREATLY APPRECIATED!!!
  2. jcsd
  3. Apr 13, 2010 #2
    Show exactly how you think you solved the problem. I'm not quite sure what you are saying here so I can't tell you if you did something wrong.
  4. Apr 13, 2010 #3
    let D=displacement vector

    dog D ground = dog D raft + raft D dog

    use the momentum of the dog and the raft to solve for one unknown variable

    let m1=dog, m2=raft

    momentum before is equal to zero

    0 = m1v1'+m2v2'
    -m2v2'=m1v1 v=d/t
    -m2d2'/t=m1d1'/t times both sides by time
    d2= -1.6m

    now put this back into the original equation

    dog D ground = dog D raft + raft D dog
    dog D ground = 8m + (-1.6m)
    dog D ground = 6.4m

    now minus this from the original 20 and it gives you the 13.6m
  5. Apr 13, 2010 #4
    I think your displacement equation is what is causing the confusion. For me you are switching reference frames way too many times. Here's how I would get a displacement equation.

    The speed of the raft in the dog's reference frame, Vr-d, is equal to the speed of the dog in the shore's reference frame, Vd-s, minus the speed of the raft in the shore's reference frame, Vr-s. In equation form:

    Vr-d = Vd-s - Vr-s

    Multiplying through by time we get

    Dr-d = Dd-s - Dr-s

    Each side only uses one reference frame. Your equation uses two different frames on one side. How did you derive it?

    Continuing the problem my way...

    First consider an observer on the shore. By conservation of momentum the observer must see


    vraft = -1/4vdog

    Now consider the dog's reference frame. To him the raft is moving by at a speed of:

    vdog + 1/4vdog = 5/4vdog

    He stops when he sees 8 m of raft go by, given by:

    5/4vdogt = 8

    t = 6.4/vdog

    Obviously the time interval is the same for the observer. He sees the dog move forward by

    vdog*6.4/vdog = 6.4 m = 20 - x

    x = 13.6 m
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook