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Linear Momentum Problem

  1. May 12, 2013 #1
    1. The problem statement, all variables and given/known data

    A boy of mass 60kg, running at 5m/s jumps on a stationary 15kg canoe and they move of together.

    1.1) Find their speed just after the boy lands on the canoe
    1.2) If the canoe comes to rest after moving through a distance of 15m, calculate the average magnitude of the retarding force on the canoe due to the water

    2. Relevant equations

    momentum before= momentum after
    Impulse = Change in momentum

    3. The attempt at a solution
    Unsure about 1.2

    1.1) 4m/s(momentum before = momentum after)
    1.2) t=d/s = 15/4 = 3.75's

    f*t = m*v
    3.75f = 75(combined mass)*4
    F = 80N

    Could someone check
  2. jcsd
  3. May 12, 2013 #2
    Hello TheRedDevil18,

    Correct !!

    Hmmm. Wrong. Constant resistive force is acting on the canoe system. You will have to use third equation of motion, v2=u2-2ad... Find deceleration a....
  4. May 13, 2013 #3
    v^2 = u^2+2ad
    16+30a = 0
    a = -0.53m/s^-2

    Fnet = ma
    = 75*-0.53
    = -39.75N

    Would this be correct?
  5. May 14, 2013 #4
    It is asking for the average retarding force exerted on the canoe system, and not the "canoe+boy" system.

    You need to consider the mass of canoe only.
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