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Homework Help: Conservation of momentum ( I Think ! )

  1. May 15, 2012 #1
    1. The problem statement, all variables and given/known data
    A car ( mass 1500kg) is traveling at 40km/hr when it instantaneously starts to accelerate at a rate of 1.5ms-2 it then cruised at this new velocity before it collides with a stationary vehicle two third its mass at a set of traffic lights and the two vehicles move off together. Due to the collision the engines of both cars stall and gears are thrown into neutral so that the combines vehicles free wheel along the road.

    Evaluate the front and rear loads before and during the acceleration (coefficient of friction = 0.35)

    2. Relevant equations

    3. The attempt at a solution
    Well the Load during the first bit of acceleration would be MA which would be 1500 x 1.5 = 2250n
    The speed it reaches would be v=u + at
    40km/hr in m/s is 11.1 m/s
    V=11.1 + 20 x 1.5 = 41.1 ms-1
    Then once the join together there new mass would be 2500KG
    But i don't really understand what they mean by loads.. can anyone point me in the right direction
  2. jcsd
  3. May 15, 2012 #2


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    Yeah that is poor wording using the terms "front and rear loads". I think it means "what forces act in the direction of the first car's motion and what forces act opposite it's motion, before and during the acceleration. During, the net force is' 2250 N. Now since the max force of friction available to move the car forward is 0.35mg or about 5000 N, (assuming friction is at it's max value), then you can calculate the rear facing force which may be due to air drag, skin friction, or even gravity if the car is going uphill. Now assume before the acceleration, when the car is moving at constant speed, that the rear force remains the same, what is the forward force? I'm just guessing about this making my own assumptions, since the question is not clear.
  4. May 16, 2012 #3
    yeah was very poor wording went to my lecturer and he apologised for the question as it is very unclear what i have to do. thanks anyway!
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