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Help with Kinematics Problem (Two bodies in motion; one direction)

  1. Oct 6, 2011 #1
    1. A train has a length of 92m and starts from rest with a constant acceleration at t= 0s. At this instant, a car just reaches the end of the train. The car is moving with a constant velocity. At t=12sec the car just reaches the front of the train. Ultimately, the train pulls ahead of car and at t=28sec, the car is at the end of the train once again. Find the magnitude of (a) the car's velocity; (b) the trains acceleration.

    One thing to note is that is at time 0, the train is at rest and while the car is already at constant velocity. The train begins motion after time 0. Therefore, both vehicles start to move at time zero which is why this problem is a bit hard for me.

    d= Vit+1/2(at2)
    Vf2=Vi2+2ad
    Vf=Vi+at


    3. How do I go about solving this. I tried splitting the problem into three parts. Part 1- car reaches the the back of the train at time 0. Part 2- car passes the train completely. Part 3- train passes the car.

    I tried doing algebraic manipulation to get like Vf and Vi but i am for the car but I am working with too many unknowns for my liking and it doesnt help.

    Any help from the physics community here would be lovely :D
     
  2. jcsd
  3. Oct 6, 2011 #2

    PeterO

    User Avatar
    Homework Helper

    Your explanation of Part 2 doesn't seem to recognise that at 12 seconds the car reaches the front of the train, but the train has by then reached the speed of the train.

    "At t=12sec the car just reaches the front of the train".

    I hope that is enough to help.
     
  4. Oct 6, 2011 #3
    yes, I understand that. This question would be easy if the train wasnt moving. But it is. Since the train is 92 meters. The distance the car would have to travel to catch up to the train at t=12 is 92+x meters. Because in those 12 seconds, the train is moving too.
     
  5. Oct 6, 2011 #4
    So, i am still confused :(
     
  6. Oct 6, 2011 #5
    help please :'(
     
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