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Homework Help: Help With Kinematics!

  1. Jan 17, 2012 #1
    1. The problem statement, all variables and given/known data

    Two train cars are on a straight, horizontal track. One car starts at rest and is put in motion with a constant acceleration of 2.1 m/s2. This car moves toward a second car that is 25 m away and moving at a constant speed of 3.6 m/s.

    (a) Where will the cars collide?
    (b) How long will it take for the cars to collide?

    2. Relevant equations

    x= x0 + V0*t
    x= 1/2(a)*t^2 + V0*t + x0

    3. The attempt at a solution

    This is a practice problem from WebAssign for our Physics course. My friend and I could not figure out how to solve this problem but we know the answer for both the parts which doesn't help since we don't know how to do the problem. If someone could help! Thanks!

    a) 49.8 m
    b) 6.89 s
  2. jcsd
  3. Jan 17, 2012 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    You should make some attempt at the problem.

    When you get stuck in kinematics, it often helps to draw the v-t graphs for each object.
    You need to find the time that the areas under the graphs is the same.... but you have to set them up right.

    List what you know for each train:
    x0, v0, a.

    What is the equation that tells you the position of the constant-speed train?
    What is the equation that tells you the position of the constant acceleration train?

    (you've actually written them down - you just have to write in the numbers you know.)
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