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Homework Help: Physics Mechanics Homework

  1. Oct 5, 2012 #1
    1. The problem statement, all variables and given/known data

    A subway train starting from rest leaves a
    station with a constant acceleration. At the
    end of 6.27 s, it is moving at 10.4082 m/s.
    What is the train’s displacement in the first
    4.59591 s of motion?
    Answer in units of m​
  2. jcsd
  3. Oct 5, 2012 #2


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    Homework Helper

    hi damombz, welcome to the physicsforums :) How far have you gotten with the problem so far?
  4. Oct 5, 2012 #3
    Thanks for the welcome! I've been stuck on this problem for awhile now, haven't started it yet.
  5. Oct 5, 2012 #4
    the to approaching this problem is knowing some differentiation. As an example, if you differentiate distance with respect to time, you create a rate of change of distance dependent upon time, more commonly know as velocity (ie miles per hour...). If you differentiate again, you then have distance per unit time squared (or rate of change of velocity) more commonly know as acceleration.

    In classical physics, distance can be determined by X(t)=Xo+V(t)+1/2A(t)^2 where Vo is the starting position, V is velocity (the t value cancels out the time variable in the denominator of V, leaving just distance), and the A corresponds to acceleration (with the 1/2 co-efficient as a result of integration, with time square to cancel out the time variable in the denominator

    extension of differentiation
    If you differentiate the X(t) equation with respect to time you then have a velocity equation in the form of:

    V(t)=V+A(t), (all variables have the form of unit distance per unit time)

    this should help start you out
  6. Oct 5, 2012 #5


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    Homework Helper

    You mean that you've been thinking through some ideas, but you think they are not the right way to do the problem? Well what are your ideas, what equations do you know?

    Edit: this is in response to the OP'er, of course.
  7. Oct 5, 2012 #6
    I just know:
    vi = 0
    vf = 10.4082 m/s
    t = 6.27 s

    I don't know what equation(s) to use
  8. Oct 6, 2012 #7
    You know the change in velocity and you know the change in x position.

    If you graph these, you build a straight line, since acceleration is constant and is the slope of your line Δv/Δx.

    Find the equation of this line in y = mx + b form and go from there

    EDIT: If you can graph something, always graph it. It's the best way to understand numerical relationships.
  9. Oct 6, 2012 #8
    Alright thanks! I have another problem that I'm stuck on.

    A cat climbs 3 m directly up a tree.
    What is the horizontal component of the
    cat’s displacement?
    Answer in units of m
  10. Oct 6, 2012 #9
    I'll certainly help you with this, but you need to show me what you've tried that didn't work.
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