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Homework Help: Moving train problem

  1. Nov 26, 2008 #1

    Can anybody offer any assistance with the following problems please:

    1. A train is moving along a straight line at constant speed V. Inside a carriage an object is thrown upwards from a height h (measured from the floor of the carriage) with initial velocity v. Determine the trajectory of the object in a reference frame which is not in motion with respect to the ground and in a reference frame which moves together with the train. Are these two reference frames inertial?
    2. Consider the same train but now assume that when the object is thrown upwards (with same velocity and height as 1.) the train has a negative acceleration A and its instantaneous velocity is V. Determine the horizontal distance from the starting point at which the object falls on the floor of the carriage. Describe the motion in the two reference frames of problem 1. Are they inertial in the present case?

    I'd appreciate any help/advice. Thanks in advance.
  2. jcsd
  3. Nov 26, 2008 #2
    Let us begin with your problem 1
    1 What is the kinetic energy defined by the startvelocity v?
    2 The object already starts with a potential energy defined by h?
    3 What is the maximal potential velocity, assumed that v is not so that the ceiling of the train can be reached?
    It is correctly of no importance what the mass is but let us call it m.

    I only answer 2: Let us call force=F , E = integral [ s=h -> s=0] (F * ds)= F*s [s=h-> s=0]
    since F = -mg is constant. E= F*0 - F*h= -F*h= mgh.
  4. Nov 27, 2008 #3


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    This is a reference frame of an observer who is stationary with respect to the ground. The observer would see the object leave the throwers hand at point A on the tracks, and wind up falling down at some distance away, point B on the tracks. In the absence of any forces except gravity acting on the object , its trajectory is in the shape of a _______??
    this is the reference frame of the thrower. Does the thrower catch the object when it fall back to the floor? What's its path with respect to the thrower?
    Look up the definition of an inertial reference frame using a google search. (Don't use Wikipedia unless you have a PhD in Physics).
    Well, the stationary frame of the observer is stationary with respect to the ground.
  5. Nov 27, 2008 #4
    _____??=parabola through A and B, but first vertically:

    s(t)=h + integral [t1=0 -> t1=t] V(t)*dt,

    ' was trying to calculate the top.
  6. Nov 28, 2008 #5
    Thanks for the help guys.
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