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Homework Help: Some mechanics questions

  1. Feb 24, 2004 #1
    We have been given a set of questions to do. Most of these I have managed fairly easily but there are some which I am really stuck on and am getting quite annoyed with now as I have been working at them for ages! Help on any of these would be really appreciated:

    The Saturn V rocket which launched the Apollo space missions had the following specifications:
    mass at lift-off = 3.0 x 10^6 kg
    velocity of exhaust gases = 1.0 x 10^4 m/s
    initial rate of fuel consumption at lift-off = 3.0 x 10^3 kg/s

    1.(a) Calculate:
    (i) the force (thrust) produced at lift-off
    (ii) the resultant force acting on the rocket at lift-off

    2.(a) An empty railway truck of mass 10000kg is travelling horizontally at a speed of 0.5m/s. (had to calculated the momentum and kinetic energy - not sure if this will help with the bit im stuck with!)
    Sand falls vertically into the truck at a constant rate of 40kg/s. Calculate the additional horizonal force which must be applied to the truck if it is to maintain a steady speed of 0.5m/s.

    Thanks for anyones help - I can usually manage these kind of questions but I'm really stuck for some reason!!
     
  2. jcsd
  3. Feb 24, 2004 #2

    ShawnD

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    [tex]Ft = mv[/tex]

    Derive both sides

    [tex]F = (\frac{dm}{dt})(v)[/tex]


    I think this means to include gravity. It should be whatever you got from part (i) - gravity.

    [tex]Ft = mv[/tex]

    Derive both sides

    [tex]F = (\frac{dm}{dt})(v)[/tex]
     
    Last edited: Feb 24, 2004
  4. Feb 24, 2004 #3
    I've not met that formula before but I do understand where its from but how can you use it because you don't know time or is it necessary to work out the time or something??
     
  5. Feb 24, 2004 #4

    ShawnD

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    dm/dt is a variable, not a division of 2 or 4 variables :wink:

    In the first question, dm/dt is 3.0 x 10^3 kg/s

    In the second question, dm/dt is 40kg/s
     
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