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Homework Help: Thermodynamics: Conservation of energy problem

  1. Sep 18, 2011 #1
    1. The problem statement, all variables and given/known data
    A closed system initially at rest on the surface of the Earth undergoes a process for which there is a net energy transfer to the system by work of 200 Btu. During the process, there is a net heat transfer from the system of 30 Btu. At the end of the process, the system has velocity of 200 ft/sec at an elevation of 200 ft. The mass of the system is 50lb and the local acceleration due to gravity is 32 ft/s2. Determine the change in internal energy of the system for this process, in Btu.

    2. Relevant equations

    [itex]\Delta[/itex]E = [itex]\Delta[/itex]U + [itex]\Delta[/itex]KE + [itex]\Delta[/itex] PE

    3. The attempt at a solution

    So I understand that the system initially has 200 Btu transfered to it. And 30 Btu taken away from the heat.

    So my understanding of that is that [itex]\Delta[/itex]E is equal to 170 Btu. Correct?

    In calculating the [itex]\Delta[/itex]KE, all I see is that initially it is 0, and after it is 1/2mv2. This gives my units in [itex]\frac{lbft^2}{s^2}[/itex]

    How do I get this in Btu? I'm not very good with units.

    Also, would I include [itex]\Delta[/itex]PE since the system is moving at the end of process?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 18, 2011 #2
  4. Sep 19, 2011 #3
    No help?
  5. Sep 20, 2011 #4
    Maybe this can help you;

    Btu / lbm = 25,037 ft2 / s2

    or you can also use any conversation software/website from pound force to btu.
  6. Sep 20, 2011 #5

    rude man

    User Avatar
    Homework Helper
    Gold Member

    I think everything you've written is good. Yes, the increase in potential energy must be included in your equation.

    I also don't like dealing in the British units system. It doesn't help that there are more than one of them - BG and EE and god knows what else (see link below). The thing you need is converting BTU to ft-pounds: •1 Btu (British thermal unit) = 778.3 ft-lb. Now everything is in pounds of mass, pounds of force, feet and seconds. In this case I suppose we're dealing with the EE system since mass is given in pounds, not slugs. So 1 lb of force = 1 lb of mass * 32.2 ft/sec2. The last number is of course g, the acceleration of gravity, which everybody but the Brits know as 9.81 m/sec2. :-)

    Actually, there IS a similar confusion with folks using kg - they use kg to mean both force and mass, exactly analogous to the misuse of pounds. Scientists of course avoid using kg for force or weight, using Newtons instead.

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