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B Question about real world examples of 2.7 megajoules

  1. Mar 29, 2017 #1
    Recently I decided to combine three of my favorite things, physics, machines that propel things, and videogames.

    The game in question: Space engineers.

    Machine in question: A device of my own creation that abuses the marvel of artificial gravity to accelerate a projectile at high speeds.

    The physics? How much energy can one of my cannons deliver in the form of kinetic energy.

    After building a small cannon for testing and dusting off my physics textbook I came up with these variables.

    Mass of the projectile: 20 tons or 18143.695 kilograms.

    Acceleration due to the gravitational forces I've enslaved: 1177.20 m/s^2

    Total distance of acceleration: 125 meters.

    After a bit more math I found that the final velocity of the slightly unrealistic 20 ton shell is 542.4942396 m/s.
    If needed I can put a link to the thread I found my velocity from acceleration and distance work on.

    Now using the lovely 1/2mv^2 I came to a kinetic energy of 2,669,844,719 j, or 2.7 mj.

    Now here is my problem. My mathematically inclined brain says that's a lot. But physics brain is saying that's not all that much energy.
    How much energy is 2.7 megajoules?
    Is there any good real world examples of something with this kind of energy?
    Many thanks in advance.
  2. jcsd
  3. Mar 29, 2017 #2
    More information about the device that wasn't pertinent to the question. What I constructed today is an infinitesimal launcher compared to some of my builds. Today's only utilized 120 generators in game. My biggest have used many thousands of generators, each outputting 9.81m/s^2 acceleration within their fields. One of these days I'll build one of these larger ones and do the math on their kinetic energy outputs. But this becomes tricky due to uneven acceleration fields along the "barrel" due to overlap of thousands of generators.
  4. Mar 29, 2017 #3


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    Staff: Mentor

    Such energies are often compared to "tons of TNT": (1 kg TNT = 4 MJ)

    The meteor of Chelyabinsk had more than 500 kilotons.
  5. Mar 29, 2017 #4


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    I think you mean 2.7 Gigajoules. 2.7 Megajoules isn't that much energy - it's about the amount of food energy in a cheeseburger (600 calories). For reference, 1 Kilowatt-Hour is 3.6 Megajoules.
  6. Mar 29, 2017 #5
    Evidently all of my school work has leaked out of my head while I slept, there was my mistake. I apparently can't differentiate between 2.7 million and 2.7 billion
  7. Mar 29, 2017 #6
    And to ask again, 2.7 gigajoules? Just how much better is that?
  8. Mar 29, 2017 #7


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    You could calculate how much water that energy can heat from room temperature to boiling point, by using the heat capacity ##C_{H_2 O} \approx 4000## JK-1kg-1.
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