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Physics question regarding a movie scene

  1. May 10, 2005 #1
    After watching Donnie Darko I was curious about a scene where a 747 Jet Engine (just the engine) falls out of the sky and onto the corner of a rather large suburban house. In the movie the house was intact (minus some damage to the side it landed on). Assuming something like this happened in reality, would it be possible for the house to remain somewhat intact if the 747 Jet Engine fell from 35,000 feet onto the corner of the house?

    I was having an argument about this with a friend of mine and figured a physics forum would be the best possible place to find an answer. :biggrin:
     
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  3. May 10, 2005 #2

    chroot

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    There are an awful lot of unknowns.

    1) What's the terminal velocity of an engine?
    2) What's the weight of an engine?
    3) What's the shear strength of 4x4 wooden beams?

    It seems reasonable to me that a pretty dense, relatively small object like an engine would plunge right through a house, breaking the frame members it directly strikes, but leaving most of the rest of the structure intact. It's much more like a bullet than a bomb.

    - Warren
     
  4. May 10, 2005 #3
    I tried looking up the weight of a 747 Jet Engine but I couldnt find it. I would assume it to be at least 2-3 tons.

    I have no idea what the terminal velocity would be. My physics stopped at high school 10 yrs ago :smile:

    Thanks for any help!
     
  5. May 10, 2005 #4
    Now if someone could dig up the approximate size of the engine we could have a classic post starting with "assume a spherical engine" in the makings.

    I guess someone must have calculated the air resistances for cylinderical or water droplet like objects so that part is solvable.

    nightshadz: terminal velocity is the maximum velocity a falling object can attain before gravity and air's resistance cancel eachother out leaving the object falling at a constant velocity.
     
  6. May 10, 2005 #5

    Pengwuino

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    I highly doubt the thing would be intact. The engine must have hit the house with a horizontal velocity of at least a hundred miles per hour or so meaning it would plow through the house along with the damage inflited simply because of its energy from falling a few thousand feet.
     
  7. May 10, 2005 #6
    I know what Terminal Velocity means, i just dont know how to derive it and how to apply it to this problem. Sure I could look it up and plug in some numbers, but those numbers won't have any meaning to me regarding this question. :rofl:

    Also, assuming it did fall from 35,000 feet, wouldnt it eventually end up dropping completely vertical by the time it impacts?
     
  8. May 10, 2005 #7

    chroot

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    Yes, I was assuming wind resistance would kill its horizontal velocity by the time it hit the ground. That may or may not be a good assumption, but I don't think a tumbling engine is all that aerodynamic.

    - Warren
     
  9. May 10, 2005 #8
    oh sorry, I misread your post.
     
  10. May 10, 2005 #9
    There are a lot of reports of cars crashing into houses at very high speeds, and the over all damage is confined to a small area. A lot times the car doesn't even make it into the house. I assume a jet engine' weight is similar (probabally less) than a car. To me it seems highly probable that the majority of the house would remain intact. As was mentoned its a projectile not a bomb.
     
  11. May 10, 2005 #10

    Danger

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    The 747 uses Pratt & Whitney JT9D high-bypass turbofans. They are 97" in diameter, 132" long, and produce from 46,300—56,000 pounds of thrust depending upon the specific model. I can't find a weight figure for them anywhere, but all aviation engines use as light a material as they can get away with, and the biggest part of that engine diameter is just the bypass fan at the front. I'm not sure that one would even reach a ton in weight.
     
  12. May 10, 2005 #11
    My god it is just a movie. Its like armaggeddon and the gravity of the asteroid. Just get over it and enjoy the moive.
     
  13. May 11, 2005 #12

    Danger

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    But I enjoy this more than movies. That's why I'm here instead of watching one. :tongue:
     
  14. May 11, 2005 #13
    Don't forget that the engine in question took a trip through a time vortex (or equivalent movie-science buzzword), which could have changed its velocity in an arbitrary way to suit any required plot line...
     
  15. May 11, 2005 #14
    I know. That's why I created this thread. To find out what would actually happen assuming a 747 Jet Engine fell from 35,000 feet (without the help of a wormhole). :wink:
     
  16. May 11, 2005 #15
    I don't know the weight of those engines, but I know the engines some B-57 bombers (jet engines) use produce about 1/10th that thrust and weigh about 1.25 tons. Assuming a direct relationship, the engine in question would weigh around 12-13 tons. Not exact, but it's a starting place.
     
  17. May 11, 2005 #16
  18. May 11, 2005 #17
    assuming the engine fell from h meters and has a mass of m

    it would have protentional energy of

    [tex]u=mgh[/tex]

    ill take a guess here and say that because the engine isn't all that big it doesn't have that much air resistance so suppose it falls in a vacuum
    so upon impact it has kinetic energy of

    [tex]e= \frac{mv^2}{2}[/tex]

    [tex]2mgh=mv^2[/tex]
    [tex]v= \sqrt{2gh}[/tex]

    suppose it doesn't lose due to air resistance more than 80% of its vacuum falling speed

    [tex]v= \frac{4 \sqrt{2gh}}{5}[/tex]
    if it falls from 35,000 feet (10,668 meters) it would have a velocity of at least
    v=366 m/sec (1318 km/h)
    im pretty sure it would simply act like a bullet penetrate through the roof and leave a big crater on the floor :biggrin:
     
  19. May 11, 2005 #18

    DaveC426913

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    747 engine: RollsRoyce RB211 : reference

    There are some widely conflicting specs for it, but this one appears to give numbers that include the cowling, not just the naked engine: reference
    Weight: 13,900 lbs :
    Rolls Royce size: ~208"x168"

    Airliner crusing speed: ~500mph
    Airliner cruising altitude: 35,000-40,000ft

    Edit: deleted obsolete calcs
     
    Last edited: May 11, 2005
  20. May 11, 2005 #19

    DaveC426913

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    OK, I found another formula that got me a better result for the test case (Got me within an order of magnitude for human v(t).)

    First, the real surprise is just how dense an aircraft engine isn't. The mass divided by the volume is slightly less than 0.1g/cm^3!! If it were sealed in a plastic bag, it would float 9/10ths out of the water!

    OK, takes sense once you think about it, by design, an engine is a mostly air-filled cavity. Boats float with most of their volume out of the water too.

    As far as air resistance, one can assume the engine stops working fairly quickly as it falls, so the blades act more as a wall than a passage for airflow. Thus, the engine acts as a solid sphere as far as air resistance goes.

    The formula for terminal velocity is:

    [tex]v_t = \frac{.222 \cdot g \cdot (d_s-d_a) \cdot r^2}{n} [/tex]

    (reference)

    where
    g = gravity 9.8m/s
    [tex]d_s[/tex]= density of object (.099g/cm^3)
    [tex]d_a[/tex]= density of air (.001239g/cm^3)
    r is the radius of the "sphere" (210cm)
    n is the "dynamic viscosity of air near the Earth's surface" (0.00018 g/cm/s)


    I get a number of approximately 50m/s, which is a mere 100 mph.


    That provides momentum of 315,000 kg-m/s. That is twice the momentum of a 3000kg car moving at 100mph.

    One other factor: the engine is moving downward, not laterally. A house can deform laterally, but it will not deform vertically since the ground is immovable. This would lessen the amount of damage it would do (since much of the impact would be transferred to the ground)

    Conclusion:

    I would conclude that a jet engine falling on a house would do amount of damage that is within 1x and 2x the amount that a car hitting it at 100mph would do.

    Thus, the effect in the movie is pretty much bang on on the plausibility scale.


    P.S. My wife loves that movie.
     
    Last edited: May 11, 2005
  21. May 11, 2005 #20
    If I recall correctly, in the movie the engine falls through the roof and comes to rest in a second-storey bedroom... I'm thinking that the ground storey must have a pretty tough ceiling!
     
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