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Bad Physics in movies.

  1. Jan 3, 2007 #1
    "Bad" Physics in movies.

    OK, so, I have to do something for my physics class involving disproving the physics of something that happens in a movie. I have come up with several ideas to make this work, including scenes from:

    * Transporter 2 (A man jumps out a window about 3 stories high, catching two vials in the air which were thrown out the window before he jumped)

    * V for Vendetta (A man throws two small throwing knives a distance of about 9-10 meters, which hit (inelastic collision) two other men, knocking them backwards a distance of 1-2 meters).

    My problem is measurements. We need to prove this mathematically. Both scenes are in "slow motion", so i can only prove the speed of the objects relative to "slow motion" seconds--i am not sure if the clips are at half speed or possibly even slower. Also, in the first clip, There is not really all that much to "prove" outside of the fact that two objects which are thrown with no "y" component of velocity out the window will move downard at the same rate, and that a man could not "catch up" to objects moving at the same rate that he is. Again, in both cases, time is in slow motion.

    My question is, can you think of any way to prove either of these, or do you have any suggestions as to other movies to look into? we have covered topics like: projectile motion, forces, collisions, and general calculus-based physics (mechanics). Thanks in advance.
  2. jcsd
  3. Jan 3, 2007 #2


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    I've not seen the film, so correct me if I'm wrong, but were the men actually made to move backwards by the sole force of the knives hitting them? It seems to me that if a knife is thrown at someone from a distance of 10m, the man hit would stumble/fall backwards a metre or so, just through the shock/pain of being hit!
  4. Jan 3, 2007 #3
    the knife (14" long or so) was thrown very straight at the guy,hit him, lifted him up off the ground and back 1-2 meters. he did not "stumble" but was literally thrown back by the thing.
  5. Jan 3, 2007 #4
    Star Wars :rofl:
  6. Jan 3, 2007 #5
    Well any movie that shows people flying left and right when beeing shot with a shotgun or regular gun.

    How about superman flying up with tremendous speed and catching someone falling down a building(nevermind him beeing able to fly lol)

    FTL travel in most sci-fi movies.

    Here is a good link for you
  7. Jan 3, 2007 #6
    movies like "star wars" and other movies that are not supposed to be based in reality are not allowed, unfortunately...
  8. Jan 3, 2007 #7


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  9. Jan 3, 2007 #8


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    How about "the fast and the furious" when they dice and just avoid getting hit by the train. You should be able to calculate the velocity of the train in that shot where you see the train passing behind the two cars as they fall. I suspect the train is going far too fast for any real train.

    Edit: Oh right, this is probably too elementary as it involves only equations of motion.
    Last edited: Jan 3, 2007
  10. Jan 3, 2007 #9


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    For the first, you'd have to consider the aerodynamics of each falling object. In the real world, you have wind resistance and each object reaches its own terminal velocity. If the man somehow found a streamlined enough position, and the vials were very un-aerodynamic with a low density, and the building were tall enough for the difference to matter (really, really tall), it would theoretically be possible for the man to catch up to the vials. I don't think three stories will give the man time to catch up unless the vials had parachutes. You'd have to look at the smallest reasonable cross section the man could occupy and the largest reasonable cross section the vials would occupy, plus estimate the weight of each.

    In the second, I'm assuming the men flew backwards because the momentum of the collision sent the mens heads flying backwards, with the rest of their body and feet following behind. Just calculate the amount of time it would take the mens heads to drop to the floor, measure the horizontal distance the mens heads travelled, and you have the horizontal velocity. For the momentum, you'll need the mass of the person's entire body. (Edit: actually, this would be a little more complicated since the feet don't travel the same distance as the head) With an estimate of the knife's mass, you can calculate the velocity it must have been travelling to impart so much momentum. If the knife were around 4 kg (or 8 lbs) or a little less (taking into consideration the feet don't travel as far, etc), someone with an arm like Nolan Ryan might be able to have the same effect (but I don't think Nolan Ryan could throw an 8 lb knife nearly as fast as a 5 1/4 ounce baseball).
    Last edited: Jan 3, 2007
  11. Jan 3, 2007 #10
  12. Jan 3, 2007 #11
    Maybe ill get this in before Chi Meson...The Core!!!
  13. Jan 4, 2007 #12
    In the movie Troy, a couple of times the sun is seen rising over the sea. But the sun rises in the East and the sea around Troy is in the North and West. That should if I'm not mistaken be fairly easy to prove considering it also appears to set in the west too :smile: Also the ships appear to be heading in the wrong direction due to this, which means that Helen was either the face that launched a thousand ships in the other direction or the physics is suspect :smile:
    Last edited: Jan 4, 2007
  14. Jan 4, 2007 #13
    Back to the Future:

    "1.21 Jigawatts [sic]?!!"
  15. Jan 5, 2007 #14
    OK, so which do you think would work. the problem with time is that both are in slow motion. for the purpose of my proof i was trying to measure time in "slow motion" seconds. For the second one, i have no clue what to estimate the weight at. in the movie, the knives were 14" long and looked pretty heavy. a "regular" type throwing knife ranges from 100-250g (looked it up), but these things were much larger and appeared to be made from a heavier metal.
  16. Jan 5, 2007 #15
    i think i pretty much figured it out. i played it on my DVD player at 2x and 4x, and at 4x time appeared to be "correct" as in at the normal speed. i am assuming that the entire scene takes place at 1/4th of the real speed, so in this way i can figure out the real times
  17. Jan 5, 2007 #16
    Edit: nevermind i kinda gave away an answer

    look up old 50s movies they're sure to give you lots of false-ness
  18. Jan 5, 2007 #17


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    So let me get this straight...you're trying to apply real world laws of physics to a movie about a super human throwing daggers?
  19. Jan 5, 2007 #18


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    Leaving it in slow motion will be more advantageous. The ideal would be to look at the video frame by frame and determine the time increment per frame.

    From the instant the knife's momentum caused there to be no support under the victim's head, the victim's head was in free fall. Estimate the height of the victim and determine how long it would take the head to fall from that height. That will give you a time reference.

    After you calculate the velocity the knife has to have to impart the necessary momentum (you have to use a rough estimate for the knife's mass, but I wouldn't assume it's density is too much greater than the knives you looked up), you can estimate the time it took to cover the distance between the thrower and the victim to see if the knife's velocity was at least in the ball park.

    You're probably not going to get precise numbers for this. Just make sure you list your assumptions and provide some kind of justification for them. You're basically figuring out if the physics of the scene are even remotely possible.
  20. Jan 5, 2007 #19
    V is not a super human.
  21. Jan 5, 2007 #20


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    Yes he is. It seemed obvious from the film that he was given some sort of super strength while in prison that allowed him to survive that fire and break through all the walls, and win what would otherwise be an awful lot of mismatched fights.

    Any film based on a comic is likely to not obey normal laws of conservation of momentum.

    The best I can think of is from the film Speed. I'm sure a careful analysis would reveal plenty of times that bus would have dropped beneath 50 MPH and that it would have been physically impossible for it to make that jump on the incomplete portion of the 110.
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